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## Über dieses Buch

These Proceedings, consisting of Parts A and B, contain the edited versions of most of the papers presented at the annual Review of Progress in Quantitative Nondestructive Evaluation held at the University of Washington, Seattle on July 30 to August 4, 1995. The Review was organized by the Center for NDE at Iowa State University, in cooperation with the Ames Laboratory of the USDOE, the American Society of Nondestructive Testing, the Department of Energy, the National Institute of Standards and Technology, the Federal Aviation Administration, the National Science Foundation IndustryiUniversity Cooperative Research Centers, and the Working Group in Quantitative NDE. This year's Review of Progress in QNDE was attended by approximately 450 participants from the US and many foreign countries who presented over 375 papers. The meeting was divided into 36 sessions with as many as four sessions running concurrently. The Review covered all phases of NDE research and development from fundamental investigations to engineering applications or inspection systems, and it included many important methods of inspection science from acoustics to x-rays. In the last several years, the Review has stabilized at about its current size. Most participants seem to agree it is large enough to permit a full-scale overview of the latest developments but still small enough to retain the collegial atmosphere which has marked the Review since its inception. The Proceedings are structured in a format to reflect the organization of the Review itself, producing a more logical organization for both the meeting and the present volume.

## Inhaltsverzeichnis

### Chemical Non-Destructive Evaluation at the Center for Process Analytical Chemistry

The chemical discipline of Process Analytical Chemistry focuses on new sensors and analyzers capable of acquiring quantitative chemical information from a process via online, in-line and non-invasive approaches. This is opposed to off-line or at-line methods that lead to sampling problems, time delays, and in general, an inability to control and optimize chemical processes. The Center for Process Analytical Chemistry (CPAC) at the University of Washington has long recognized that the preferred approach to process analysis is non-invasive and has pioneered a variety of process analyzers and associated multivariate data analysis tools to exploit the relatively few physical and chemical phenomena available to the non-invasive approach. This work is similar to research at the Center for Non-Destructive Evaluation (NDE) except that the CPAC emphasis is on quantitative chemical information. Both centers are Industry/University Cooperative Research Centers initiated by grants from the National Science Foundation. This report will include topics on non-invasive analysis, or chemical NDE, selected from the CPAC research program. Additionally, the chosen topics represent a progression from the standard spectral measurements to more complex combinations of both spectral and spatial information. The combination of spectral and spatial domains has been, in part, facilitated by the development of chemometric techniques geared towards handling complex multidimensional data sets. The first topic involves extracting both chemical and physical information from infrared emission (IRE) spectra using multivariate methods from the field of chemometrics.

B. R. Kowalski, J. E. Koch

### Optical Sensors for Direct Measurements in Chemical Processes

The benefit of employing continuous, and ideally, non-destructive analysis during chemical manufacturing processes is widely recognized as providing very high rates of return to industry. As markets become increasingly more competitive, feedstocks more costly, and environmental issues escalate, the need for robust, stable, and affordable on-line process analysis systems continues to grow. Among the possible technologies employed in these process applications, optical methods are frequently used. The University of Washington Center for Process Analytical Chemistry (CPAC) has had a core effort in the investigation of sensor systems based on optical waveguide technology since its founding in 1984. These efforts have been broadly based, involving both the use of non-invasive direct optical analysis for species or parameter identification and the investigation of minimally invasive extractive approaches utilizing reagent chemistries. This work is further leveraged through strong interactions with another core CPAC program focusing on the development and use of chemometric multivatiate data analysis techniques. The three examples presented below: fiber optic evanescent wave spectroscopy using extractive coatings, the use of integrated grating and waveguide structures, and applications of low coherence high precision reflectometry for process monitoring, represent on-going optical sensor work at CPAC and demonstrate the synergy between this program and chemometrics.

L. W. Burgess

### Data Fusion for Chemical Process Monitoring, Control and Optimization

On-line sensors have long played a key role in process operations. The most common use is monitoring — sensor outputs are displayed for the plant operators, who take any required corrective action. Equally important are feedback control systems, which use the sensor outputs to take automatic corrective action. As automation becomes more prevalent, sensors are seeing even wider use.

N. L. Ricker, M. Eaton

### The Use of Spectral Methods in Predicting the Reflection and Transmission of Ultrasonic Signals Through Flaws

Finite difference solution of the wave equation will produce excellent results when the numerical procedure employs time increments and spatial discretization resulting in a Courant number of 1 for all elements. This ideal situation is difficult to achieve with reasonable mesh density when the modeling requires: 1) non-uniform grid discretization, 2) different materials or 3) more than one spatial dimension.

W. Dauksher, A. F. Emery

### Elastic Wave Diffraction at Cracks in Anisotropic Materials

Ultrasonic inspection is used to confirm that there are no defects of concern in various regions of a nuclear reactor primary circuit. All materials are naturally anisotropic, but if the grains are small relative to the ultrasonic wavelength and are also randomly oriented, then the material will appear as homogeneous and isotropic as in ferritic steel. The ultrasonic wavelength is chosen as a compromise between resolution of defect size and acoustic noise from grain boundaries. In austenitic steel, the wavelength chosen will typically be smaller than the grain size, at least in one direction. The grains are not randomly oriented but exhibit macroscopic patterns which depend on the welding process, and the material is neither homogeneous nor isotropic.

P. A. Lewis, J. A. G. Temple, G. R. Wickham

### Modeling of Ultrasonic Signals from Weak Inclusions

Recent research efforts aimed at improving the detection of hard-alpha inclusions have emphasized the need for accurately modeling the responses from such weakly-reflecting inclusions. The need arises because of the rare natural occurrence of hard-alpha inclusions, and consequently, the lacks of suitable experimental samples. These difficulties lend impetus to the application of signal modeling to augment and extend the experimental data in assessing detectability. Currently, a new approach is being developed for the purpose of predicting time-domain echoes from inclusions of specified morphology. This work is the continuation of our previous study of flat-bottomed holes [1–2] in constructing a methodology for estimating the probability of detection of flaws in titanium alloys based on a combination of physical and statistical models.

Chien-Ping Chiou, Frank J. Margetan, R. Bruce Thompson

### Ultrasonic Modeling of Real-Life NDT Situations: Applications and Further Developments

In order to cope with real-life NDT situations, we have to develop numerical time domain modeling tools which are based on direct numerical methods because only very few idealized canonical NDT situations can be modeled with analytical methods. Well known direct numerical procedures for the time domain modeling of ultrasonic waves (elastodynamic waves) are given in the references [1]–[6]. One of these, the Elastodynamic Finite Integration Technique (EFIT) [6, 7], has been applied to several real-life NDT situations. A subset of six 2D EFIT simulations include the NDT of a welded high pressure vessel, ultrasonic pipeline inspection, NDT of concrete (NDT-CE) (the reader is referred to [8]), NDT of fiber reinforced laminates, NDT of fiber reinforced T-stringer, and transducer modeling.

R. Marklein, K. J. Langenberg, S. Klaholz, J. Kostka

### Mechanical Reciprocity Principles and Ultrasonic Measurement Models

An electromechanical reciprocity relation derived by Auld [1] has become a powerful tool for modeling many ultrasonic NDE experiments. Auld’s relation has also served as the foundation for developing more explicit models of ultrasonic systems, such as the quasi-plane wave measurement model of Thompson and Gray [2], which has been used for a variety of quantitative calibration, classification, and flaw sizing applications. Here, we will develop a relationship similar to that of Auld’s but using simpler mechanical reciprocity relations. One side benefit of this mechanical reciprocity approach will be an explicit statement of the manner in which ultrasonic transducers are mechanically reciprocal to one another.

Lester W. Schmerr, Alexander Sedov

### Numerical Simulation of Pulsed Pressure Waves in Attenuative and Dispersive Media

This paper proposes a method of simulating pulsed pressure waves in attenuative fluids as typically exist in biological medium. The numerical algorithm is based upon an explicit time domain formulation which is capable of determining the form of an acoustic wave as it evolves in both time and space.

Margaret G. Wismer, Reinhold Ludwig

### Theoretical Predictions and Experimental Measurements of Echo Responses from Tilted Flat-Bottomed Holes

In some earlier work, semi-analytic methods were proposed to predict echo responses from point-like targets [1] and normally aligned flat-bottomed holes in solids (FBH) [2]. A new, more general formulation has recently been developed (hereafter, the “elastic model” [3]), capable of predicting responses from defects of arbitrary shape. In this paper, theoretical predictions using the new model are compared with experimentally measured echo responses from tilted FBH’s, where the hole bottom is tilted relative to the transducer axis. A related study [4] has already been presented in which experimental echo responses from tilted disks immersed in a fluid were compared with results predicted by an earlier model [5]. In that earlier model, mode conversion was not taken into account and only compression waves were considered (the “acoustic model”).

Alain Lhémery, John P. Weight

### Distance Amplitude Correction Factors for Immersion Ultrasonic Measurements through Curved Surfaces

Near net-shaped forgings offer significant advantages for component manufacture, including less material waste and reduced costs for machining to final shape. However, curved entry surfaces on near net shape forgings create complications for ultrasonic inspection methods. In immersion ultrasonic testing, entry surface curvature causes ultrasonic beam focusing or defocusing, which affects the detection sensitivity to interior material flaws, such as voids and inclusions, as compared to inspection through planar surfaces.

Tim Gray, Mike Garton, Paul Zombo

### Defect Sizing Using Distance-Gain-Size Diagrams for Flat-Bottomed Holes in a Solid: Theoretical Analysis and Experimental Verification

Although there are a number of potential pitfalls, the classical method of relating defect area to echo amplitude is still the most widely used method to size defects using ultrasonic pulse-echo techniques. In 1959 Krautkramer [1] was the first to introduce a set of curves (DGS diagrams) showing the variation of echo amplitude with range and target size. As Krautkramer made clear, such curves are dependent on transducer pulse shape. For the very far field he gave theoretical results assuming a fluid-like medium of propagation, but he had to resort to a large number of experimental measurements to construct the near field portion of the curves. Well known problems in using DGS diagrams include the sensitivity of echo amplitudes to target angular and lateral alignment and the need to construct a new set of curves for each transducer pulse shape. Furthermore, when sizing targets in solids there are likely to be errors if curves constructed assuming a fluid medium are used. In 1987, McLaren and Weight [2] gave an impulse-response method to predict echo amplitudes for arbitrary target position in the field and for any transducer pulse shape. Normally-aligned, flat-ended cylindrical targets and a fluid medium were assumed. More recently, Schmerr and Sedov [3,4] have calculated single frequency DGS diagrams for flat-bottomed holes (FBH’s), for both direct and water coupling, but the holes are assumed to be in a fluid-like material. Their method takes account of diffraction and refraction effects but not mode conversion. A more exact treatment of the effect of a solid medium of propagation on DGS diagrams has been given by Sumbatyan and Buyove [5] who developed DGS diagrams for disc-like targets using a boundary element method to solve the elastodynamic equations, but again, only for the case of continuous sinusoidal waves. One disadvantage of such an approach is that the calculations can be rather time consuming.

J. P. Weight, S. A. Hussein

### Elastic Wave Scattering by an Interface Crack in Layered Materials

Interfaces play an important role in structural performance of composite materials, which are widely used in many industrial applications. Composite materials are usually made in layered structure, where two adjacent materials are bonded together along their common faces. Therefore, the inspection technique for determining the quality of the bonding interface is of great interest. The elastic wave scattering method for characterization is often used for this purpose[1].

M. Kitahara, J.-H. Yin

### Numerical Model of Elastic Wave Interactions with a Diffusion Bond

Solid state bonds are being extensively developed and used in aerospace structures. As with any other bonding technology, a reliable method for estimating bond strength is needed in order to establish confidence levels in the process routes and the finished bonded products. Ultrasonic nondestructive evaluation (NDE) methods are currently being investigated for detection and characterisation of anomalies at diffusion bonds between similar or dissimilar metallic layers [1], There is large gap in the understanding of the nature and behaviour of imperfect bonds. These are diffusion bonds which do not show up as good reflectors of ultrasonic energy, but at the same time have very low bond strengths. A purely experimental approach to developing an NDE method is often hampered by the difficulties in fabricating samples with controlled defects, and other experimental uncertainties. The objectives of this study are thus to develop a computer model to determine whether ultrasound could be used for characterisation of imperfections at a diffusion bond, and what would be the optimum set-up for obtaining the most reliable information about the condition of the bond line.

Nader Saffari, Jianwei Zhou

### Spring and Asymptotic Boundary Condition Models for Study of Scattering by Thin Cylindrical Interphases

Specially designed fiber-matrix interphases are created in modern composites to improve fracture toughness, chemical compatibility and matching of thermal expansion coefficients between composite constituents [1, 2, 3]. Since the interphase transfers the load from matrix to fiber, the interphase elastic moduli, thickness and the quality of bonding with the surrounding fiber and matrix are essential in determining composite mechanical performance. Such interphase conditions can be sensed by ultrasonic waves due to strong interphase effects on wave scattering from fibers. However the interphase properties (elastic modulus and thickness) are in-situ parameters and are often difficult to define. One way to get around this is to introduce simplified boundary condition (B.C.) models to describe the displacement and stress fields across the interphase directly. In this paper we will address this problem with emphasis on spring and asymptotic B.C. models as a representation of a thin fiber-matrix interphase when studying wave scattering from fibers.

W. Huang, S. I. Rokhlin

### Coherent and Incoherent Scattering Mechanisms in Air-Filled Permeable Materials

Ultrasonic evaluation of porous materials can take advantage of some very specific acoustic phenomena that occur only in fluid-saturated consolidated solids of continuously connected pore structure. The most interesting feature of acoustic wave propagation in such media is the appearance of a second compressional wave, the so-called slow wave [1,2]. The slow compressional wave represents a relative motion between the fluid and the solid frame. This motion is very sensitive to the kinematic viscosity of the fluid and the dynamic permeability of the porous formation. Certain material properties such as tortuosity, permeability, porosity, and pore size, shape and surface quality are inherently connected to the porous nature of the material and can be evaluated best from the propagation properties of the slow compressional wave.

Peter B. Nagy

### Modeling and Parameter Estimation of Ultrasonic Backscattered Echoes

Ultrasonic backscattered echoes represent not only the impulse response of the ultrasonic transducer, but also contain information pertaining to the inhomogeneity of the propagation path, effect of frequency dependent absorption and scattering, dispersion effect, and geometric shape, size and orientation of reflectors. Therefore, a well-defined modeling of the backscattered echoes leading to the estimation of arrival time, echo skewness, center frequency, and bandwidth is highly desirable for the nondestructive evaluation of materials [1]. In this paper, we model the backscattered echoes, assuming that all parameters describing the shape of the echo are unknown. Then, iterative parameter estimation techniques such as the Gauss-Newton method [2] or Simplex-Search method [3] have been applied to estimate echo parameters. These algorithms have been evaluated in terms of rate of convergence, sub-optimal estimation due to local minima, and presence of noise.

R. Demirli, X. M. Jin, J. Saniie

### A Computationally Efficient Modeling Code for Sh-Waves in Austenitic Welds Using an Explicit Space-Time Green-Function

For ultrasonic inspection of austenitic welds and cladded components horizontally polarized shear (SH) waves — as generated by electromagnetic acoustic transducers (EMATs) — have certain benefits compared with quasi-vertically polarized shear and quasi-pressure waves. SH-waves suffer the least distortion of all three wave modes when propagated through anisotropic weld material and no energy is lost through mode conversion at the steel/free surface or base metal/weld interfaces. To explain experimentally observed phenomena and to predict the cases where SH-waves might be best employed, modeling of the respective wave propagation effects is useful. In this contribution, a computationally efficient modeling code is presented for SH-waves propagating in transversely isotropic media, thus particularly applicable to ideally fiber-textured austenitic weld material. An explicit space-time domain far-field representation of Green’s dyadic function has been derived with respect to the wave type under concern, the fiber direction being included as a free parameter. The obtained relationships have been applied to the Generalized Point-Source-Synthesis method (GPSS [1,2]) to model radiation, propagation and scattering effects. The code thus improved — SH-GPSS — is characterized by a considerable reduction of computer run-time and is therefore particularly convenient in view of a respective extension to inhomogeneous weldments. Numerical results are presented for both continuous wave and time-dependent rf-impulse modeling for austenitic weld metal specimens, covering field profiles as well as wave front snapshots for a phased array EMAT-probe.

Martin Spies, Michael Kröning

### Mathematical Modelling of the Rayleigh Wave Reception by the System Whith Elastic Waveguide

Elastic waveguides are often applied in the acoustic nondestructive evaluation systems. Even though elastic waves in the waveguides are studied in the literature rather in detail, the very important problem of the incident wave interaction with the waveguide and its re-emission into the waveguide practically hasn’t been studied.

V. M. Shikchman, S. P. Pelts

### Green’s Function for Lamb’s Problem and Rayleigh Wave Propagation in General Transversely Isotropic Materials1

Composite materials have gained considerable industrial importance, being widely applied e.g. in aerospace industries. The need for their proper testing in view of delaminations, inclusions and other defects has correspondingly stimulated the interest in describing wave propagation in such anisotropic media. In this study, Lamb’s problem of determining the disturbance resulting from a point source in a half-space [1] is investigated for the case of transversely isotropic (TI) symmetry, which is characteristic for unidirectional fiber composites and extruded metal-matrix composites, but also for fiber-textured columnar-grained steels. Using the dyadic and triadic full-space Green’s functions obtained previously in their 2d-space-time spectral representations [2], a corresponding representation of Green’s dyad for the half-space has been derived exploiting the boundary condition of the stress-free surface. The resulting dyadic function is the solution of the elastic wave equation with point forces applied at the surface or within the uniform half-space, the fiber orientation being variable. First numerical evaluations have been performed with respect to Rayleigh-surface wave propagation by determining the zeroes of the corresponding Rayleigh function, which is included in the analytical expressions. Resulting slowness and wave curves are presented for several materials. The work presented can be further applied, e.g., to determine Rayleigh wave directivity patterns for point sources on the half-space as well as to model laser-generated wave propagation in composites. Application in the field of seismic wave propagation is also possible.

Martin Spies, Michael Kröning

### Dispersion of Guided Circumferential Waves in a Circular Annulus

Fatigue cracks have been found to initiate and grow in the radial direction in many of the annulus shaped components in aging helicopters. Those include some of the most critical components such as the rotor hub, connecting links and pitch shaft, etc. At the present time, detection of such radial fatigue cracks relies mostly on visual inspection. A more systematic, automated, and efficient method to detect these cracks must be developed.

Jianmin Qu, Yves Berthelot, Zhongbo Li

### Geometry and Diffraction Effects in Acoustic Beam Reflection Studies from a Fluid-Loaded Plate

The near coincidence of the guided wave modes of a plate with zeroes of the reflection coefficient has been used often in the past [1–4] to estimate the plate’s guided wave mode spectrum. Schoch [5] in his quantitative treatment of acoustic reflection from plates expresses the reflected field as a one-dimensional spectral integral over the incident field weighted by the reflection coefficient (RC) and by a propagator term which accounts for diffraction in the incident plane. Bertoni and Tamir [6] later evaluated this integral approximately for an incident Gaussian beam in their analysis of leaky Rayleigh waves. This procedure was later extended to reflection from plates, by Pitts, et al. [7]. Comparisons between numerical or analytical evaluations of this integral formulation of the reflected field and experimental measurements have been made by several authors. A missing element in essentially all these prior treatments, however, is a rigorous analysis of the influence of the receiving transducer.

O. I. Lobkis, A. Safaeinili, D. E. Chimenti

### Lamb Wave Propagation Across a Lap Joint

Lap joints are common elements of aircraft and other engineered structures. They are often subject to hidden defects which are caused by corrosion and fatigue, and are very difficult to detect. Development of accurate and efficient methods for the early detection of corrosion and fatigue cracks in lap joints is of considerable current interest. Ultrasonic techniques using guided waves offer the possibility of improving the technology of detecting and characterizing flaws within lap joints. It is well known that the characteristics of guided waves can be used to detect defects in plates [1]. However, the geometry of the lap joint makes it difficult to extend these techniques to lap joints. In this paper we consider the theoretical problem of the propagation of guided waves across a simple model of the lap joint in an effort to understand the interaction of the guided waves with the geometrical features of the lap joint. The geometry of the lap joint, including the vertical stress free boundaries, the rectangular corners, and the change in thickness, makes it impossible to derive a closed form solution to the problem of wave propagation across it. The problem can only be attacked by numerical methods. Conventional finite element methods fail when dealing with problems with infinite domains. In order to handle problems with local inhomogeneities or irregular shapes in an infinite domain, hybrid methods must be used.

Zensheu Chang, Dawei Guo, Ajit K. Mal

### The Existence of Low Loss Lamb Modes in Highly Attenuative Media

A great deal of work on Lamb waves has been devoted to their application to materials which are assumed to be elastic and hence have negligible material attenuation (see e.g. [1–2]). However for materials such as polyethylene, the assumption of elasticity may be invalid as the amplitude of the Lamb waves will decrease with propagation distance, a phenomenon not accounted for in the elastic treatment. From a dispersion curve point of view, the interest lies in determining whether the inclusion of attenuation will greatly change their characteristics. In most, if not all, circumstances the bulk shear wave attenuation for any given material is considerably larger than that for the bulk longitudinal wave. Given that Lamb waves can be modelled by the superposition of such bulk waves, it is believed that the shear wave attenuation will be the dominant factor on the character of the dispersion curves.

Che-Wan Chan, Peter Cawley

### Scattering Analysis and Simulation for Lamb Wave Ultrasonic Testing

Lamb wave ultrasonic testing has been practically used as a nondestructive method to detect flaws in a thin plate. Accuracy of the ultrasonic testing has, however, not been evaluated quantitatively since propagation and scattering processes of Lamb waves depend on various experimental conditions such as the frequency, the plate thickness, the wave mode and the flaw's properties. As shown in Fig. 1, the Lamb wave ultrasonic testing includes three wave phenomena: 1) conversion of ultrasonic waves driven by a transducer into Lamb waves and vice versa, 2) Lamb wave propagation between a transducer and a flaw, and 3) scattering of Lamb waves by a flaw. The relatively classical elastodynamic theories have been applied to understand the former two wave phenomena, 1) and 2). The conversion process of ultrasonic waves to Lamb waves can be interpreted as the reflection and transmission of acoustic-elastic waves at the interface of dissimilar materials. Also, the propagation of Lamb waves in a plate is one of the fundamental problems in the classical elastodynamics. On the other hand, there are few studies on the scattering of Lamb waves, although the Lamb wave scattering is of importance to establish a quantitative Lamb wave ultrasonic method. Coen et al. [1] showed that the symmetric Lamb wave is more sensitive to flaws around the center plane of the plate, while the antisymmetric Lamb wave has better responses to flaws in the vicinity of the top and bottom surfaces of the plate. In order to investigate such interaction of the Lamb wave with a flaw, it is necessary to analyze the scattering problem of Lamb waves.

Sohichi Hirose, Masaki Yamano

### Lamb Wave Scattering from Rivets

For structures with large surface areas, a full integrity evaluation can be a time-consuming operation. Lamb wave techniques allow this evaluation to be performed with waves propagating along one dimension of the inspection area while the probing transducers are moved in the perpendicular dimension, giving information about the presence of flaws within the entire scanned area. For riveted structures the scattering of the Lamb waves from the rivets is often the dominant feature in the measured response, masking the more subtle effects of Lamb wave interactions with the flaws of interest [1]. In this paper we consider the scattering of lowest mode symmetric and antisymmetric Lamb waves from model rivets, and derive analytic expressions for the scattered fields. With solutions of this type the disruptive effects of the rivets can be “processed out” of measured data in order to expose the signals which are due to the flaws in the structure.

Mark K. Hinders

### Guided Wave Behavior Analysis in Multi-Layered Inhomogeneous Anisotropic Plates

Guided waves behave much differently in inhomogeneous anisotropic plates than in homogeneous anisotropic plates. It has been reported before that guided waves in inhomogeneous plates tend to follow preferred directions based on the location of ply-groups as well as the orientation of the fibers [1]. The pattern obtained by imaging the leaked energy into the surrounding fluid (earlier called as Plate Wave Flow Patterns) has been shown to indicate fiber orientations [2]. In this paper, a model based on the Thomson-Haskell transfer matrix is employed to obtain the internal distributions of the energy vector within the inhomogeneous plate. Based on the theoretical results, the plate wave flow patterns can be predicted and compared with the experimental results. The results provide insight into the understanding of the generation mechanism of guided wave mode patterns in inhomogeneous plates.

Yuyin Ji, Rani Sullivan, Krishnan Balasubramaniam

### Guided Wave Mode Propagation Influence for a Shear Horizontal Wave Source in An Anisotropic Viscoelastic Layer

Sensors are used in a variety of different manufacturing systems in order to improve product quality and process control. In a recent development by Menon et al. [1], guided waves are used to monitor part quality. The purpose of this paper is to explore the possibility of using shear horizontal (SH) waves for an anisotropic viscoelastic layer.

S. P. Pelts, S. M. Menon, J. L. Rose

### Lamb Wave Mode Sensitivity to Detect Various Material Defects in Multilayered Composite Plates

Ultrasonic method has become a very popular nondestructive characterizing technique because of its versatility and ease of operation. It can usually detect internal cracks and inclusion type defects in homogeneous or layered materials without any difficulty. However, it has its own shortcomings. It is not very effective in detecting cracks which are vertical to the plate surface. This is because the ultrasonic signal is not reflected by the crack when the signal propagation direction is parallel to the crack surface. The back scattering technique and acoustic microscopy technique are used for detecting such vertical defects. However, when these defects are not located very close to the surface these techniques also encounter difficulties. Use of Lamb waves to detect such defects may be a viable alternative to the currently practiced methods. Theoretical studies by Kundu and Blodgett [1], Yang and Kundu [2,3] and Yang [4] have shown that different Lamb wave modes produce different levels of excitation in various layers in a multilayered solid plate. This phenomenon is exploited here to experimentally generate C-scan images of different layers of a composite plate by propagating Lamb waves of different modes through the plate. The C-scan image generated by receiving the leaky Lamb waves is denoted as the “L-scan” image in this paper.

Tribikram Kundu, Prasanna Karpur, Theodore E. Matikas, Perikles D. Nicolaou

### Leaky Lamb Wave Along VCR Magnetic Tapes

High recording density with the home-use digital VCRs requires the use of narrow tracks, short recording wavelength, and thin magnetic tapes. Knowledge of Young’s modulus of the tape is essential for the precise positioning of the tape on the rotating drums and then a stable tape-to-head interface. The magnetic tapes usually show different Young’s moduli for the machine direction (MD) and the transverse direction (TD) [1]. The anisotropy develops mainly in the base film of polyethylene terephthalate (PET) through the partial crystallization and the crystallite orientation alignment during the stretching process on the tapes [2], while the original PET sheet, from which the tapes are cut, shows much less anisotropy. This situation requires the determination of Young’s moduli for both MD and TD of the tape. The tapes on play are straightened by tensile loads, which should be controlled with Young’s modulus for the MD. Too much load may distort the recorded tracks or damage the tape. Besides, the vertical load is applied onto both edges of the running tape by the guiding rollers. Again, too much load may induce the tape buckling. Critical load is proportional to the Young’s modulus in the TD. Large moduli are desirable for both directions.

M. Hirao, K. Yokota, H. Fukuoka

### Utility of Lamb Waves for Near Surface Crack Detection

Ultrasonic waves have a long history in detection of surface breaking cracks. Attempts are being made to use guided waves as a defect detection tool in aging skin structures in aircrafts and in the power generation industries as these waves offer a great advantage over conventional bulks waves. Guided waves can be excited at one position and allowed to propagate considerable distances before attenuating. Depending on the configuration employed for defect detection, reflected or received waveforms give information regarding the integrity of the structure along the line of sight. This description makes the technique look rather simple. Particularly, NDT utilizing Lamb waves is more complex due to the existence of two or more modes at any given frequency. Success was reported by several authors on defect detection using Lamb waves. Brief or no explanation was given on the reasons behind the choice of specific excitation frequencies and incident angles. The emphasis was solely on the defect detection aspects.

Krishna M. Rajana, Younho Cho, Joseph L. Rose

### Lamb Wave Modes Propagating Along Arbitrary Directions in an Orthotropic Plate

The recent interests [1–5] on the Lamb wave study of orthotropic sheet materials have been motivated by nondestructive evaluation (NDE) and characterization of the fiber composite plate. The dispersive properties of Lamb waves in an isotropic plate have been well known since the original work of Lamb [6]. However, the problem of Lamb waves in anisotropic plates is, as yet, largely unexplored. For the propagation of elastic waves in composites reinforced with large diameter fibers, such as the SiC fibers employed to reinforce both ceramic and metallic matrix materials, the dynamic effects of the microstructuring must be considered if the fiber is large enough to equal a longitudinal acoustic quarter-wavelength in the range of 10 to 20 MHz. However, in most fiber composite systems, the fiber diameter is small enough to permit modelling of the material as a homogeneous, but anisotropic, medium which retains the symmetry of the composite, but ignores its microstructural nature.

J. C. Cheng, S. Y. Zhang

### Lamb Wave Propagation in Thermally Damaged Composites

The use of composites in primary and secondary structures of aerospace vehicles is important for increased performance with little weight penalty. Determining the response to thermal damage is necessary for a complete understanding of the total use environment of these materials. The objective of the research presented here is to provide a method of quantifying the amount of thermal damage in composite materials. Components which have non-visible damage, but have degraded performance on the order of several percent, are of interest. At this level of damage the safety margin designed into the structure may be compromised.

Michael D. Seale, Barry T. Smith

### Perturbation and Finite Difference Solutions for Wave Propagation in Composites with Periodic Stiffness Variations

Wave propagation in composite materials with discrete changes in properties has been extensively studied and is well understood. In contrast, wave propagation in composites with smooth continuous periodic stiffness variations has only begun to be studied [1]. Use of direct analysis techniques for wave propagation in a composite material with varying stiffness has lead to mathematical contradictions and has indicated the need for a different approach [2]. The present study investigated wave propagation in a composite with smooth continuous periodic stiffness variations using perturbation techniques and a model simulation with a refined finite difference method.

Joseph S. McIntyre, Maurice L. Rasmussen, Ronald A. Kline, Charles W. Bert

### Axisymmetric Waves in Layered Anisotropic Fibers and Composites

The complicated morphology of the new generation of advanced fibrous composites gave further impetus to the study of the interaction of ultrasonic waves with multilayered concentric cylindrical systems. Typically, the fiber consists of a cylindrical core embedded in a cladding region followed by a distinct interface zone separating the fiber system from the host (matrix) region. In addition, the cladding region itself often consists of subregions which can be identified as distinct layers. Each individual layer can posses certain degree of microscopic anisotropy adding to the macroscopic anisotropy produced by the presence of layering and imperfect interfaces. Relatively few efforts have been spent upon the study of free and immersed homogeneous anisotropic rods [1–5]. These works are insufficient to model real situations encountered in materials characterization of advanced fibrous composites. In order to better model advanced fibrous composites at least three major effects need to be accounted for. These are the inhomogeneous nature of the structure as reflected in its multilayering, the inherent microscopic anisotropy of some of the constituents and finally the quality of the interfaces. In this paper we briefly describe a unified analytical treatment of wave propagation along the fiber direction of multilayered coaxial fibrous systems embedded in a host material. A more detailed discussion of this general treatment will be presented elsewhere [6]. Figure 1 shows typical geometric situations including (a) a single multilayered fiber, (b) a single multilayered fiber either immersed in an infinite fluid or embedded in an infinite solid, and an infinite composite material with periodically distributed multilayered fiber.

Adnan H. Nayfeh, Peter B. Nagy

### Efficient Modeling of Finite Acoustic Beam Excitation and Detection of Interface and Bulk Waves on Planar and Cylindrical Fluid-Solid Structures

Ultrasonic (UT) nondestructive evaluation (NDE) of fluid-immersed bulk or layered elastic materials is commonly carried out with a single or a pair of acoustic transducers used in pulse-echo or pitch-catch modes. Applications range from determining material properties to identifying interior and/or surface defects. Some of the configurations often encountered in UT-NDE, and that are considered in this paper, are depicted in Figure 1. These sketches show a transmitting transducer radiating a continuous or pulsed finite beam that excites interface or bulk waves within the elastic part. Acoustic energy radiated back by the elastic part into the fluid is collected by a receiving transducer which converts it into a voltage. Quantitative modeling of this class of experiments, even under assumptions of ideal conditions (e.g. homogeneous and isotropic layers and defect-free structures), is important for design optimization purposes and for understanding and interpreting the data acquired. It also provides a first step towards tackling non-ideal configurations. There is a large body of work that address this objective through various approaches (analytical, numerical, hybrid, etc); the reader is referred to References in this issue and in past issues of the Proceedings of this conference. This paper presents recent developments in the application of analytic methods to comprehensive and efficient modeling of the type of configurations depicted in Fig. 1. Comprehensive in the sense that the methodology used can account for 1) arbitrary three-dimensional (3D) diffraction and orientation of transmitting and receiving transducers; 2) interface and layering wave effects such as the excitation of surface and modal waves in the structures inspected.

Smaine Zeroug

### Aperture Integral Ultrasonic Pulse Transmission Model

This paper discusses a numerical algorithm and supporting formulation for evaluating ultrasonic pulse transmission through non-planar component geometries. The algorithm is engineered to model experimental configurations where irregularities in surface geometry preclude the use of less rigorous approaches, such as a field expansion about a single entry point. The algorithm formulation represents the transmitted pulse as a surface integral coinciding with a pulse origin aperture, employing the Green function for the water-component system. The model explicitly considers the component surface geometry over the footprint of the incident pulse, thus allowing consideration of smooth yet non-expandable (i.e. in power series about a single point) geometries, such as adjoining flat and fillet surfaces. A computationally efficient algorithm results from use of asymptotic Green function approximations. Approaches are also discussed under conditions where the asymptotic Green function expressions are singular or invalid, due to focusing by surface concavity or transmission near critical angles. Consideration of pulse time dependence represents an extension of previous work [1], as also does treatment of surface concavity and critical angle transmission. The following sections summarize theoretical formulation and algorithmic implementation, followed by the presentation of illustrative computations.

R. A. Roberts

### Finite Element Modeling of Transient Wave Phenomena at Solid/Fluid Interfaces

The solid/fluid interface appears in many ultrasonic measurement systems. Models for the system must take account of the interface. Analytical models for wave phenomena at the interface (especially curved interfaces) are either difficult or subject to severe approximation. The finite element method is ideal for this especially when the problem domain is bounded. A survey of this subject has been given by Kalinowski [1]. In this paper, an axisymmetric finite element model is developed for a solid medium and a fluid medium in contact. Displacement is used as the primary variable in the solid media and pressure in the fluid. The scalar pressure in the fluid medium makes the total degrees of freedom less than if displacement is used. The global mass matrix and stiffness matrix are rendered symmetric by introducing a potential variable for the fluid medium [2]. The final finite element equations are solved by the explicit integration approach.

T. Xue, W. Lord, S. Udpa, L. Udpa, M. Mina

### Ultrasonic Wave Propagation through Nozzles and Pipes with Claddings around their Inner Walls

The inner walls of pipes and nozzles used in nuclear power plants are susceptible to degradation by mechanism such as stress corrosion cracking. To protect these inner walls against excessive corrosion and the formation of cracks, a cladding layer is used. The clad layer generally has anisotropic elastic properties and complex surface topology.

A. Minachi, R. B. Thompson

### Absorbing Boundary Conditions with Zero Reflection Angles for Compressional and Shear Incident Waves

The technique of artificial absorbing boundaries is often applied in the numerical modelling of wave propagation and other related numerical studies of ultrasonic NDE. For a physical problem with an unbounded spatial domain, the introduction of artificial boundaries to bound the computational domain can be vital for a successful numerical solution. Even for problems with bounded spatial domains, the introduction of artificial absorbing boundaries that localize the area of interest can reduce the computing cost and allow limited computer resources to achieve more. It is particularly so in ultrasonic NDE where the field near the scatterers or transducers is normally of interest.

Jianwei Zhou, Nader Saffari

### Lamb Waves Propagation in Aluminum Honeycomb Structures

Honeycomb sandwich, formed by adhesively bonding metallic or non-metallic thin plates with honeycomb core, is one of the composites used extensively in the industry. The common types of honeycomb cores presently being produced are aramid fiber reinforced, glass fiber, and the metallic core such as titanium, corrosion resistant steel and aluminum. The facing materials are usually aluminum, fiberglass or carbon fibre laminates. With the core carrying the shear loads and skins carrying the bending load the honeycomb panel enjoys many applications in the aerospace industries and have been found in many structural components due to their extremely light weight, high stiffness and strength-to-weight ratio.

N. Guo, M. K. Lim

### Recent Developments in Modeling Eddy-Current Probe-Flaw Interactions

A number of industries have been traditional users of eddy-current technology in nondestructive evaluation (NDE). The traditional mode of eddy-current inspection has been ‘monostatic,’ in which a single probe is used as both a ‘transmitter’ and ‘receiver’ Research in these industries now indicates the value of using ‘bistatic,’ or even ‘multistatic’ probe configurations, in which a single probe is used as a transmitter, and one or more probes are used as receivers. The probes may be either air core, or ferrite core, or perhaps a combination. Some examples of bistatic configurations are the split-core differential probe, and remote-field probes. The industry is turning to computer codes that are based on sophisticated computational electromagnetics algorithms in order to design these probes, and to interpret the signals that arise from the interaction of these probes with flaws.

Harold A. Sabbagh, R. Kim Murphy, Lai Wan Woo, Elias H. Sabbagh, Kenji Krzywosz

### Extended Magnetic Potential Method for Quasi-Static Electromagnetism and Eddy Current Phenomena

In-service eddy current (EC) inspection of aircraft engine components is one of the most demanding NDE applications. Any defect in such high-performance components will have serious consequences, and hence reliable periodic inspections are required. Yet, it is known that significant improvements to the current implementation are desirable in various aspects. One of our recent modeling activities is directed toward developing a software simulator that is able to predict the performance of inspections and, hence, to examine any improvement ideas such as new probe designs on computers.

N. Nakagawa, J. Chao

### Forward and Inverse Processing in Electromagnetic NDE Using Squids

Electromagnetic NDE has been successfully applied to the detection of surface cracks and is routinely used to locate flaws in airframes, pipelines and in steel offshore oil platforms. However, there are still many problems to be solved, particularly in the aviation industry, which require the detection of deeper flaws such as corrosion in multi-layered structures and cracks around rivet holes which are obscured by the head of the rivet. Most systems use coils as detectors (though Hall probes are occasionally used), which have low sensitivity at low frequencies due to the fact that the induced voltage is proportional to the rate of change of magnetic flux through the coil. Unfortunately it is necessary to use low frequencies to detect deep subsurface flaws on account of the skin-depth effect, otherwise the electromagnetic field cannot propagate down to the depth of the flaw. SQUID (Superconducting Quantum Interference Device) sensors are ideally suited to overcome the deficiencies of coils, because they are primarily detectors of magnetic flux which, together with their high sensitivity, makes the detection of deep flaws more likely. SQUIDs have been successfully used for measuring very low magnetic fields, particularly in the field of biomagnetism, and it is hoped to exploit this sensitivity to detect flaws at large stand-off distances for example in pipelines which are surrounded by thick layers of cladding.

D. McA. McKirdy, A. Cochran, G. B. Donaldson, A. McNab

### Eddy Current Flow Near an Edge: A Comparison between Stratton-Chu and Magnetic Potential Formulations

This paper presents a comparative study of two different boundary integral equation (BIE) formulations applicable to eddy current (EC) problems.

J. Chao, D. Lehther, J. C. Moulder, N. Nakagawa

### Edge Crack Detection: A Theoretical and Experimental Study

Detection of cracks close to an edge by conventional eddy current techniques is difficult, owing to the significant background signal from the edge. It is necessary to develop methods for minimizing the effect of the background signal, thereby increasing the probability of detection. The edge signal is known to be influenced by a number of factors and it is essential to characterize these in order to minimize its influence. This study aims at developing a good understanding of these factors so as to facilitate the development of such techniques. A boundary element method (BEM) approach was used to model the signal due to the edge and to compare with experimental measurements. Experiments were conducted on electro-discharge machined (EDM) slots in the vicinity of an edge using both absolute and differential probes. The influence of orientation of a differential probe on the signal from the crack and the edge was also studied. We report on the development of improved methods to reduce the influence of signal due to the edge by appropriate use of differential probes and with the aid of signal processing. An inexpensive physical technique which results in a improved detectability was also developed.

D. Lehther, J. Chao, N. Nakagawa, J. C. Moulder

### Analytical Solutions to the Problem of Eddy Current Probes Consisting of Long Parallel Conductors

Eddy current testing is currently used to determine conductive specimen physical characteristics and to detect defects by measurements of electrical impedance of an eddy current probe. In general, reliable quantitative NDE requires accurate measurements and a theory to interpret them. In some NDE applications related to coated metals inspection eddy current probes which induce planar or linear currents in a sample are used for flaw detection, thickness and conductivity measurements [1]. For probes of this type, e.g. rectangular or meander like coils, there still exists the need of theoretical modeling.

B. de Halleux, O. Lesage, C. Mertes, A. Ptchelintsev

### Wavelet Expansions in Volume Integral Method of Eddy-Current Modeling

Eddy current nondestructive evaluation is a widely used in-plant NDE technique in which the flaw information is extracted from the impedance change of a coil placed above the metal testpiece. To obtain quantitative information about flaw size and shape, we would like to have a theoretical model which is able to predict the impedance change of a coil for different flaws in the test geometry. Because of its importance, this eddy current forward problem has been studied extensively for many years. For simple flaw shapes and geometry, it is possible to obtain analytical solutions. However, for flaws with irregular shapes and complex geometry, an analytical solution usually is not available so we must find a numerical solution. There have been several numerical models in the literature, e.g., the finite element method[l], the boundary element method[2], the volume integral method[3–5] and methods based on variational formulas[6].

Bing Wang, John P. Basart, John C. Moulder

### Three Dimensional Simulation of Remote Field Eddy Current Nondestructive Testing Phenomena

The remote field eddy current (RFEC) technique[1], [2] is widely used as a nondestructive evaluation tool for inspecting metallic pipes and tubing. Essentially, the RFEC phenomenon can be observed when an AC coil is excited inside a conducting tube (see Fig. 1). The RFEC signal can be sensed by a pick-up coil located 2–3 diameters away from the excitation coil. The signal is closely related to the tube wall condition, thickness, permeability, and conductivity. Particularly the signal phase is approximately linearly related to the tube wall thickness.

Y. Zhao, M. J. Chen, M. X. Qu, Y. S. Sun, J. T. Si

### Optimal Multidimensional Multifrequency Eddy Current Mixing Techniques

The received signals from eddy current testing (ET) sensors are dependent on a large number of variables. These include conductivity, permeability, geometry, and defects in the material being tested, as well as sensor liftoff and orientation. In order to isolate the effects of any one of these properties from the others, multiple inspection frequencies are often used. This paper describes novel adaptations of a standard technique for combining (mixing) the data from multiple frequencies in order to isolate signals of interest. The adaptations were designed for the optimization of signal-to-noise ratio (SNR), where the “signal” is the information of interest and the “noise” is information from other system variables. For example, for detecting cracks in the presence of geometrical changes, the “signal” is the crack information and the “noise” is the information from the geometrical changes.

K. A. Bartels, J. L. Fisher

### Depth-Selective Squid Eddy Current Techniques for Second Layer Flaw Detection

Detecting hidden damage at the second layer of lap joint structures in aircraft by using eddy current techniques is difficult because the cracks are usually located beneath the head of the rivets, which cause large signals that mask the relatively smaller signals from the second layer cracks. In addition, using lower frequency excitation for deeper penetration of the eddy currents reduces the signal-to-noise ratio. Several new techniques have been tried, such as using a sheet inducer to increase the penetration depth or using sensors that have a frequency independent response, to improve the signal to noise ratio [1, 2]. To better extract the information due to the cracks beneath the rivet at the second layer, we have developed a phase analysis of the magnetic field produced from the eddy currents induced by the sheet inducer.

Yu Pei Ma, John P. Wikswo

### Thickness and Conductivity of Metallic Layers from Pulsed Eddy Current Measurements

Coatings and surface treatments find a wide range of technological applications; they can provide wear resistance, oxidation and corrosion protection, electrical contact or isolation and thermal insulation. Consequently, the ability to determine the thickness of coated metals is important for both process control and in-service inspection of parts. Presently ultrasonic, thermal, and eddy current inspection methods are used, depending on the circumstances. A number of commercial instruments for determining the thickness of nonconducting coatings on metal substrates are based on the fact that the impedance change of the coil decreases exponentially with the distance of the coil from the metal (the lift-off effect). However, these instruments are not suitable for determining the thickness of metal layers on conducting substrates.

Cheng-Chi Tai, James H. Rose, John C. Moulder

### Effect of Phase Delay on Low Frequency Operation of Flux Focusing Eddy Current Probe

The operation of the Flux Focusing Eddy Current Probe has been found to yield critical information on the thickness of the material being inspected [1–2]. The design of the probe forces the low frequency magnetic fields to diffuse through the sample in order to link with the pickup coil. An attenuation of the magnetic field results such that the pickup coil output is inversely related to the material thickness [2]. In extending the technique to thicker and/or layered materials, however, an apparently anomalous behavior is sometimes seen in which a small increase in the probe output occurs with increasing material thickness. This paper will clarify the underlying principles involved with the probe during low frequency operation and explain the apparent anomaly in terms of the phase shifting of the magnetic field with diffusion depth. A phasor addition model will be presented which accounts for the observed experimental results, and implications of the phenomena on material testing will be discussed.

Buzz Wincheski, Jim Fulton

### Optimization Tool for X-Ray Radiography NDE

The quality of x-ray radiography NDE is highly dependent on a large number of inspection parameters. Examples of these parameters are the generator settings: tube potential, tube current, exposure time,…; object parameters: orientation, thickness, distance from the source…; detector parameters: film type, source to detector distance, etc. Optimization of an x-ray nondestructive testing can be achieved by testing different settings and choosing the set of parameters that produces the best possible radiograph. The large dimensionality of the optimization problem, however, makes the process of manually changing the inspection parameters, and investigating the radiographs, not an easy task. The time and cost requirements of the optimization process can be reduced by resorting to advanced x-ray computer simulators.

I. Elshafiey, J. N. Gray

### Underwater X-Ray Tomography Using Compton Backscatter Imaging

X-ray backscatter tomography (XBT) is a relatively new radiographic NDE technology that is unique among x-ray methods by requiring access to only one side of an object. The object is interrogated by a collimated x-ray beam and collimated detectors to measure the Compton scatter signal produced by each volume element. The acquired signal can be directly imaged to represent the density of the material as a function of position. We have investigated several XBT applications that exploit this one-sided capability. One such application is the inspection of large composite naval sonar domes. The current method, radiography, requires not only costly drydocking but also dome removal from the ship. Previously, we have reported on our feasibility study and our development of a prototype dome inspection system leading to the successful demonstration of in-situ dome inspection in drydock [1, 2]. Since then, we have developed and demonstrated an underwater system. The benefits of x-ray tomography can now be realized in the underwater environment.

E. C. Greenawald, C. Draper, J. Chow, L. Levenberry, C. F. Poranski, Y. S. Ham

### A Model of X-Ray Film Response

In the 100 years since Roentgen produced the first X-ray radiograph, many useful images have been produced for medical and industrial applications. To ensure high quality and reproducibility, standards have been developed to describe different types of film and methods of exposure and development[l]. It is desired to relate these film properties and other X-ray inspection parameters to a probability of detection for a certain type of flaw in a given object.

T. Jensen, T. Aljundi, J. N. Gray, R. Wallingford

### A Practical Algorithm for Reconstruction From X-Ray Backscatter Data

Although numerous applications of x-ray backscatter tomography (XBT) have been demonstrated, only a few have been fully developed to practical implementation [1–5]. In some applications the images produced by direct data acquisition and display methods are plagued with superposition artifacts that can interfere with interpretation [6]. Non-homogeneous materials such as composites or layered structures are particularly susceptible. Reconstruction methods have been proposed to correct the datum from each volume element (voxel) by exploiting the information in data from overlying voxels [7]. Practical inspection systems, however, present a more challenging problem than the monoenergetic highly collimated laboratory demonstration systems. In particular, the use of a bremmstrahlung source and a fan beam, or slit collimated, detector geometry, deprives us of knowledge of the backscattered photon energies and paths that are needed for a true reconstruction. In this paper, we present our work towards a reconstruction using data from a commercial XBT system (Philips ComScan) and a real composite inspection application. Our approach uses pre-processing to remove system artifacts, a priori information about the material, and an iterative method to determine the composition of each voxel.

Young S. Ham, C. F. Poranski, E. C. Greenawald

### Temporal Response of Terbium Glass Scintillator Used for X-Ray Tomography and Radiography

An important characteristic of any scintillator is its temporal response to an impulse of radiation. Ideally, the response time for the induced luminescence is much shorter than the time interval between data acquisitions. As the response time approaches this time interval blurring results in the acquired images. The presence of a long secondary decay component is typically referred to as afterglow. In order to avoid conditions under which such blurring may occur, a study of the scintillator’s temporal characteristics is required. This is especially important for x-ray computerized tomography where an object is constantly in motion.

Mike S. West, William P. Winfree

### Heavy Metal Contamination Detection Using X-Rays

Within the DOE complex there are large quantities of radioactive and hazardous chemical waste that exist in a broad variety of forms, toxicity, and storage conditions. There are 3700 contaminated sites, with 500 facilities now surplus, and as many as 7000 expected to be declared surplus in the coming decades[1]. Most of these facilities will require cleanup of hazardous waste before decommissioning. Efficient, safe, cost-effective methods of characterization are needed to assist in the timely cleanup of these sites. Due to the hazardous nature of the contaminants, a nondestructive non-invasive technique is preferred for characterization and for monitoring the decontamination processes.

T. Aljundi, T. Jensen, J. N. Gray, D. Robinson

### Segmentation and Density-Evaluation of Fiber-Reinforced Materials by Dual-Energy Computerized Tomography

This work has been prepared within the Brite-Euram project “DUALETO”, whose purpose is the development of a high-resolution Dual-Energy computerized tomography system (DE-CT). It is dedicated to non-destructive inspection of fiber-reinforced ceramic parts. The DE-measurement will be acquired simultaneously by an energy- and position-sensitive line detector. Using an appropriate calibration function, these DE-acquisitions are used to calculate the distributions of mass density of the ceramic’s two components and with them, the volumic fraction (VF) of fibers in the matrix material. Due to several reasons, these density data are very noisy. This paper presents a method for obtaining mass density data with a lower level of noise. For this, structural information about object, defects and bundles of fibers are derived from the low-energy measurement and stored in label images. Structure information is then used to perform a non-linear filtering on mass density data, in order to estimate the volumic fraction.

U. Hassler, P. Rizo, L. Garnero

### Dual-Energy Computed Tomography for Ceramics and Composite Materials

This work is part of the Brite-Euram project BRE5535 “DUALETO”, whose purpose is to set-up a Dual-Energy Computed Tomography (DE-CT) system suited for the examination of fibers-reinforced composites. DE measurements are obtained by a microfocus X-ray source, and a multi-wire proportional chamber which simultaneously collects projection data at two different energies over a section of the object [1].

C. Robert, J. M. Dinten, P. Rizo

### Application of 3D X-Ray CT Data Sets to Finite Element Analysis

Finite Element Modeling is widely used by the research community and industry in order to grasp a better understanding of real-world applications through computer simulations. This engineering tool is becoming more important as industry drives towards concurrent engineering. A Finite Element Analysis (FEA) is typically made up of three steps, namely the creation of a mesh, the computer simulation, and the post-processing of the results. A fundamental hindrance to fully exploiting the power of FEM is the human effort required to acquire complex part geometries. This preprocessing can represent up to 80% of an engineer’s time. Therefore, the need to minimize the amount of interactivity cannot be over stressed. Besides the need for a speed-up, the meshing should be based upon actual “as-built” geometries, which should be preferred to as-designed geometries from CAD models or generic descriptions. A CAD model may not accurately account for all the changes to the initial design that take place during the manufacturing process. A CAD model may simply not be available if the manufacturer has gone bankrupt or is part of the competition. In biomechanics applications, the generic descriptions of human parts are not precise enough, which limits the use of Finite Element Models.

P.-L. Bossart, H. E. Martz, H. R. Brand, K. Hollerbach

### X-Ray Computed Tomography Application Research

The value of CT in the medical community has been well documented. Medical computed axial tomography (CAT) scans can provide physicians with valuable information such as the presence, location, size and growth patterns of tumors and abnormalities within the body. This information can be used to judge the severity of the problem, aid in removal through surgery and detect the onset of a problem at an earlier stage than might have been possible otherwise. Outside the medical community CT has been very successful as an NDT modality, such as for the inspection of rocket motors and turbine blades. However its full potential to industry has not yet been realized. This paper will introduce several innovative areas for the application of CT that have proven successful and discuss their potential benefits to the Department of Defense and to industry.

S. Trent Neel, Robert N. Yancey

### Mathematical Modelling of Transient Thermography and Defect Sizing

The principle employed to obtain an image of a sub-surface defect by transient thermography is deceptively simple. A surface is heated by powerful flash lamps and subsequent thermal transients are recorded by an infrared camera. Defects cause perturbations in heat flow which are revealed by the camera. Whilst there is now a considerable body of practical experience of the application of the technique, there is rather less precise quantitative information about the image formation process that could lead to reliable defect sizing. In earlier papers [1,2] one of the authors considered circular air gap defects by treating them as buried uniformly heated disks. The thermal edge-effect occurring at the tip of a perfect crack-like defect was dealt with analytically by adapting the well established Wiener-Hopf [3] solution for the scattering of light or sound from the edge of a semi-infinite half-plane. The problem was solved in the frequency-domain, i.e. to obtain a thermal wave solution, and then a time-domain solution was obtained by a suitable transformation. The analysis showed an edge-effect amounting to a decay in temperature contrast over a distance of about a thermal diffusion length from the edge of the crack. A crucial feature of the edge-effect was the decay of thermal contrast to zero at the crack tip. This, and the edge-effect as a whole, is caused by the flow of heat around the crack tip from the hot upper surface of the crack to the cold under surface. The symmetry of this process ensures that there is no net flux increase for material in front of the crack tip.

M. B. Saintey, D. P. Almond

### Applications of Lockin-Thermography Methods

The basic idea of nondestructive testing is that a sample is characterised by its response to a certain kind of excitation. The excitation may be an electric or elastic field with a time pattern described by a step, a pulse, or a sine wave type. The field can be applied locally to characterise the area around a certain sample spot. A raster scan image is then performed by measuring many spots one after the other.

D. Wu, J. Rantala, W. Karpen, G. Zenzinger, B. Schönbach, W. Rippel, R. Steegmüller, L. Diener, G. Busse

### Early-Time Pulse-Echo Thermal Wave Imaging

We describe the early time behavior of reflected thermal wave pulses from planar subsurface scatterers, and describe methods for making depth images, independently of the lateral size of the scatterer.

Xiaoyan Han, L. D. Favro, P. K. Kuo, R. L. Thomas

### Precise Thermal NDE for Quantifying Structural Damage

We have developed precise thermal NDE as a wide-area inspection tool to quantify structural damage within airframes and bridge decks. We used infrared cameras and image processing to produce precise temperature, thermal inertia, and cooling-rate maps of flash-heated aircraft skins. These maps allowed us to distinguish major structural defects from minor flaws which do not warrant costly repairs. We quantified aircraft skin corrosion defects with metal losses as low as 5% with 3% overall uncertainty [1–6]. We proved the feasibility of precise thermal NDE to inspect naturally-heated asphalt-concrete bridge decks. To this end, we quantified structural damage within asphalt-concrete slabs by locating the sites, and determining the relative volumes, of concrete displacements from 2-inch deep and 4-inch deep synthetic delaminations in asphalt-concrete slabs [4–8].

Nancy K. Del Grande, Philip F. Durbin

### Thermal Ellipsometry: A Tool Applied for in-Depth Resolved Characterization of Fibre Orientation in Composites

Composites with a thermoplastic resin reinforced with short fibres or platelets are commonly processed by injection molding. As far as the fibre orientation is concerned, a stratification generally appears during this operation: near the mould surfaces the fibres are oriented in the direction of the flow, whereas in the middle they are oriented perpendicularly to this direction. The flow parameters obviously determine the relative thickness of this core region and consequently the longitudinal and transverse properties of the composite material. The moulding process has therefore to be optimized so that these anisotropy variations contribute the composite to fulfil the mechanical performance specifications. The in-depth resolved characterization of the filler local orientation as well as its control are thus of prime importance.

J.-C. Krapez

### Time-Resolved Microwave Thermoreflectometry for Infrastructure Inspection

This paper describes the development of time-resolved microwave thermoreflectometry (TRMT) as a potential NDE technique for the detection of corrosion of reinforcing bar (rebar) in concrete structures. The approach in this technique is as follows: rebar inside a concrete structure is heated in a noncontacting fashion using an induction heating sourcethe temperature of the surface of the rebar is monitored directly through the concrete using a microwave reflectometerdegradation of the structural integrity of the bond between the concrete and the rebar affects heat flow from the heated rebar to the surrounding concrete.

J. W. M. Spicer, R. Osiander, Y. Chang, R. Hildebrand

### Simulations on the Accuracy of Laser-Flash Data Analysis Methods

The Laser-Flash thermal diffusivity measurement method can be considered one of the most succesful applications of photothermal techniques. This due to the phenomenological and experimental simplicity and ease of reaching better than 1% accuracy over a wide temperature range. The method is based on observing the temperature rise of the sample back face resulting from the absorption of a laser pulse at the other face. There are various approaches for the data reduction and, especially for high temperature measurements where heat loss effects need to be accounted for, they are based on approximations. This is because the inverse function relating thermal properties and heat exchange conditions with the temperature rise temporal shape is not available in closed form. Therefore, detailed error propagation calculations analyses that would take into account all the steps of the data analysis procedures have not in general been performed for data. In this work, simulations of the noise sensitivity and accuracy of selected data reduction schemes were studied using synthetic data. The work was done in connection with the design of a high temperature laser-flash instrument for the measurement of ceramic composites for fusion reactor applications.

M. Oksanen, A. Volcan, P. Fenici, L. Fabbri

### Linearized MFL Model for Embedded Flaw Detection in Rails

Numerical models simulating magnetic flux leakage (MFL) phenomenon in ferromagnetic materials are nonlinear in nature and hence require excessive computational effort. This paper describes an approach for developing an equivalent linear model (ELM) where the ferromagnetic region is appropriately partitioned into different domains with each domain being assigned a constant permeability value depending on the magnetization level and the flaw size. The nonlinear behavior of the multi-layered object is then modeled using a linearized MFL model. The model is first validated and then applied to the flaw detection problem in a rail geometry.

Zuorong Zhang, Lalita Udpa, Satish S. Udpa, Atul S. Athavale, David Utrata, Jiatun Si, Yushi Sun

### 3D Finite Element Methods for Modeling MFL Inspection of Pipelines

It is estimated that there are about one million kilometers of gas and liquid transmission pipelines operating across the globe today. Pipelines, owing to their strategic role of transporting gas and liquid fuels, are of immense capital value. Potential degradation and failure of pipelines is a sensitive issue both with the public and legislative bodies, since the consequences of failure could include injuries and death. In addition, pipeline failures have severe financial consequences. More than half the pipelines in use today are 30 or more years old and invariably have experienced some deterioration. Preventive maintenance using nondestructive evaluation (NDE) techniques plays an important role in ensuring safe pipeline operation [1].

G. Katragadda, J. T. Si, W. Lord, Y. S. Sun, S. Udpa, L. Udpa

### Magnetic Particle Inspection Simulation Model

The Magnetic Particle Inspection (MPI) method is generally used to detect surface and near surface flaws. In a MPI test, a ferromagnetic specimen in unmagnetized state is sprayed with magnetic particles in an aerosol suspension. The particles are generally ferromagnetic oxides coated with fluorescent pigments and the suspension is a petroleum distillate of low viscosity. The specimen is then magnetized. In the presence of a flaw transverse to the direction of the applied magnetic field, leakage fields are established on the surface of the specimen. These fields exert a translational force on the ferromagnetic particles in addition to a rotational torque. These two in combination accelerate the particles toward the flaw increasing the density of particles in the vicinity of the flaw. When excited by ultraviolet light, the particles emit visible radiation indicating the location of the flaw. This paper attempts to model the physical principles underlying the MPI method including imaging techniques to recreate the dynamics of the particles prior to their reaching an equilibrium around the flaw. The proposed approach has the capability to predict the time to equilibrium of the magnetic particles and the efficiency of the method in terms of the fraction of the total number of particles in the MPI image.

Atul Athavale, Lalita Udpa, Satish Udpa, Zuorong Zhang

### Laser Ultrasonic Thermoelastic/Ablation Generation with Laser Interferometric Detection in Graphite/Polymer Composites

Ultrasonic signals have been generated and detected in graphite/polymer composites by optical methods. A Doppler interferometric technique was used for detection. The output voltage of this type of interferometer is proportional to the surface velocity of a sample area which is illuminated by cw laser light. Ultrasonic signals were generated by thermoelastic and ablation processes which occur as a consequence of laser pulses incident on the opposite surface of the sample. The evolution of the magnitude and shape of the detected signals was measured as a function of the pulse energy of the generating laser. Low-energy laser pulses generated ultrasound without causing obvious surface damage. At higher energies surface damage was observable in post inspection but could also be detected by observing (through protective goggles) bright flashes near the illuminated area. The energy at which these processes first occur is qualitatively referred to as the ablation threshold. Changes in the observed waveform were evident at energies above the ablation threshold. The higher-energy waveforms were found to consist of a superposition of a thermoelastic component and an ablatic component, whose relative magnitudes changed with laser power. A delay in the initiation of the ablatic wave relative to the thermoelastic wave was observed to be of the order of 0.3 μs, consistent with observations in pure polymer. [1] Photoelectric detection measurements of the ablation plume also showed a clear threshold and a time scale for growth of the ablation products with a characteristic time scale on the order of 0.3 μs.

James N. Caron, James B. Mehl, Karl V. Steiner

### Characterization of Surfaces and Coatings Using Laser-Generated Ultrasonic Surface Waves

Metallic coatings are produced for various purposes, and methods employed for testing their quality vary accordingly. Many tests have been developed in the past to determine the various properties of the coatings. Wear-resistance tests are commonly carried out on metallic deposits, plated specifically as a protection for the metal substrate against wear and erosion. A typical example is chromium plated on steel substrates.

A. Abbate, S. C. Schroeder, B. E. Knight, F. Yee, J. Frankel, P. Das

### Narrow Band Laser Ultrasonic NDE

The generation and detection of ultrasound with lasers has evolved to become a useful laboratory tool. An extensive compilation of laser ultrasonic technology and applications is available [1]. These laser ultrasonic techniques are complimentary to other methods such as piezoelectric transducers. Since they are non-contact and have very high resolution, these techniques provide otherwise unavailable NDE options. They are unfortunately relatively inefficient in their signal to noise ratio (SNR) in comparison to other NDE techniques limiting their value to potential applications where traditional NDE methods are unable to perform satisfactory. The work presented here demonstrates progress in overcoming some of the problems which have prevented laser ultrasonics from becoming useful in industrial environments.

C. E. Duffer, C. P. Burger

### Shear Wave Wedge for Laser Ultrasonics

Ultrasonic shear waves are a useful tool for determining the mechanical properties of various materials. One example is the use of shear waves to measure viscosity. The viscosity can be determined from the shear wave reflection coefficient. The reflection coefficient from a solid-liquid interface is a function of the viscosity and density of the liquid, as well as the angle of incidence and the material properties of the solid. [1,2]

R. Daniel Costley, Vimal Shah, Krishnan Balasubramaniam, Jagdish Singh

### Laser Ultrasonic Detection of the Solidification Front During Casting

A real-time sensor that directly measures properties of the solidification front would be a valuable aid to the metal casting industry. Information needed includes solidification front location, shape, and growth dynamics. The use of contacting probes is often undesirable because it can cause contamination and probe deterioration. Noncontacting laser ultrasonics offers an attractive solution to these problems, particularly if access to the free liquid surface is available. This paper presents results of laser ultrasonic measurements of the solidification front in tin and a tin-lead alloy. The ultrasonic waves were generated and detected at the liquid surface. Tin was selected for its low melting point and the availability of a suitable furnace. Results are presented for reflections from stationary and moving solidification fronts.

J. B. Walter, K. L. Telschow

### Injection Locked Laser Sensor for Ultrasonic Sensing

Optical ultrasonic sensors have been commonly used in research laboratories long before Monchalin’s classic review paper entitled “Optical Detection of Ultrasound” [1] in 1986 and yet, optical ultrasonic sensors have not made a successful transition from the laboratory to industry. This sensing technology has had plenty of time to mature. In fact, the components which comprise the optical sensors have been significantly enhanced during this time period. Despite the large gains in gas laser, solid state laser, diode laser, photodiode, fiber optic, non-linear optic, electronic design, data acquisition, and general sensing technology since 1986, optical ultrasonic sensors have not made it out of the laboratory and into industry.

J. A. Smith, C. P. Burger

### A Laser Interferometric Method for Small- and Finite-Amplitude Ultrasonic Waves’ Detection in Transparent Media

The acousto-optic interaction affords a convenient way of optically probing ultrasonic waves in medical diagnosis and nondestructive evaluation. The effects of ultrasonic waves on the light transmitting through transparent media arise from the refractive index variations produced by ultrasonic waves. The index variations may be detected by optical deflection, diffraction or interference methods [1–4]. In Raman-Nath regime, the acoustic waves act as a moving phase grating and diffract the light into different orders. Schlieren visualisation derived from this mechanism has been extensively used to ultrasonic measurements in liquids. In solid media, the acousto-optic effects become more complicated because of the induced optical birefringence. The usual photoelastic method consists in detecting the change in the polarization state of the light caused by ultrasonic waves [5]. Both of the methods are only amplitude-sensitive to ultrasonic waves.

Xiaoping Jia, Gérard Quentin, Laszlo Adler

### Heterodyne Detection of Ultrasound from Rough Surfaces Using a Double Phase Conjugate Mirror

Ultrasonic excitation of a solid sample (optically opaque) can be detected by directing a laser beam at one of its surfaces. Surface motion causes a transient phase shift upon the scattered light, which has to be demodulated into an intensity variation prior to its detection by a photodetector. Classical reference beam interferometry (homodyne or heterodyne) is a well-known technique for performing this demodulation. It is characterized by a broad detection bandwidth, but is, following the antenna theorem [1], essentially limited to the detection of one speckle, when used on rough surfaces. In order to circumvent this limitation (i.e., in order to increase the étendue of the interferometer), two different approaches for adapting the signal and reference wavefronts have been considered. The first approach proceeds by creating a reference beam that matched the wavefront of the signal beam. This can be done by using a Fabry-Pérot (FP) [2] which is a self-reference interferometer and means that the reference beam is generated by the signal beam. It can also be done by using two-wave mixing (TWM) in a photorefractive crystal [3,4]. In this case, the reference beam is created by the diffraction of a plane wave pump beam by the hologram written by both pump and signal beams. Alternatively the signal beam wavefront can be adapted to the reference wavefront, which requires, since the reference beam can usually be approximated by a plane wave, the transformation of the speckled beam to a beam with a plane wavefront. Devices using externally pumped [5] or self-pumped phase conjugate mirrors (SPCM) [6] have been reported.

Philippe Delaye, Alain Blouin, Denis Drolet, Jean-Pierre Monchalin

### Specifications of an Ultrasonic Receiver Based on Two-Wave Mixing in Photorefractive Gallium Arsenide Implemented in a Laser-Ultrasonic System

Optical techniques for ultrasonic measurements present several advantages over conventional piezoelectric methods. First, they are remote sensing techniques and can be, for example, used for the inspection of materials at elevated temperature or products moving on a production line. Secondly, surfaces of complex shape can be easily probed since these techniques work with scattered light. For specific applications, these advantages compensate the usually lower sensitivity of optical techniques.

Denis Drolet, Alain Blouin, Christian Néron, Jean-Pierre Monchalin

### Fiberized Sagnac Interferometer for Ultrasound Measurement

Laser-based ultrasonics (LBU), i.e. the generation of ultrasound by laser illumination and the measurement of ultrasonic signals by laser interferometric techniques, has many advantages for applications to nondestructive evaluation (NDE). These include non-contact generation and detection, remote placement of equipment using fiber-optics, easy scanning, absolute displacement calibration, both broad band-and narrow band signal generation, wide frequency band measurements, and applicability to curved surfaces. Both laser generation of ultrasound and the subsequent detection of the ultrasonic waves using a laser interferometry are areas of active research [1-6].

Pavel Fomitchov, Sridhar Krishnaswamy, Jan D. Achenbach

### Optical Techniques to Examine the Scattering of Rayleigh Surface Waves

This paper presents the results of an experimental investigation of the interaction of Rayleigh waves with corners. The experiments are done with a dual probe interferometer to measure the interaction of a normal incident Rayleigh wave with corners of varying angles. The incident Rayleigh waves are optically generated with a pulsed laser. The effects of the corners on the Rayleigh waves are then characterized and a finite impulse response (FIR) filter is designed to remove them.

Douglas A. Bruttomesso, Laurence J. Jacobs

### A Fiber Optic Ultrasound Sensor for Monitoring the Cure of Epoxy

Epoxy is a common matrix material in fibrous composites and is frequently used with: kevlar, glass, carbon, and boron fibers. Composite materials with an epoxy matrix are processed in an autoclave which applies temperature and pressure to the part during cure. Temperature and pressure facilitate the crosslinking of epoxide groups which react with amine groups present in the epoxy resin. Current methods of hardening fiber/epoxy composites utilize predetermined values of temperature and pressure in the cure cycle. These values are specified by the manufacturer and do not account for batch to batch variations in the epoxy resin which affect the cure. Possible variations in the epoxy resin include: chemical composition, water content, resin fiber content, temperature history, and humidity [1].

John Dorighi, Sridhar Krishnaswamy, Jan D. Achenbach

### Phase Stepping Shearography for Testing Commercial Aircraft Structures: An Application Review of Advanced Image Processing Techniques for Shearography

Interferometric NDT methods have a long history in the laboratories of DAIMLER BENZ AEROSPACE Airbus, even first papers about early approaches to shearography go back into the late seventies, when in the Lemwerder Labs of former VFW-Fokker the first shearography setup was assembled [1]. That time using film or thermoplast camera the method proved to be too complicated for in field applications. During the following years the acceptance of interferometric NDT methods remained poor as the typical test results were represented by complex fringe patterns. These patterns could be interpreted only by well trained specialists. It was sometimes impossible to extract the defect information from complex fringe patterns caused by the geometry of the test specimen. In the spectrum of different approaches to overcome this problem the phase stepping fringe analysis method showed great potential in conjunction with the capabilities of shearography.

Wolfgang J. Bisle, Dieter Scherling, Gustav Tober

### A Compact Digital Phase-Stepping Shearography System

Electronic shearography has proven to be a valuable tool in nondestructive evaluation. With this technique, correlation fringes corresponding to a deformation or movement of an object are produced electronically. In general, these correlation fringes are composed of random intensity fluctuations due to speckle, making it difficult to analyze the fringes and calculate the phase distribution accurately. If such quantitative information regarding the object is desired, direct interpretation of the fringe pattern is often attempted. Although various methods have been devised to accomplish this task, manual fringe counting is still often performed—a tedious job requiring much operator experience, yielding sparse data, and usually producing inaccurate results.

Benjamin A. Bard, Shudong Wu, Bernhard R. Tittmann

### Detection/Location of Small Flaws in Composite Structures Using Microwaves

The use of high-frequency electromagnetic radiation (“microwaves”) in a variety of NDE scenarios has been receiving increased attention recently [1], largely as the result of advances in microwave instrumentation, computer technology and digital signal processing. Microwave images of both surface and interior features of non-conducting samples can be generated either by focusing the energy on the smallest possible spot and scanning either the target or the measurement probe (analogous to an ultrasonic C-scan), or by using coherent processing to form an image from multiple exposures of the entire target [2,3]. The latter approach, as implemented in an “Inverse-Synthetic-Aperture Radar” (ISAR) system has the significant advantage of being able to image complex, large and oddly shaped targets from a large stand-off distance (many meters). The results presented below give an indication of the great sensitivity of the IS AR technique in its ability to detect and locate very small flaws in a complex target.

R. P. Flam, D. J. Farina, J. M. Liu

### Preliminary Results of Microwave Non-Contact Detection and Depth Determination of Disbonds in Low Permitivity and Low Loss Thick Sandwich Composites

To ensure manufacturing quality and safe use of thick dielectric composite structures it is essential to utilize a nondestructive testing technique for inspecting their structural integrity. As the thickness of these composite structures increase, most of the nondestructive testing (NDT) techniques become less capable of detecting defects. Microwave signals can penetrate deep inside dielectric materials and interact with their inner structure. They are also sensitive to changes associated with boundary interfaces, which makes them very attractive for disbond detection in composite structures [1,2]. In a thick sandwich composite structure a disbond can occur between any two layers (i.e. in the place of an adhesive fine). The results of an electromagnetic model investigating the potential of a microwave NDT method for detecting disbonds and the potential of determining their depths in a multi-layered sandwich composite is presented. The model describes the interaction of microwave signals, radiating out of an open-ended rectangular waveguide, with a multi-layered composite structure. The composite structure under consideration includes thirteen layers of various materials (three layers of foam with two skin laminates and two similar substrates in between the foam layers) and two layers of air (standoff in front of the sample and free-space backing). Each layer is bound to another by a thin layer of adhesive. A layer representing a disbond is considered to be present in between any given two layers, replacing the adhesive line. The goal of the modeling is to arrive at optimum measurement parameters (frequency and standoff distance) for detecting a disbond and providing information about its depth.

N. Qaddoumi, S. Ganchev, R. Zoughi, G. W. Carriveau

### Microwave Sensors for Imaging Moisture and Flaws in Advanced Composites

Maps of moisture contamination of advanced composite structures have been created using microwave open reflection resonator sensors. By optimizing the resonator’s shape significant advances have been made in sensitivity, penetration depth, quality factor and lateral resolution.

Michael Werner, Ray King

### A Microwave Transducer for Detecting Conductive Surfaces Beneath Protective Coatings

This paper describes a new device, named the EMI Tester, that nondestructively detects the conductivity of surfaces beneath protective paint of varying thickness. This device utilizes a microwave stripline placed in contact with the surface to detect surface conductivity. It provides field maintenance personnel with an easy way to evaluate the EMI configuration of aircraft outer moldline surfaces. The device is small enough to be hand-held by service personnel and compatible with the field service environment.

Stephen C. Buckner, John J. Domalewski, Garrett G. Heil, Jeffrey K. Hoyt, Arthur C. Lind, Frederick C. Wear

### Air-Gap Detection in Dielectric Materials by a Step-Frequency Microwave Technique

Most microwave NDE has been performed using continuous wave excitation and reception, due to the general availability of such equipment and the acceptable procedure of extracting information from the amplitude and phase of such signals. With the availability of sources that can be swept over a band of frequencies, the amplitude and phase information can be converted via the Fourier Transformation to the impulse response in the time domain. Instead of searching for changes in the amplitude and phase of microwave reflected from boundaries, interfaces, and defects, this time domain approach concentrates on the recognition of “echoes”. This approach is implicit in some of the more complex approaches in microwave imaging [1–3]. It is the purpose of this paper to demonstrate that this approach facilitates the detection of internal defects using microwave, in a manner similar to the practice of pulse-echo ultrasound. The time delay for a microwave “echo” is related to the location, and the Fourier transformed amplitude is related to some characteristics of the defect inside the material.

John M. Liu

### Effects of Lift-Off on Microwave NDE Using an Open-Ended Rectangular Waveguide

Some authors have demonstrated recently the capability of microwave imaging of subsurface defects and material characteristics by scanning a suitably excited open-ended rectangular waveguide over a multi-layered composite [1,2]. This near-field approach of microwave NDE is in some aspects similar to the conventional eddy-current based techniques for defect detection and imaging in metals [3]. Of course, in non-metallic materials, it is the discontinuities in the dielectric property, instead of the electrical conductivity, that gives rise to the defect signal. Bahr [4,5] has reported various techniques for the detection, discrimination, and processing of microwave signals, which are also applicable in the lower frequency regime typical of eddy current testing.

John M. Liu, M. Allen Matteson

### Electromagnetic Modeling of the Interaction of Cracks in Metallic Surfaces with Open-Ended Waveguides Using an Equivalent Magnetic Current

In previous research a mode matching approach was used to describe the electromagnetic properties of a system formed by an open-ended waveguide and a crack [1]. A more versatile formulation for evaluating the electromagnetic properties in such a system is possible by using a moment solution approach, which eliminates the necessary distinction between empty, filled and finite length cracks, independent of the relative position of the crack within the probing waveguide aperture. The reflection coefficient at the waveguide aperture can be expressed with a generalized scattering matrix, thus making this approach versatile for any incident excitation field [2].

C. Huber, H. Abiri, S. Ganchev, R. Zoughi

### Near-Field Analysis of Rectangular Waveguide Probes Used for Imaging

Near-field microwave imaging of composite structures has received considerable attention recently. The success achieved on the experimental level motivated the development of a theoretical model to describe the high quality images obtained using near-field microwave imaging [1–4]. This theoretical model will also help in building an intuitive understanding of the behavior of the fields inside dielectric materials in the near-field of an open-ended rectangular waveguide probe. A near-field microwave image is the result of several factors such as probe type (example rectangular waveguide, circular waveguide or coaxial line), field properties (i.e. main lobe, sidelobes and half power beam width, etc.), geometrical and physical properties of both the defect and the material under inspection. Thus, in order to characterize a defect, the effect of all non-defect factors needs to be taken out of an image. One of the dominant non-defect factors which influences an image significantly is the radiator field properties. Thus, it is essential to formulate the properties of the fields radiating out of an open-ended rectangular waveguide in its near-field. This knowledge will aid in formulating the forward problem when imaging a defect, and will be used to solve the inverse problem for obtaining defect properties. In this paper fields radiating out of an open-ended rectangular waveguide, into an infinite half-space of a dielectric material, are calculated and used to explain some of the features observed in experimental near-field microwave images.

N. Qaddoumi, H. Abiri, S. Ganchev, R. Zoughi

### An Application of Wavelet Signal Processing to Ultrasonic Nondestructive Evaluation

In this paper we present a flaw signature estimation approach which utilizes the Wiener filter [1–5] along with a wavelet based procedure [6–15] to achieve both deconvolution and reduction of acoustic noise. In related ealier work by Patterson et al. [6], the wavelet transform was applied to certain components of the Wiener filter, and coefficient chopping was used to reduce acoustic noise. In the approach that we present here, the wavelet transform is applied individually to the real part and to the imaginary part of the scattering amplitude estimate determined by application of a sub-optimal form of the Wiener filter. This wavelet transform takes the real and imaginary parts, respectively, from the typical Fourier frequency domain to a wavelet phase space. In this new space, the acoustic noise shows significant separation from the flaw signature making selective pruning of wavelet coefficients an effective means of reducing the acoustic noise. The final estimates of the real and imaginary parts of the scattering amplitude are determing via an inverse wavelet transform.

Alan Van Nevel, Brian DeFacio, Steven P. Neal

### Wavelet Transform Signal Processing Applied to Ultrasonics

The wavelet transform (WT) provides a new tool for processing transient signals and it can be considered as an alternative to the classical Short-Time Fourier Transform (STFT) for describing the time-frequency evolution of such signals. The purpose of the present paper is to provide an overview of the applicability of the wavelet transform to ultrasonic signal analysis. The WT will be briefly introduced with emphasis on the aspects which make it suitable for our applications. In particular the WT has been utilized to enhance the ultrasonic signal detection in presence of background noise, and the application of this technique for flaw detection will be presented. Improvements in detection were quantified using steel samples of different thickness and with simulated flaws. Furthermore, the self-adjusting window structure of the WT results in a time-scale representation of signals which can display the temporal variation of the spectral components with varying resolution. This property is extremely useful in the study of dispersive wave propagation, in particular for extracting the dispersion relation of the Pseudo-Lamb wave velocity in thin coatings. Numerical simulations and experimental results will be presented.

A. Abbate, J. Frankel, P. Das

### Echo Extraction from an Ultrasonic Signal Using Continuous Wavelet Transform

Wavelet transform has already been shown as a useful tool for the interpretation and the enhancement of ultrasonic data in the context of nondestructive evaluation [1–3]. Main applications of the wavelet transform are signal analysis in the time-frequency domain, data compression and now signal processing. Comparisons with other time-frequency representations like short time Fourier transform [1] and Wigner-ville transform [2] have shown the usefulness of the continuous wavelet transform for signal analysis: this method is well adapted to localize in time both high and low frequencies and does not introduce interference terms. Another important property is that signal reconstruction can be achieved from wavelet decomposition. This ability allows one to do signal processing in the time-frequency plane. Earlier work has shown the possibilities to use the wavelet transform as a filter for signal-to-noise ratio enhancement [2] by reconstructing the signal after applying energy thresholding in the time-frequency domain. This reconstruction does not involve global averaging in time or frequency domain because of the good localization of the wavelet coefficients in both domains.

O. Roy, J. Sallard, S. E. Moubarik

### Improvement of Time Reversal Processing in Titanium Inspections

We have explained in previous papers [1,2] a completely different NDE technique applied to the titanium alloy billet, the time reversal process, which allows conversion of a divergent wave issuing from a defect into a convergent wave focusing on it. The time reversal method is a self adaptive technique which produces a focussed beam matched to the defect shape and generates an unfocussed wave in the case of a speckle noise source. The results have showed the ability of this technique to focus on defects in a scattering media through a complex interface. However, this technique is subject to the principal NDE systems problem: the false alarms. Indeed, high levels of grain noise can mask signals from smaller or lower acoustic reflectivity flaws and than a grains configuration can be confused with a small defect. In case of the inspection of large titanium parts such those found in the aircraft engine industry (thickness > 5″), the classical techniques are more sensitive to this problem in the deep zones: the grain density often increases with depth du to the forging technique and the backpropagated defect echoes are weak in account of the attenuation.

Véronique Miette, Mathias Fink, François Wu

### Singular Value Decomposition of Wigner Distribution for Time-Frequency Representation of Ultrasonic Echoes

Wigner distribution (WD) is an effective tool for characterizing non-stationary signals where different frequency components arrive at different times. WD was proposed in 1932 by Wigner with applications in quantum mechanics [1]. WD offers high frequency resolution, and satisfies important properties such as marginals and time/frequency shifts. However, in spite of these advantages, WD creates spurious frequency information (cross-terms) when it is applied to signals consisting of multiple echoes or to a signal corrupted by noise. The cross-terms interfere with and often mask the true time-frequency information associated with echoes of interest. Due to the random and complex nature of backscattered ultrasonic echoes, and because the echoes are not exactly Gaussian in shape, the WD of ultrasonic signals is corrupted by the cross-terms. In this paper, singular value decomposition (SVD) is used to analyze and reduce the cross-terms.

X. M. Jin, M. A. Malik, J. Saniie

### Optimum Filter Based Techniques for Data Fusion

The growing complexity of inspection needs in the industrial workplace has contributed to an increasing interest in data fusion techniques. The interest in such methods have been fueled by a perception that classical approaches, involving the use of a single inspection methodology, are sometimes inadequate for capturing all the information necessary for characterizing the test specimen. It is often possible to employ two or more inspection techniques or measurement conditions for evaluating the specimen. Each test may provide a limited but slightly different perspective of the state the test object. Operators have traditionally combined the information from the test results “mentally” to draw conclusions. Recent developments in the field of data fusion, however, allow the information to be integrated on a more systematic basis. The use of such data fusion techniques can potentially improve the probability of detecting flaws and contribute to improved defect characterization results.

J. Yim, S. S. Udpa, M. Mina, L. Udpa

### Quantitative Evaluation of Defects by Fuzzy Reasoning Based on Principal Component Analysis

Ultrasonic waveforms reflected by various kinds of defects were calculated by numerical analysis based on elastic wave theory. By using these calculated results as a knowledge base, development of nondestructive evaluation system which provides quantitative information, such as types, locations and sizes of the defects is expected. In previous studies, a neural network was applied to construct the knowledge base [1–3]. However, there are some problems in a neural network approach for the quantitative evaluation of defects. A neural network is able to characterize the waveform data effectively, but it does not necessarily interpolate the learning data accurately. In addition, there is an inevitable error caused by the different conditions in numerical analysis and experimental results.

T. Ogi, T. Mandai, Y. Yabe, M. Kitahara, J. D. Achenbach

### Testing for Nongaussian Fluctuations in Grain Noise

In ultrasonic nondestructive evaluation (NDE), grain noise corrupts the scattered wave field from a flaw in a polycrystalline material. Many probabilistic approaches associated with flaw detection and characterization utilize stochastic models in which grain noise is assumed uncorrected and zero-mean Gaussian distributed. Typically, the Gaussian assumptions is justified via heuristic arguments based on the central limit theorem. This paper presents the kurtosis test and the Shapiro-Wilk W test as methods to quantitatively test time domain noise ensembles for deviations from Gaussian statistics. We will establish, through the application of these hypothesis tests to grain noise, a quantitative tool which can be used to consider “how Gaussian” grain noise signals must be for Gaussian noise based signal processing procedures to out perform alternative approaches.

Steven P. Neal, Kevin D. Donohue

### Crack Parameter Characterization by a Neural Network

A neural network with binary outputs is presented to determine the angle and the depth of a surface-breaking crack from ultrasonic backscattering data. The estimation procedure is divided into two steps: 1.The angle of the crack is estimated in the range from 10 to 70 degrees with a precision of 5 degrees. To improve the accuracy of estimation, information on the integral of the backscattered signal is utilized.2. 2. When the angle of the crack has been estimated, the depth of the crack is determined with a precision of 0.5mm in the range from 2.0mm to 4.0mm. This determination is achieved by employing sets of neural networks corresponding to various angles of the crack.

M. Takadoya, J. D. Achenbach, Q. C. Guo, M. Kitahara

### Fuzzy Inference Systems for Invariant Pattern Recognition in MFL NDE

Defect related information present in NDE signals is frequently obscured by the presence of operational variables inherent in the system. A typical NDE system comprises of an energy source, a test specimen and a sensor array. Operational variables include uncontrollable changes in source signal strength and/or frequency, variations in the sensitivity of the sensor and alterations in the material properties of the test specimen. These operational variables can confuse subsequent signal interpretation schemes, such as those relying on artificial neural networks. Invariant pattern recognition methods are required to ensure accurate signal characterization in terms of the underlying defect geometry. This paper describes a generalized invariance transformation technique to compensate for operational variables in NDE systems. An application to magnetic flux leakage (MFL) inspection of gas transmission pipelines is presented. The technique is employed to compensate for variations in magnetization characteristics in the pipe wall.

S. Mandayam, L. Udpa, S. S. Udpa, W. Lord

### Registration Issues in the Fusion of Eddy Current and Ultrasound NDE Data Using Q-Transforms

Data fusion methods are finding increasing application in nondestructive evaluation (NDE) [2, 3, 4] for enhancing the reliability of inspection. These techniques typically combine information from two or more NDE modalities to improve the probability of detecting flaws and enhance specimen characterization results [1]. Eddy current methods rely on diffusion for propagating energy. Ultrasonic methods, in contrast, rely on wave propagation. Consequently, the two tests rely on different material/energy interaction processes and can potentially provide complementary perspectives of the flaw in a specimen. This paper proposes a novel phenomenological approach using Q-transforms for addressing the registration issue in the fusion of eddy current and ultrasonic data. Specifically, ultrasonic signals are Q-transformed to the diffusion domain. The transformation allows the superposition of the transformed field on the eddy current field as shown in Figure 1. It is anticipated that the resulting field will have a lower signal-to-noise ratio.

Kai Sun, Satish Udpa, Lalita Udpa, Tianji Xue, William Lord

### Introduction of the Signature Classification Development System

Discontinuities of various types and sizes may be introduced into a material during the manufacturing process and during the service life of the component. Discontinuities generated during the manufacturing process may include stress cracking, slag inclusions, lack of fusion, and gas or shrinkage porosity, whereas typical discontinuities generated during the service life may include fatigue cracks and corrosion [1]. Before the component can be accepted for inventory or continued service, an accurate nondestructive evaluation of the component must be performed.

Lawrence M. Brown, Jeffrey S. Lin

### Edge Detection on Eddy Current Image to Increase Defect Characterization

As the nuclear yard ages, the control of steam generator tubes (SGT) must deal with new problems. In fact new defects appear, especially in the area of the tube sheet, of the tube support or at the U-bend area. Eddy current testing using Rotating Probe Coil (absolute mode) gives a better resolution. These measurements allow smaller defects to be detected along different orientations, especially defects along the circumference in the rolling transition. Signals collected during the exploration of the tube internal wall with this coil contain the useful information which is represented in the form of a cartography (or image) for each of the signal complex components.

B. Benoist, P. Attaoui, R. La, R. Lengellé, P. Gaillard, J. Reuchet

### Eddy Current Image Restoration Using the Groshong Parametric Model

French PWR cooling systems include austenitic/ferritic stainless steel cast elbows. Due to the aging of these components, there is an increasing need for monitoring possible fabrication flaws near the elbow inner and outer surfaces. Various non destructive techniques are being evaluated by the EdF (French Electricity Board) for defect detection and characterization. One of these, the eddy current (EC) technique, is usually very efficient for the detection of flaws near the surface. However, large local variations in the ferrite ratio induce high structural noise thus reducing the EC detection efficiency. Image restoration using the Groshong parametric model is proposed as a means of improving the signal-to-noise ratio (SNR) and defect characterization.

T. Sollier, J.-M. Philippe, H. Maury, D. Villard

### Conductivity Imaging in Plates Using Current Injection Tomography

The task of reconstructing an unknown distribution of electrical conductivity is widely recognized as a central theoretical problem in eddy-current nondestructive evaluation [1]. Rather than using an eddy-current method, we address this problem using DC injection of current into conductive materials. Experimental methods of the magnetic imaging of injected currents using high-resolution SQUID magnetometers have been described elsewhere [2]. In this paper we describe a tomographic method for using magnetically-imaged, injected currents to reconstruct distributions of electrical conductivity. Much of what we describe should also be applicable to data obtained using uniform colinear eddy currents induced by means of planar sheet inducers [4, 5].

Daniel J. Staton, Sergei V. Rousakov, John P. Wikswo

### High Resolution Image Reconstruction of Polymer Composite Materials Using Neural Networks

A neural network is an artificial intelligence technique inspired by a simplistic model of biological neurons and their connectivity. A neural network has the ability to learn an input-output function without a priori knowledge of the relationship between them. Typically a neural network consists of layers of neurons, whereby each neuron in a given layer is fully connected to neurons in adjacent layers. Figure 1 shows such an arrangement with three layers, called the input, hidden and output layers. The connection strengths between neurons, often referred to as weights, are modified by a training phase. The training phase used here utilizes an error back propagation algorithm [1]. During training the neural network is presented with input which propagates through the network producing a corresponding output. A comparison of the actual output with the desired or target output generates an error which is used to adjust the neural network’s weights according to an error gradient descent technique [2]. This procedure is repeated for many different input and desired output pairs allowing the neural network to learn the input-output function.

A. C. Pardoe, D. A. Hutchins, J. T. Mottram, E. L. Hines

### Computed Tomography Using Laser-Based Ultrasonics

Computed tomographic techniques provide the ability to image defected regions of a structure from outside the area containing the defects. This is often advantageous when the defected area is not directly accessible. Recently, computed tomography techniques using Lamb waves and surface acoustic waves have been proposed and investigated [1–3].

Y. Nagata, J. Huang, J. D. Achenbach, S. Krishnaswamy

### Ultrasonic Imaging of Anomalies in Truck Tires

Ultrasonic imaging provides an effective and efficient method to detect and size defects in tires. Defects may be delaminations, belt fractures and fatigue cracks. The interaction of ultrasound with defects produces reflected and transmitted signals which can be used to produce images of the crown, shoulders and sidewalls of the tire to visually reveal the presence of defects.

A. Cheng, J. D. Achenbach, R. Rogers, M. Peterson

### Development of a New Method for the Inspection of a 3D Volume of Ultrasonic Data

NDT technologies by ultrasounds are in constant progress. The advance is considerable for sensors and acquisition systems. The means for data processing follow this evolution. In order to meet the demand for processing very high volumes of data, we propose in this paper a method of volume automatic thresholding. The application concerns the testing of austenitic steel or else, for the nuclear industry. The thresholding techniques from the image histogram are inadequate as the ultrasonic image histogram is unimodal. The study of the image with the cooccurrence matrix which is a two dimensionnal histogram allows to clearly show the noise-defect transition. Several authors have elaborated on thresholding techniques of an image from the cooccurrence matrix. These vary according to the type of exploited image [1–2]. We showed the good results of thresholding by cooccurrence matrix on images with a defect. We develop in this paper a two-part study. Firstly, after a brief account of the matrix exploitation, we show the limits of this method in bearing simulated images. We then describe the chosen approach to extend the method to the volume thresholding problem. We give results from a data volume obtained on austenitic steel testing.

J. Moysan, G. Corneloup, F. Guerault, O. Roy

### A New Technique for Distinguishing Internal Voids from Solid Inclusions

An acoustic microscope has been proven to be a very effective tool for visualization and characterization of small internal defects in solids[l]. The distinction of internal defects such as cracks and voids from solid inclusions is sometimes necessary for material evaluation. For example in case of light metal casting alloys ultrasonic scattered echo from pores and heavy metal inclusions used for strengthening purposes can give the ultrasonic signal of the same order of magnitude [2]. In this paper it is shown how the phase information of the reflected echo can be used to distinguish void signals from solid inclusion signals. Conventional acoustic imaging techniques that use only amplitude information and ignores the phase information can not distinguish between voids and inclusions.

K. I. Maslov, T. Kundu, O. I. Lobkis

### Magneto-Optic Image Classification Using Neural Networks

The aging aircraft program sponsored by the FAA is keen on improving existing technologies and developing new techniques for the detection of cracks and corrosion in aircraft structures. This program is a part of the overall scheme of addressing the global aging infrastructure problem. The authors have been involved in the aging aircraft program through the NASA Langley Research Center (LaRC).

S. Nath, J. P. Fulton, B. Wincheski

### Virtual Reality for Nondestructive Evaluation Applications

Gas transmission pipelines are often inspected and monitored using the magnetic flux leakage method [1]. An inspection vehicle known as a “pig” is launched into the pipeline and conveyed along the pipe by the pressure of natural gas. The pig contains a magnetizer, an array of sensors and a microprocessor-based data acquisition system for logging data. The data is subsequently retrieved and analyzed offline. The pipeline inspection results in the generation of a vast amount of data — in excess of 4 GB, even in compressed form. It is important that these data are presented in a suitable manner for evaluation by trained operator. Virtual reality (VR) display techniques represent an attractive mechanism for presenting this huge amount of data effectively. The application of VR techniques enables the operator to explore the virtual environment generated by the computer. This technique can serve as an important bridge between human operator and the computer. In this paper, we present some preliminary efforts in achieving this interface.

Jaejoon Kim, S. Mandayam, S. Udpa, W. Lord, L. Udpa

### A Novel Image Processing Algorithm for Enhancing the Probability of Detection of Flaws in X-Ray Images

Application of Digital Signal Processing (DSP) techniques in x-ray radiography is a field that is gaining a rapidly growing interest. Dealing with digital x-ray images and enhancing these images using DSP techniques allow the automation of x-ray inspection, which offers several advantages over the traditional film-based inspection. These advantages include reducing the inspection time and cost requirements, obtaing a consistent decision regarding the integrity of the object under test, and allowing the use of real-time inspection [1]. Typically processing of x-ray images to detect and size flaws involves edge detection. In this paper, we primarily focus on an image processing algorithm that is based on a new Gaussian weighted image moment vector edge operator. Application of this operator enhances image edges and suppresses the noise, which results in a significant improvement in the probability of detection of flaws in x-ray images.

Nawapak Eua-Anant, Ibrahim Elshafiey, Lalita Udpa, Joseph Nahum Gray

### Imaging Flaws under Insulation Using a Squid Magnetometer

Superconducting QUantum Interference Devices (SQUID) are the most sensitive instruments known for the measurement of magnetic fields. An all niobium two-hole homemade SQUID can easily achieve sensitivities of 10-4 Ф0/√Hz (Ф0 = 2.07 × 10-15 Wb). Our complete system has a sensitivity of 50 × 10-15 Tesla √Hz, and more sophisticated systems can reach sensitivities one order of magnitude higher. Due to its high sensitivity, and to the advent of high temperature superconductivity, SQUID systems presents new opportunities for its use in nondestructive evaluation of electrically conducting and ferromagnetic structures, mainly when the area to be inspected is difficult to be reached.

A. C. Bruno, C. Hall Barbosa, J. E. Zimmerman, P. Costa Ribeiro, E. Andrade Lima, L. F. Scavarda, C. Kelber, J. Szczupak, C. S. Camerini

### Improvement to Defect Detection by Ultrasonic Data Processing: The DTVG Method

In the case of coarse grained austenitic stainless steel, an important diminution in the defect detection possibilities is noted. The wave amplitude is attenuated and can also be deviated, according to the importance of anisotropy and/or heterogeneity.

G. Corneloup, J. Moysan, D. Francois

### Imaging of Creep Damaged CrMo Steel Piping Using Magnetic Parameter Variations

Creep damage is a leading cause of failure in pressure vessel steels exposed to high temperature and stress. The development of a nondestructive inspection method for visualizing and detecting the level of creep damage throughout the service life of these pressure vessels is critical to safe power plant operation. If the damage is detected at an early stage and monitored throughout its progress, replacement costs and downtime can be reduced, and catastrophic failures can be avoided.

M. A. Negley, D. C. Jiles

### Strategies for Characterizing Transducers and Measurement Systems

The primary objective in calibrating ultrasonic transducers and measurement systems is to ensure the interchangeability of data. Thus, one wishes to create a situation in which experimental results, e.g. A-Scans or C-Scans taken at one time and place, can be directly compared in a quantitative sense to similar data taken at another time and place. The presently available tools are not sufficient to this task. As noted by Burley [1], there has been no single type of calibration standard that is suitable in all ultrasonic applications and inspection procedures. Instead, there is a wide range of reference standards that are used in different situations to calibrate transducers and measurement systems. Although these have served the community well in bounding the results which can be obtained by different systems, they do not ensure uniformity of results. A striking example for the case of eddy currents can be found in Fig. 4 of Ref. [2].

R. B. Thompson, D. O. Thompson, L. W. Schmerr

### Determination of the Absolute Sensitivity Limit of a Piezoelectric Displacement Transducer

Many ultrasonic techniques, such as conventional acoustic emission [1], waveform-based acoustic emission [2] and ultrasonic testing of high-stiffness reinforcing fibers [3], require highly sensitive, broadband, displacement transducers. Optical probes, such as interferometers, offer both large dynamic range and a very wide bandwidth, but do not exhibit the sensitivity needed for such applications [4, 5]. Piezoelectrics, on the other hand, have approximately 40 dB higher sensitivity than optical probes and can be designed to exhibit an acceptably wide bandwidth [6, 7]. In our applications we typically use transducers that exhibit flat frequency response from 10 kHz to 1 MHz on metals. This paper details a procedure by which the noise floor of one of our “HFHS”, piezoelectric transducers can be measured and compared to a model.

E. S. Boltz, C. M. Fortunko

### Metal Diffusion Bonded Transducers for Resonant Ultrasound Spectroscopy (RUS)

Ultrasonic transducers for Resonant Ultrasound Spectroscopy (RUS) need to be produced in a variety of sizes and designs in order to optimize the signals, minimize the effect on the object being measured and deal with odd shaped objects, limited access, and different environments. [1] This continuous requirement to build new transducers for different projects led to the construction technique shown in fig. 1. A piezoelectric element, selected according to the frequency range which is to be covered and the size of the sample, is glued to both an inertial backload and a support, usually a metallized kapton (plastic) diaphragm. Electrical connections to the piezoelectric element are also made with thin metallized kapton with a signal connection glued between the piezo element and the backload and a ground connection made via the diaphragm at the front.

T. W. Darling, A. Migliori, R. D. Dixon

### The Effect of Dust and Surface Roughness on the Efficiency of Solid Coupled Wheel Probes

Recent improvements in the performance of solid coupled wheel probes and other solid coupled ultrasonic devices [1] have opened up the possibility of their wider application. If such devices are to become general purpose NDE tools then it is important that the parameters which limit their use be understood. Experience gained at Imperial College by using such devices on real engineering structures indicated that the presence of high surface roughness and dust/dirt severely affected their performance. It is the aim of this paper to quantify these effects and gain a better understanding of their origins. This increased understanding will then allow for improved design of dry coupled devices.

Bruce Drinkwater, Peter Cawley

### Wideband Nonuniformly Excited Focused Transducers — Theory and Experiments

In an earlier study [1], we discussed the interest of wideband nonuniform transducers in nondestructive testing (NDT). Only flat transducers were studied. Since that, we have developed a new technique that combines in a single probe three different modes: a conventional uniform mode, a mode favoring the plane wave component leading to a diffractionless beam and a mode favoring the edge diffracted wave leading to a focusing effect. This original technique is fully described in [2]. Here, we study the influence of nonuniform source profile on the characteristics of the field radiated by wideband focused transducers, as compared to conventional focused transducers with uniform profile. Classically, the transient field radiated by a conventional spherically focused transducer is described as the superposition of a geometric wave (a spherical wave converging to the focal point then diverging) and of diffracted edge wave [3]. Two nonuniform profiles are studied, both theoretically and experimentally which favor either one or the other contribution.

Alain Lhémery, Daniel De Vadder, Nicolas Gengembre

### Fidelity of Michelson Interferometric and Conical Piezoelectric Ultrasonic Transducers

The motivation for this research is our ongoing effort in the development of ultrasonic, waveform-based, materials characterization techniques. Having developed a three-dimensional representation of the elastodynamic Green’s Function for anisotropic plates [1], we now seek to verify the applicability of this representation. With elastic properties measurement as our end goal, we also seek a transducer for measurement of theoretically predicted waveforms. Our waveforms typically exhibit a large range in both amplitude (40 to 60 dB) and frequency (20 kHz to 2 MHz). The transducer used must exhibit both high-sensitivity and high-fidelity so that multiple reflections can be detected and identified. In a companion paper we describe a transducer developed at NIST for acoustic emission (AE) studies [2]. In that paper we determine that this transducer has a displacement sensitivity of approximately $$\frac{{5 \times {{10}^{{ - 17}}}m}}{{\sqrt {{Hz}} }}$$ in the 250 kHz to 1 MHz frequency region on aluminum. This transducer appears to be a good candidate for waveform-based materials characterization. In this paper we evaluate this transducer’s fidelity by comparing it with both theoretical results and measurements from a path-stabilized, Michelson interferometer. We conclude that the current transducer design does not have sufficient fidelity for waveform-based materials characterization and discuss the reasons for its shortcomings and potential solutions to these problems.

E. S. Boltz, V. K. Tewary, C. M. Fortunko

### A New Ultrasonic Transducer Combining Three Modes: High Axial Resolution, High Transverse Resolution and Standard Modes

Ultrasonic methods of nondestructive testing become more and more quantitative. They aim not only to detect but also to identify and characterize defects. This results in contradictory requirements for transducer design. One sought improvements in axial resolution for a given frequency range in reducing the pulse duration, leading to low sensitivity and failing in the case of small defects close to the transducer. Good transverse resolution with a planar transducer is obtained by using it in its pseudo-focal zone in which the ultrasonic beam is narrow, that is, at a range corresponding to the limit of the near-field, even if this range is ill-defined in the case of a broad band transducer.

Daniel De Vadder, Alain Lhémery, Nicolas Gengembre

### Observation of Shear Wave Modes in the Echoes of the Longitudinal Wave Transducer

Longitudinal and shear waves are the fundamental elastic wave modes in an infinite unbounded solid and their velocities are determined by the mass density and elastic moduli of propagationg medium. Thus, elastic moduli could be determined by measuring the phase velocities of longitudinal and shear waves using the corresponding mode transducers.

Young H. Kim, Jeong Ki Lee, H. C. Kim

### Transient Analysis of a Line-Focus Transducer Probing a Liquid/Solid Interface

The use of a line-focus ultrasonic transducer in a vertical scanning reflection acoustic microscope system is well known for quantitative materials characterization [1]. The technique relies on the measurement of the reflected radio frequency tone burst echo amplitude, V, as a fonction of amount of defocus, z, and analysis of the interference minima of the V(z) curve to obtain various interface wave speeds. The technique uses well developed theory [2,3,4] representing fixed frequency ultrasound generated and detected by a cylindrical lens in the frequency domain. We have developed a large aperture lensless line-focus transducer which is highly efficient and has a bandwidth wide enough to allow the generation and detection of narrow transient pulses [5]. From this transducer placed in water near a solid sample, the resulting echo waveforms have multiple features which can be interpreted as the arrival of a specularly reflected axial ray and leaky surface waves. Using this transducer, we have developed a time-resolved and polarization-sensitive testing technique for materials characterization [6]. The objective of this paper is to provide a theoretical basis for interpretation and analysis of these time domain waveforms.

N. N. Hsu, D. Xiang, S. E. Fick, G. V. Blessing

### Acoustic Waves Generated by a Transient Point Source in an Anisotropic Medium: Comparison Between Theory and Experiment

Many authors did study Lamb’s problem for an anisotropic half space belonging to the hexagonal system of symmetry. The first application of the Cagniard — de Hoop method to study the line source configuration is due to Kraut [1]. Following this pioneering contribution, Suh et al. [2] have extended such method to the point like source case. More recently, Payton [3] further developed to analytical calculations of the epicentre displacement fields.

B. Audoin, A. Mourad, M. Deschamps

### Modeling the Ultrasonic Radiation of Shear Wave Angle Beam Transducers

Weld inspections of fabricated plate and pipe assemblies made with shear wave angle beam transducers are a common and important application of ultrasonic NDE. It is now possible to develop complete models of such angle beam inspections (see, for example, the measurement model of Thompson and Gray [1]) for many practical configurations. One important element in these models is the calculation of the fields generated by the angle beam transducer, i.e. the transducer beam model. To date, three different beam models have been derived and studied. They are the Surface Integral model [2], the Boundary Diffraction Wave (BDW) Paraxial model [2], and the Edge Element model. Each has certain advantages and disadvantages associated with it, as will be seen in later sections of this paper.

Terence P. Lerch, Lester W. Schmerr, Alexander Sedov

### Broadband Fields Radiated in a Solid by Water-Coupled Transducers: A Comparison of Approximate Models, Numerical Approaches and Experiments

In number of configurations, ultrasonic tests in the French nuclear industry are made using water-coupled focused transducers. To study the influence of the various parameters involved in transducer/piece configurations, model-based predictions of the field radiated by transducers are very useful. A model (called Champ-Sons) has been developed at the French Atomic Energy Commission (CEA) to calculate the field radiated by focused or unfocused transducer through liquid/solid interface at normal or oblique incidence [1]. It can deal with radiating surface of complex (3-D) shape (spherical focusing, Fermat’s surfaces, multiple-elements [2] etc.). The calculation is done directly in the time domain for broadband sources and in the frequency domain for narrowband sources. In its present form Champ-Sons deals with either plane or cylindrical interfaces between a fluid and an isotropic solid. It is implemented in a user-friendly software developed at the CEA called CIVA [3] for NDT data processing (eddy-current, ultrasonics, neutrongraphy, radiography). Since non-canonical configurations are considered and pure numerical schemes are too computer intensive, the model treats the refraction at the fluid/solid interface in an approximate way. It has been validated experimentally [1].

Pierre Calmon, Alain Lhémery, Jérôme Nadal

### Interrogating a Thin Layer of Heterogeneity with Confocal Transducers

This is an expository summary of our work [1] building a mathematical model of scanned acoustic imaging of complicated solid-solid interfaces comprised of scatterers at several length scales, many of which are less than a wavelength. We construct an approximate two dimensional model of a scanned confocal acoustic imaging arrangement operating in a transmission or reflection mode using anti-plane shear or SH waves. Further, we suggest how the sound scattered from the interface is mapped into the sound collected by the transducers. The scalar approximation, while restrictive, still captures many of the basic ideas, ideas that we are at present extending to a three dimensional calculation.

John G. Harris, Douglas A. Rebinsky, Gerry Wickham

### Design and Optimization of a Multifunctional Phased Array Search Unit

Dual-element angle beam transducers exhibit operating characteristics suitable for a large range of practical field applications, particularly for the interrogation of highly attenuative materials such as stainless steel. These capabilities can be improved by operating such transducers as phased array search units, consisting of many transducer elements. Each of the array elements is pulsed with the appropriate time delays, thus controlling the shape and the sound beam direction on a large scale. This study describes the design and optimization of a multi-mode T/R (transmit/receive) phased array search unit generating longitudinal, shear and subsurface longitudinal (’creeping’) waves. The elaborated design can be operated with 12 or 16 transducer elements generating multiple wave modes for certain inspection ranges. This search unit is particularly suited for detection of surface connected planar discontinuities associated with circumferential or vertical weld seams of pressure vessels or pressure vessel components (core shroud) in the range of 1.5 inches (~ 40 mm) wall thickness. Based on the well-known physical processes inherent to ultrasound generation in isotropic media, the Generalized Point-Source-Synthesis method (GPSS) [1,2] has been used to model and optimize the probe. The three-dimensional transducer build-up simulation includes the reflection/refraction process at the wedge-to-material sound entry interface considering fluid coupling. The established search unit parameters consist of wedge angle, roof angle, transducer element length and width by given search unit housing dimension and a fixed inspection frequency.

Martin Spies, Wolfgang Gebhardt, Michael Kröning, Michael H. Dalichow

### The Response of an Air-Coupled Ultrasonic Transducer for Various Backplates and Apertures

Development of air-coupled capacitance ultrasonic transducers that operate above 100kHz have recently been of interest. The main applications for these devices include the location and imaging of objects, and in the nondestructive evaluation of engineering materials. To control the response of either as a source or a receiver, we needed to investigate the effect of using different backplate materials and manufacturing techniques. Backplates were produced in a range of materials such as silicon, copper and stainless steel. Various techniques of micromachining such as chemical etching, laser machining and ion beam machining were used.

A. G. Bashford, D. A. Hutchins, D. W. Schindel

### Non-Contact Ultrasonic Tomography Imaging Using Air-Coupled Capacitance Transducers

Tomographic reconstruction [1] is a method of imaging by illuminating the object in many different directions in the plane of interest, using X-rays or ultrasound. An image is formed from changes in a physical variable occurring in the planar cross section. Typically, changes in propagation delay or arrival time are used to reconstruct an image of the slowness variations (where slowness is the inverse of velocity), or changes in signal amplitude are used to produce an attenuation image.

W. M. D. Wright, D. A. Hutchins, D. W. Schindel

### Air-Coupled Ultrasound and Leaky Lamb Waves in Composites

Recently, improvements in instrumentation have provided the option of gas- or air-coupled ultrasonic testing as a realistic alternative to immersion or contact testing. In this paper we present theoretical and experimental results on resonant sound transmission methods mediated by ultrasonic coupling directly through ambient air. Although these methods are not new [1], they have only recently been considered for MHz applications. Advances in transducer technology [2–5] have further improved signal to noise ratios.

A. Safaeinili, O. I. Lobkis, D. E. Chimenti

### Quantitative Materials Characterization of Elastic Plates Using Air-Coupled Leaky Lamb Waves

The signal-to-noise penalty of air or gas-coupled ultrasonics has so far limited its application principally to feasibility demonstration experiments and qualitative assessments of material condition [1–3]. In most cases to date, use of air-coupled ultrasound to estimate material properties has been performed by observing the phase-matched transmitted signal amplitude and correlating this occurrence with the plate wavespeed. While this approach has approximate validity, other experimental conditions, including geometry and frequency, as we have shown [4], can significantly alter the estimate of material properties made only on the basis of phase-matched amplitude extrema. In addition, since the acoustic impedance mismatch is so large in air-coupled measurements, material damping plays a greatly enhanced role in determining the nature of the transmitted or reflected field.

D. E. Chimenti, A. Safaeinili, O. I. Lobkis

### Air-Coupled Ultrasonic Bulk Waves to Measure Elastic Constants in Composite Materials

The ultrasonic immersion technique, using a water coupling medium, is now a standard method for the non-destructive inspection of anisotropic composite materials. Using a goniometer to investigate a composite material in any direction of propagation, their anisotropic elastic l,2 or viscoelastic 3,4 properties can be determined. A comparative analysis of the through-transmission ultrasonic bulk wave methods was recently published by Y.C.Chu and S.I.Rokhlin 5.

Bernard Hosten, David A. Hutchins, David W. Schindel

### Air-Coupled Ultrasonic Transducers for the Detection of Defects in Plates

In order to minimise the problems due to the acoustic impedance mismatch between solids and air, the non destructive testing of materials using ultrasonic transducers generally requires either contact transducers or immersion transducers to be used [1]. Air-coupled transducers however would be very advantageous for testing structures which must be not contaminated with couplant and also for all in-situ industrial applications. Although the propagation of ultrasonic waves from laser generation [2] involves air-coupling, the difficulties due to the experimental set-up of this technique and the financial investment it implies are two major disadvantages.

M. Castaings, P. Cawley, R. Farlow, G. Hayward

### A New ECT Probe with Rotating Direction Eddy Current

In the eddy current testing, various kinds of noise are generated by variations of many factors such as the probe lift-off and the material configuration. A lot of efforts have been made to develop new probes with little noise[1,2]. The authors think that it is necessary to develop a new noise free ECT probe in order to conduct the quantitative nondestructive testing.

H. Hoshikawa, K. Koyama

### Passive Pulsed Eddy Current Inspecticn of Sheet Metal

Sheet metal is known to contain various amounts of nonmetallic inclusions. If inclusion content becomes large enough the end product can be adversely affected. Some of the more common types of nonmetallic inclusion material are silicon dioxide and aluminum oxides. Inclusions are not found uniformly distributed throughout sheet stock. They tend to appear in clusters, usually elongated strings due to the rolling and drawing processes involved in making sheet stock.

R. Hockey, D. Riechers, R. Ferris, R. Kelley

### Computer Assisted Eddy Current Probe Design

For many years, the inspection of components using eddy current (EC) techniques has been playing an important role in the nondestructive evaluation (NDE) industry. There are many factors which affect the probability of detection (POD) of flaws. One important consideration is how to select suitable EC probes to inspect a given test component with certain class of flaws. Commonly, the performance of an EC probe design for a scan is evaluated experimentally by physically constructing prototype probes and perform test scans. This approach is both time-consuming and expensive. In particular, the EC probe has to be reconstructed each time the probe design is changed and the design cycle typically requires a number of iterations before a satisfactory performance is achieved.

J. C. Chao, A. N. S. Prasad, N. Nakagawa, D. Raulerson

### Application of Self-Nulling Eddy Current Probe and Spin-Off Sensor Technologies to Airline Industries and Beyond

As the existing commercial air fleet of the airline industry ages, the major demand on the NDE community is to develop simple, cost effective methodologies of higher detectability and reliability. To satisfy such a demand a focused R&D effort has been performed for the past several years through NASA Airframe Structural Integrity Program (NASIP). This particular program concentrates on the development of methodologies applicable to NDI of aircraft fuselage, which, being a thin metallic structure, is best suited for inspection by various electromagnetic techniques. Such is the direct motivation for the development of the self-nulling eddy current probe technology.

M. Namkung

### High Sensitivity AC Field Measurement Using Rhombic Inducer

The principle behind the ac field measurement (ACFM) technique has been explained in [1]. In this technique, a thin-skin eddy current is induced in the metal under test. The current is perturbed by defects in the metal surface and the result is reflected in the magnetic field above the surface. A probe is used to detect perturbations in this field.

D. Mirshekar-Syahkal, R. F. Mostafavi

### The Emergence of Extended Field Technology in the Air Force

The wide-field eddy-current probe was developed to fulfill the inspection requirement of the broach slots in Pratt & Whitney F100-PW-220 Stage-1 and Stage-3 Compressor Disks. The flaw-detection requirement is to inspect an area 0.07 in. from the inner radius of the slot and 0.10 in. from the top and bottom edges for determining the presence of 0.006-and 0.010-in.-deep axially oriented flaws (Figure 1). While the slot is relatively small, the time required for inspection using the previous method is considerable. With this method a D20 (0.029-in.-diameter) split “D” coil is scanned in a “sew-stitch” fashion in the axial direction of the slot. After each scan the probe is indexed one quarter of the coil diameter. A total of 88 scans per slot is required for complete coverage, the time per slot being 20 min; with 40 slots the total inspection time per part is 14 hr when gain-calibration time is taken into account.

T. J. Braun, E. E. Keppler, L. S. Price, G. Nuxoll, M. Gehlen, D. Lyders, J. Henderson, R. Stone, B. Arispe

### Inspection of Metallic Plates Using a Novel Remote Field Eddy Current NDT Probe

The remote field eddy current (RFEC) technique was invented in 1951 [1], [2] and is widely used as a nondestructive evaluation tool for inspecting metallic pipes and tubing. Essentially, the RFEC phenomenon can be observed when an AC coil is excited inside a conducting tube (see Fig. 1). The RFEC signal can be sensed by a pick-up coil located 2–3 diameters away from the excitation coil. The signal is closely related to the tube wall condition, thickness, permeability, and conductivity. The signal phase, especially, has approximately linear relationship with the tube wall thickness.

Y. S. Sun, S. Udpa, W. Lord, D. Cooley

### Magnetoresistive Eddy-Current Sensor for Detecting Deeply Buried Flaws

One of the trends in eddy-current (EC) NDE is to probe deeper by working at lower frequencies. In aircraft NDE, frequencies as low as 300 hertz have been used to inspect lap joints involving several layers of aluminum plate [1–5]. Remote-field EC works at a few tens of hertz to penetrate the walls of ferromagnetic tubes such as oil-well casings or heat-exchanger tubes in power plants [6,7].

William F. Avrin

### Advances in the Theory and Practice of Squid NDE

The superconducting quantum interference device (SQUID) holds great promise for electromagnetic nondestructive evaluation (NDE) because it offers high sensitivity -permitting high lift-offs or very small excitation signals — and maintains this sensitivity from DC to high frequencies [1]. In eddy current NDE, this allows an induction coil to comprise only a few turns, or even a single filament, without a high permeability core, and makes forward modeling and inverse processing easier, since the induction source is well defined and the SQUID itself closely approximates an ideal sensor. However, the SQUID also has practical drawbacks, including the need for cryogenic temperatures and for differential configurations for measurements in environmental fields. Until very recently, almost all SQUID NDE systems were based on low temperature superconductors (LTSs), but the first measurements with high temperature superconductor (HTS) SQUIDs are now being reported [2–4].

A. Cochran, G. B. Donaldson, C. Carr, D. McA. McKirdy, M. E. Walker, U. Klein, J. Kuznik, A. McNab

### Magneto-Optic/Eddy Current Imaging of Subsurface Corrosion and Fatigue Cracks in Aging Aircraft

The rate of production of new aircraft, as well as the availability of funds to purchase them, are limited. As a consequence of these two factors, only a small percentage of the many thousands of aging aircraft in existence today (both civilian and military), will be replaced by new aircraft in the foreseeable future. Because replacement is not an option in many cases, the only practical way to maintain operations is to extend the useful service lifetimes of these aircraft by means of proper maintenance, repairs and upgrades [1]. An important component of proper maintenance is periodic inspection for subsurface corrosion and fatigue cracking in airframes.

G. L. Fitzpatrick, D. K. Thome, R. L. Skaugset, W. C. L. Shih

### Bulk Wave Characterization of Laminated Composites

Composite materials are currently seeing wider use in the aerospace and automobile industries. Composites offer many advantages over conventional materials, such as a greater strength to weight ratio and the ability to engineer their mechanical properties to a specific task. The major problems associated with composites are cost and reliability. Like virtually all engineering materials, composites can have flaws which may compromise their strength and reliability. The ability to detect these flaws in a reliable, cost effective fashion is significantly essential in the utilization of composite materials in critical structural areas. Currently, nondestructive evaluation using ultrasonic wave amplitude analysis, is most often used to inspect materials for flaws. This method can detect gross macroscopic flaws such as delamination or cracks, but more subtle flaws in the individual layers of a composite such as incomplete cure or low fiber volume ratio, cannot be found using conventional inspection techniques. Full stiffness modulus reconstruction, using acoustic wave velocities, is an alternative way to nondestructively determine the exact mechanical properties of a given composite part. Much research has been done in the area of modulus reconstruction of single layered composites [1–3]. The objective of this paper is to develop schemes for modeling multi-layered composites commonly seen in practice. Two basic methods of modeling composites are presented here; the layered method and the averaged method. The layer method treats each ply as a separate material. The averaged method consists of taking all the layers and averaging their material properties together. This paper will look at the differences between these two methods and will show how the relationship between the wavelength and the ply thickness determines which theory will apply.

B. Kennedy, R. Kline

### Low Frequency Guided Plate Wave Propagation in Fiber Reinforced Composites

The use of composite materials has increased steadily during the past two decades, particularly for aerospace, underwater and automotive structures. This is largely because many composite materials exhibit high strength-to-weight and stifihess-to-weight ratios, which make them ideally suited for use in weight-sensitive structures. The elastic properties of composite materials may be significantly different in specimens manufactured under the same general specifications and the bulk material properties may be different from those of the lamina. The elastic properties degrade as a result of aging, environmental and other effects (e.g., matrix cracking) resulting in overstress and eventual failure of the material. The elastic properties determine the performance of the material and it is necessary to assure the conformance of these properties with design requirements. Conventional destructive techniques for determining the elastic stiffness constants can be costly and often inaccurate. This is particularly true for the through-the-thickness properties. Nondestructive determination of these properties offers a better alternative for material characterization and for assuring structural performance.

Shyh-Shiuh Lih, Yoseph Bar-Cohen, Ajit K. Mal

### Validity of the Elastic Constant Approximations for the Ultrasonic Evaluation of Multi-Layered Inhomogeneous Composites

Several ultrasonic wave mechanics models which are based on the effective elastic properties of the overall fiber reinforced composites are currently being used in the industry and academia. Due to the repetition of the ply orientation and the lay-up sequence traditionally used in the industry, these models which often “smear” the individual ply elastic properties to obtain effective elastic properties of the entire composite. [1,2] This is an acceptable approximation for long wave length waves propagation in homogeneous or quasi-homogeneous ply lay-ups media and has provided useful results in the non-destructive evaluation process.

Krishnan Balasubramaniam, Yuyin Ji, Toxie Givens

### Application of Shear Waves for Composite Laminate Characterization

The layup sequence in a composite laminate greatly effects its properties. If one ply is misaligned in the layup sequence, it can result in the part being rejected and discarded. At the present time, most manufacturers cut a small coupon from the waste edge and use a microscope to optically verify the ply orientations on critical parts. This can add a substantial cost to the product since the test is both labor intensive and performed after the part is cured. A nondestructive technique which could be used to test the part after curing and require less time than the optical test would be very beneficial, and one that could be performed prior to curing would be extremely desirable. Preliminary tests demonstrate a high probability that the model and tests developed in this paper can be used for characterizing uncured layups as well.

Brent A. Fischer, David K. Hsu

### Dynamic Effects in Ultrasonic Determination of Composite Moduli Using Velocity Data

Ultrasonic techniques show great promise for nondestructive characterization of high temperature composites which are usually manufactured with specially designed fiber-matrix interphases. Ultrasonically measured composite elastic moduli are important mechanical characteristics, and the problem of interphase characterization is also often related to the measured composite moduli. The characterization of interphase is critical since the interphase transfers load from the fiber to the matrix and its properties significantly affects the overall mechanical performance of the composite. Chu and Rokhlin have recently developed methods to determine fiber-matrix interphase elastic properties from ultrasonically measured composite moduli using static micromechanical models [1, 2, 3, 4]. Since their method is based on measurements of ultrasonic wave velocities in different directions in the composite and on relating them to the static composite elastic moduli, error may be introduced in determining the static composite moduli from wave velocity data if dispersion is not negligible. We have performed experimental measurements and theoretical studies of dispersion and attenuation for waves propagating along and normal to fibers in a SiC/Ti unidirectional metal matrix composite [5, 6, 7]. In this paper we focus on the effect of fiber-induced dispersion on determination of the composite and fiber-matrix interphase moduli.

W. Huang, S. I. Rokhlin

### Acoustic Emission Analysis of SCS-6 Fiber Fracture in Titanium Matrix Composites

One aspect of successful composite design involves development of a detailed knowledge of damage evolution. In metal matrix composites, cracking and/or plastic deformation of one or more constituents together with fiber-matrix interfacial debonding and sliding generally occur prior to catastrophic failure [1, 2]. The nature and severity of these damage processes controls mechanical performance. In ductile matrix systems having a low fiber-matrix interfacial strength, the failure process can involve successive fragmentation of the fibers with increasing load. Broken fibers shed load (equally among the unbroken fibers in the case of global load sharing) until the fiber fracture density reaches some critical value and the sample catastrophically fails. Characterization of damage development has been slowed by a lack of NDE techniques. Here, the use of acoustic emission (AE) techniques is explored to further understand and quantify failure processes of this type.

David J. Sypeck, Haydn N. G. Wadley

### Ultrasonic Velocities of C/AL Composites Determined by Laser Ultrasonic Method

Composite materials have received wide-spread attention because of their enhanced mechanical properties and suitablility for specific applications. The fiber or particulate reinforced aluminum matrix composites are rapidly developing and considered as the best packaging materials in future[1,2]. They have outstanding properties of low density, high ratios of strength and stiffness to density, as well as can be tailored specifically for electronic packing.

X. R. Zhang, D. Fei, C. M. Gan, S. Y. Zhang, X. B. Yu, G. D. Zhang, R. J. Wu

### Characterization of Static- and Fatigue-Loaded Carbon Composites by X-Ray CT

Computed Tomography methodologies were investigated to better understanding their possibility to improve the knowledge and a correct understanding of the behavior of thin Carbon-Polymer composites when static or fatigue loaded to failure. We applied CT to study a set of six aerospace grade carbon fiber/thermoplastic or fiber/thermoset matrix composites. The samples were subjected to either static or high-stress fatigue loading in tension. Both notched (central circular hole) and unnotched specimens were examined. We investigateed a high-temperature thermoplastic polyimide composite sample by acquiring CT data sets before, during (at set intervals), and after full-reversal (tension-compression), low-stress fatigue loading at the upper use temperature.

Valentina Savona, Harry E. Martz, Hal R. Brand, Scott E. Groves, Steven J. DeTeresa

### Use of Ultrasonic Lamb Waves for In-Process Porosity Inspection of the Pultrusion Process: Theoretical Velocity Calculations

Substantial growth potential for composite materials exists in the private sector, where high-volume, low-cost production is required. Post processing inspection can represent a significant percentage of the cost of composite products. Alternate ways of assuring quality must be examined. In-process inspection can be effectively applied to continuous composite manufacturing techniques such as the pultrusion process. Recent research efforts at the Center for Composite Materials has focused on ways of evaluating pultruded composites on-line by using ultrasonic non-destructive evaluation (NDE) techniques. The most accurate method to date is based upon Lamb wave velocity measurements. This paper focuses on the theoretical analysis used to predict the Lamb wave velocities and compares the results with experimental measurements.

D. L. Fecko, J. W. Gillespie, K. V. Steiner

### The Effect of Porosity Density and Configuration in Composite Materials on the Ultrasonic Waveform

Current practice is to accept or reject composite parts based upon ultrasonic C-scan results. Normally, this is based only on ultrasonic attenuation data. However, attenuation data alone does not account for variations in porosity distribution or type, and ignores the fact that other variables can influence attenuation besides porosity. This work was directed at determining additional parameters which can be used to define the defect structure in a composite.

David Akers, Chris Vaccaro, Skip Ellsworth, Don Pettit

### Ultrasonic NDE Techniques and the Effects of Flaws on Mechanical Performance in Multi-Directionally Reinforced Textile Composites

Developmental efforts in composite technology have included a wide variety of secondary and primary structure in industries ranging from sporting goods to high performance aircraft. In addition to potentially significant weight savings, composites offer the potential benefits of increased fatigue life, corrosion resistance, and acquisition or life-cycle cost savings. Conventional two-dimensional composites, however, have poor interlaminar strengths and are susceptible to damage.

R. D. Hale, D. K. Hsu, D. O. Adams

### Eddy Current Detection of Fatigue Microcrack Distributions in Al-SiC Composites

The modern philosopy of materials reliability hinges upon the ability to detect flaws before they reach a critical size that might lead to the failure of a component before the next inspection opportunity. It is the role of nondestructive evaluation (NDE) to provide this capability. In advanced materials such as Al-SiC composites, however, fatigue failure under high stress is governed by the nucleation and growth of a large number of distributed microcracks 1, often much smaller than the detection threshold of conventional NDE techniques (i.e. 200 03BCm). The “fatal” crack forms through the linking of several microcracks. A given fatal crack can spend as little as 5 percent of the fatigue life as a crack detectable by NDE in these materials since some ductility is often sacrificed to achieve their higher strength (i.e. crack instability occurs for crack sizes much smaller than would be expected in conventional alloys). The difficulties that arise for the NDE of advanced materials are that at one end of the fatigue spectrum the microcracks are too small to detect for most of the component’s life. At the other end of the spectrum, by the time a crack long enough to detect appears, failure is imminent.

E. Y. Chen, L. Lawson, M. Meshii, J. C. Moulder, T. Khan

### Quantitative Ultrasonic Characterization of Metal Matrix Composite Fiber/Matrix Interfacial Failure

The transverse properties of unidirectional titanium matrix composites (TMCs) are dominated by the fiber/matrix interfacial properties, residual stresses and the matrix mechanical response. Nimmer et al. [1] have pioneered the research on the role of the interface when the composites are under transverse loading. In their work, a characteristic “knee” has been observed in the transverse tensile stress-strain curve of a Ti-6A1-4V/SCS-6 composite. This “knee” occurs well below the stress level at which the matrix yields extensively. The comparisons of experimental results with finite element modeling indicate that the “knee” is due to the failure of a weak interface under the transverse loading.

Shoufeng Hu, Douglas B. Gundel, Theodore E. Matikas, Prasanna Karpur, Gregory S. Clemens

### Thermography for Honeycomb Panel Inspection

Many of the secondary structures on commercial airplanes are fabricated from composite material with honeycomb core and use is increasing on new airplane models. Under adverse conditions, moisture can enter the honeycomb cells through microcracks on the surface of the part or around fasteners or fittings. Therefore, it is necessary to inspect the aircraft structures that are prone to water ingression as part of a typical maintenance program. If the moisture goes undetected, it can accumulate in the honeycomb cells. If enough water fills the honeycomb cells, the water expands upon freezing at altitude, which can lead to disbonds and, in extreme cases, the subsequent loss of the skin. Radiography and electronic thermography are currently approved NDI techniques in the Boeing NDT manuals. This paper discusses the benefits of using thermography for initial water detection and the subsequent repair inspection and describes a low cost chemical thermography method that employs liquid crystals. Both the electronic thermography and liquid crystal methods have been approved for use as alternatives to radiography.

Jeffrey G. Thompson

### Characterization of Composite Materials from Temporal Thermal Response

Fiber reinforced composite materials are increasingly used in applications that require high strength to weight ratio and resistance to high temperatures. Recent works specifically concerning thermal transfer have led to a better understanding of the relationship between constituents and the relative thermal properties. One focus is to obtain the effective thermal properties of an equivalent homogeneous medium that gives the same averaged thermal response as the composite [1, 2, 3]. Another one is the interfacial thermal barrier effect in heat conduction in heterogeneous media [4, 5, 6]. In the present work, an experimental setup used for the nondestructive characterization of multilayered flat plates [7] was modified to image the thermal response of fiber reinforced composite materials. The technique consists in rastering a laser beam, the heat source, at the surface of the specimens. At each point, the temperature is measured as a function of time. A multi-image, composed of the temperature time history at each pixel, is obtained. A model predicting the temperature response of such composite materials is presented and compared to the experimental data.

P. R. Emeric, W. P. Winfree

### Influence of the Frequency Behavior of Arall and Glare on the Ultrasonic C-Scan Inspection

Fibre Metal Laminates (FML’s) are new types of materials which are of interest to the aerospace industry. Two examples are ARALL and GLARE. The materials are composed of thin layers of aluminum bonded together with aramid (ARALL) or glass (GLARE) fibre reinforced layers (which will be called the aramid or the glass layer, see figure 1). The materials are lighter than aluminum sheets of the same (total) thickness and have a superior fatigue behavior.

Theo M. Modderman

### Guided Waves for Composite Patch Repair of Aging Aircraft

Techniques that have been developed for adhesive bond inspection of metal structures are now being extended to composite doublers. The widespread use of these structures has necessitated strict requirements for bond quality; hence, many non-destructive testing methods have been developed[l]. Although point by point ultrasonic methods have been successfully developed, these methods tend to be tedious and are relatively insensitive to a weak interface or kissing bond. A global, guided wave inspection technique for bond inspection has been developed by the authors. This technique has proved to be successful for inspecting the integrity of structures with metal to metal adhesive bonds; such as tear straps and lap splice joints[2,3].

Joseph L. Rose, Krishna M. Rajana, James N. Barshinger

### Evaluation of Adhesion Integrity in Graphite Epoxy Bonded Structures Using Ultrasonic Reflection Factor Analysis

With the evolution of light and stiff composite technology as a viable alternative to metals, wide-spread applications of bonded composite structures are being accepted in the aerospace industry. Although several bonded composite structures are currently in-service, the certification cost often negates the demonstrated benefits of composite material based structures. One of the key requirements for the certification and the subsequent implementation of the new composite-composite bonded structure is the assurance of the integrity of the adhesive bond joints. In this paper, a reflection frequency spectrum of the reflected ultrasonic oblique wave (in a pitch-catch mode) from an adhesively bonded graphite-epoxy aircraft wing-spar structure is obtained and mapped. Different types of programmed defects such as inclusions, voids, and interface contamination at the adhesive substrate were evaluated using the reflection factor technique.

Rani W. Sullivan, Krishnan Balasubramaniam, George Bennett

### The Use of Temperature Pulses to Detect Debonding of Honeycomb Sandwich Panels

The objectives of this study were twofold:1)Develop a combined finite difference thermal evaluation scheme and a rapid structural deformation assessment technique for a circular facesheet delamination in a honeycomb sandwich panel subject to incident thermal irradiation.2)Use the method of 1) to evaluate the effectiveness of a thermal delamination inspection tool.

W. Dauksher, A. F. Emery

### Ultrasonic Evaluation of Titanium Alloy Diffusion Bonding

During the diffusion bond process, parts are heated to about one half the absolute melting point, pressed together at a stress below the macroscopic yield stress, and conditions maintained for a specified time. Bonding proceeds through three steps: local yielding of contact points upon initial application of stress; creep deformation on the bonding plane to yield discontinuous voids; and closure of voids by vacancy diffusion. Presently, nondestructive evaluation emphasizes detection of residual unbonds and voids, from incomplete void isolation or closure.

E. J. Nieters, M. F. X. Gigliotti, L. C. Perocchi, R. S. Gilmore

### Measurement of Adhesion Strength of Bonds Using Nonlinear Acoustics

It is well known that the nonlinear elastic behavior of solids modulates transmitting ultrasonic wave [1], Higher harmonics of the fundamental frequency are generated, especially second harmonics describing the deviation from the linear elastic behavior of the solid. Some effects were also observed in ultrasonic velocity [2] and transmission of ultrasound [3] due to the nonlinear elastic behavior of interfaces. In general, reduced bonding of joined materials causes a strong nonlinear elastic behavior [4]. We exploit this for the observation of the modulation of the ultrasonic wave at interfaces [5,6]. There exist several models based on a macroscopic description of the interface restoring forces using for example the extended Hooke’s Law [7] or nonlinear spring relations [3,8].

S. U. Fassbender, W. Arnold

### Ultrasonic Evaluation of Quality Attributes in Live Beef Animals Using Real-Time B-Mode Ultrasound Imaging

In the Unites States, beef carcasses are subjectively graded by certified inspectors from the United States Department of Agriculture (USDA). The primary factors in determining beef quality grades are the amount and distribution (or marbling) of intramuscular fat. The intramuscular fat level is estimated by visual inspection of texture pattern in a cross-sectional area of the Longissimus dorsi (ribeye) muscle between 12th and 13th ribs. The four primary grades, from high to low marbling, are Prime, Choice, Select, and Standard. There is a growing demand in the meat industry for an objective system of evaluating the quality of beef carcasses as well as live animals. With some sort of instrument grading system, the beef industry could move towards a long desired goal of value-based marketing. Also, objectively and accurately evaluating beef quality attributes in live animals can be applied for sorting feedlot cattle and making genetic improvements in breeding stock. This could have a great impact on the future of the beef industry.

Viren Amin, Mercedes Izquirdo, Doyle Wilson, Gene Rouse, Ronald Roberts

### Backscatter from Specific Regions of Human Hearts Obtained from Standard Echocardiography Views

Further development of quantitative diagnostic procedures will enhance the selection and implementation of specific therapies that can reduce the damage to the heart muscle when applied to the victims of a heart attack. Ultrasonic imaging is one modality that provides real-time images of the beating heart that may be useful in forming diagnoses and evaluating therapies applied as well as providing insight into the underlying physiology. In characterizing the state of the cardiac muscle after a heart attack a clinician would like the ability to differentiate a segment of myocardial tissue with an old infarct (scarred tissue) from a region with acute ischemic injury where the muscle tissue may still be viable and could potentially be salvaged with the application of an appropriate therapy. Furthermore, once a specific therapy has been applied, it may be useful to monitor the reperfusion of the affected myocardial region. Our goal in ultrasonic tissue characterization is to provide an assessment of the state of the tissue based on quantitative analyses of the ultrasonic signal returned from a specific region of tissue. Implementation of quantitative ultrasonic tissue characterization procedures will complement conventional ultrasonic imaging which provides information regarding the dimensions and motion of the heart.

Mark R. Holland, Ann E. Finch-Johnston, Hiie M. Gussak, Joel Mobley, Olivera Petrovic, Kirk D. Wallace, Christopher S. Hall, Scott M. Handley, Julio E. Perez, James G. Miller

### Determination of Elastic Parameters of Pharmaceutical Materials Using a Fiber Interferometer

In the development of pharmaceutical products the elastic parameters of excipients are studied to assume drug safety. The excipients and their coating affect the emission properties of the drug. To determine the elastic parameters, the powder bond material is first pressed into the shape of a rectangular beam. In commonly used 3-point bending measurements, there is a tendency to move from the elastic bending regime to the plastic bending regime which necessarily changes the result for the elastic parameter in question [1]. Another available method is based on photoacoustics, but it is limited to the elastic regime [2]. The goal of this study is to create a system that would allow for high accuracy testing of these materials from the low force elastic to high force plastic regimes while using the same apparatus.

Jarmo Hietanen, Markku Oksanen, Pekka Raatikainen, Mauri Luukkala, Pasi Raatikainen, Jarkko Ketolainen, Petteri Paronen

### Linear and Nonlinear Ultrasonic Properties of Fatigued 410Cb Stainless Steel

The assessment of pre-crack fatigue conditions is a problem of some importance in the nondestructive evaluation of turbine blades. Of particular concern to the electric power industry is the state of fatigue of steam driven stainless steel blades during the latter stages of service life prior to the nucleation of fatigue cracks. We report here measurements of acoustic attenuation, longitudinal velocity, and nonlinearity parameters on 410Cb stainless steel specimens provided by Virginia Power Company. Both virgin specimens and specimens from retired turbine blades were studied. From the virgin material a series of samples were prepared according to ASTM standards and cyclically fatigued from 0 to 10 million cycles at a tensile stress level of 85% of the yield strength of the material with an R value of zero. The results of the measurements are compared to that obtained from the retired turbine blade material.

Jeong K. Na, John H. Cantrell, William T. Yost

### Plastic Strain Ratio and Texture Coefficients in Orthotropic Sheets of Cubic Metals

Since first demonstrated by Stickels and Mould [1], there has been ample evidence that some elastic and plastic anisotropic parameters of cold-rolled and annealed steel sheets are correlated. These correlations suggest that some formability parameters of steel sheets (e.g., the average (plastic) strain ratio $${\bar r}$$, the planar anisotropy Δr, etc.), which are usually determined by destructive tests, could possibly be inferred from nondestructive measurements of elastic anisotropic parameters. Indeed, much work has been done lately in exploring the possibility of on-line determination of r-value of steel sheets by ultrasonic techniques. [2–5]

Chi-Sing Man, Qingbo Ao

### Nonlinear Acoustic Assessment of Precipitation-Induced Coherency Strains in Aluminum Alloy 2024

There currently exists no direct in situ method for nondestructively assessing the optimum heat treatment time for precipitation strengthening of bulk alloy material. We report here progress on our investigation of nonlinear acoustic methods as a nondestructive means of determining maximum material strengthening during the artificial aging of metallic alloys. We present the elements of an analytical model that predict the effects of precipitation-induced coherency strains generated during heat treatment on the magnitude of acoustic second harmonic signals. The predictions of the model are compared to experimental measurements of aluminum alloy 2024 artificially aged from T4 to T6 temper.

John H. Cantrell, William T. Yost

### A Nondestructive Observation of the Mechanism of Plastic Deformation in Orthogonal Cutting

Machining introduces a large amount of plastic deformation in the workpiece material and chip. This plastic strain is nonuniform, and therefore residual stresses are induced in the workpiece surface and subsurface throughout, and slightly below, the depth of plastic deformation. Thus, residual stresses are often an undesirable but unavoidable byproduct of machining. It is known that the mechanism of plastic deformation in metal is dislocation movement.

Young W. Park, Clayton O. Ruud, Paul H. Cohen

### Applications of SH Mode for Measurement of Anisotropic Properties of Solids

Shear-horizontally (SH) polarized transverse (T) modes are uncoupled from the longitudinal (L) (quasilongitudinal (QL)) mode and transverse (quasitransverse (QT)) mode of shear-vertical (SV) polarization, which are generally coupled with each other. This offers certain advantages to using the SH modes for investigation of the phenomena related to the arrivals of transverse modes, because their arrival and amplitude are not affected by the preceding arrivals of the L (QL) modes and the head waves (HW) in some directions and therefore can be clearly identified and measured in the signal generated by a small pointlike source (PS). In this work the SH modes propagating inside a specimen are generated using a pointlike or a line-segment shear (S) mode piezoelectric Pb(Zr,Ti)O3 (PZT) transducer attached on the surface of a (001)-oriented cubic silicon disk, a (001)-oriented transversely isotropic zinc disk, and a thin orthorhombic poly ether ether kethon (PEEK) composite plate oriented in a principal symmetry plane. The SH modes are detected on the surface of the specimen by a pointlike S mode PZT detector which is polarized in the same direction as the source transducer. A novel technique using the SH modes for measurements of shear elastic moduli, such as C44, C55, and C66, is presented for all these specimens. Investigated also using the SH modes is a focusing of fast transverse (FT) modes in silicon. The focusing pattern in silicon is in sharp contrast to the SH mode radiation pattern observed in an isotropic glass plate.

Kwang Yul Kim, A. Richard Baker, Wolfgang Sachse, Arthur G. Every

### Crack Size Detection Capability of Acoustic Emission

This paper reports on progress of research to define the detectability of cracks and related structural defects using acoustic emission (AE). This work is in response to increasing code acceptance of AE, such as the recent Department of Transportation exemption [1] allowing the use of AE to qualify railroad tank cars in lieu of hydrostatic testing, and the pending Coast Guard acceptance of AE for qualification of ammonia barges. Each of these standards requires the establishment of a minimum detectable defect size. Because of the irreversible nature of the mechanisms that cause AE, traditional round robin testing is not possible.

Timothy J. Fowler, Thomas N. Crump, Charles A. Barnes

### Structure of Magnetic Fields in High Temperature Superconductors with Columnar Defects

Superconductivity is expected to invade our daily life shortly. Technology involving the application of superconductivity will soon be found in different instruments, devices and machinery using a new family of high-temperature superconductors (HTS) where the critical temperature below which the material becomes superconductor has been raised to around half of room temperature. An essential issue that needs to accompany the spread of use of superconductivity is the development of NDE tools that can be used for such new technology.

M. Benkraouda, M. Ledvij

### Material Properties of Aged and Unaged Inconel 718 as Determined from Nondestructive and Destructive Tests

Most if not all materials undergo deterioration of their mechanical properties during service through one mechanism or another with eventual failure occurring due to this deterioration. As structures remain in service then, it becomes increasingly important that periodic evaluation of the material properties be accomplished in order to allow repair or replacement of structural members before catastrophic failure occurs. A group of Nickel-based alloys, collectively known as Inconel, is such a structurally important material. Inconel 718, in particular, shows signs of a “thermal embrittlement” as a function of operating temperature and time exposed to this temperature.

D. J. Barnard, D. K. Rehbein, J. Kameda, O. Buck, J. A. Johnson, W. G. Reuter

### Type discrimination of Various Welding Defects Created During Production and In-Service Use

Verification of the quality of welded structures, both post-production and in-service, has long been a concern of the engineering, materials and nondestructive communities. Nondestructive techniques have been developed and refined to address this verification problem. Each technique has inherent limitations, however, that in certain situations preclude its use or severely restrict the type of information that may be obtained particularly concerning the flaw type. Of these techniques, x-ray investigation is probably the most versatile in determining flaw type and size at present. The equipment required for x-ray examination, however, place constraints on its use, particularly for in-service structures, from the standpoint of accessibility and personnel safety.

D. K. Rehbein, T. A. Gray

### Determination of Elastic Constants by Line-Focus V(Z) Measurements of Multiple Saw Modes

Line focus acoustic microscopy (LFAM) provides a method to determine the elastic constants of homogeneous materials and thin-film/substrate configurations, see Refs. [1–5]. The elastic constants are determined from the velocities of surface acoustic waves, which are obtained from measurement of the V(z) curve. Generally more than one elastic constant has to be determined. It is interesting to note that the procurement of sufficient data is sometimes more complicated for isotropic materials. For anisotropic solids the velocity can be measured as a function of the angle defining the propagation direction in the surface to yield a sufficiently large data set. For thin-film/substrate configurations measurements at various frequencies or for different film thickness may be carried out to obtain sufficient data. There are, however, obvious advantages to work with a single specimen and at a single frequency. This can be done by considering the contributions of more than one leaky SAW mode to the V(z) curve.

Wei Li, Jan D. Achenbach

### Elastic Constants Determination of the Thin Cold-Rolled Stainless Steel by Acoustic Resonance Method

Stainless steel has a good corrosion resistance and a good high temperature mechanical properties. In this study, temperature dependence of elastic constants of thin cold rolled stainless steel has been measured by using the acoustic resonance method[1]. Identification of the vibration mode has been examined numerically and experimentally. The elastic constants at room temperature have been also measured by Pulse Echo method. In addition, the texture effect on the elastic constants has been analysed by assuming the specimen as orthorhombic structure.

Seung Seok Lee, Un-Sig Min, Bong Young Ahn, Seung Hyun Yoo

### Materials Characterization by a Time-Resolved and Polarization-Sensitive Ultrasonic Technique

Since first developed by Lemons and Quate in 1973 [1], scanning acoustic microscopy has been able to obtain images comparable to those from a high quality optical microscope [2]. In the meantime, many investigators [3–7] have developed that technology to determine the microscopic properties of materials. Among those developments, the line-focus-beam (LFB) acoustic microscopy work of Kushibiki and Chubachi [6–7] in the early 1980’s has been most widely recognized [8–10]. Since the LFB technique is a directional measurement, it can be used to study material anisotropy and stress.

D. Xiang, N. N. Hsu, G. V. Blessing

### Ultrasonic Attenuation Measurements at 300 MHz

In the CANDU™ (CANada Deuterium Uranium) nuclear reactor, the primary heat transport containment is a matrix of cold-worked Zr-2.5Nb pressure tubes containing pressurized heavy water, which extracts heat from the fuel bundles in the tube. In the last few years, periodical tests on removed tubes have established a significant tube-to-tube variability in fracture toughness due to variability in the as-installed tubes [1].

M. Dubois, M. Viens, A. Moreau, C. K. Jen

### Thermal Effects in the Fracture of Rubber-Modified Polystyrene

We describe an experimental procedure aimed at examining the spatial and temporal distribution of energy dissipation during crack initiation and propagation in a ductile polymer of high impact polystyrene. The experiment combines a table-top tensile tester and an infrared imaging system. Because of the intense heating of the crack tip region, direct observation of the temperature rise not only provides an accurate measurement of the position of the crack tip, and thus the propagation velocity, but also much information for the analysis of plastic deformation, energy conversion, and evaluation of thermal properties of polymer materials.

Yingxia Wang, S. A. Telenkov, Zhouling Wu, L. D. Favro, P. K. Kuo, R. L. Thomas, H. Van Oene

### A Critical Analysis of the Theoretical Basis of Ultrasonic Scattering Measurements

There are three elements involved in the backscattering from inhomogeneous media; the scattering properties of a single particle or scattering element, the scattering associated with a group of such particles and the relationship of the scattered wave to the measured signal. Ideally it should be possible to obtain information about the material microstructure from ultrasonic backscattering measurements. However, a number of assumptions and approximations must be made before the problem becomes tractable, and it is the purpose of the present investigation to compare the various approaches available in the literature in an attempt to quantify the errors involved with some of these approximations.

R. C. Chivers, L. W. Anson, S. A. M. Foister, S. G. McKenzie

### Variance of the Ultrasonic Signal from a Defect Beneath a Rough Surface

Rough surfaces degrade phase-sensitive ultrasonic immersion inspections differently depending on the relative size of the transducer, the wavelength, the rms height and the correlation length of the roughness, as well as the size and shape of the defect. In a series of papers, the present authors, often in collaboration with P. B. Nagy, have examined the effects of surface roughness on ultrasonic inspections; see e.g. [1–3]. In these papers, we simplified the problem by computing the average signal; obtained, in principle, by measuring the same defect beneath many different realizations of the rough surface. Of course, an inspector is interested in one particular signal from a defect beneath one particular surface. An interesting question is “When will the average signal be useful for describing the observations of the inspector?” Consider the two following extreme examples, which are based on the fact that the average signal will be greatly reduced by roughness when δk h > 1, where h is the rms height of the roughness and δk is the change in wavenumber upon crossing the surface. First, suppose that the correlation length, L, is relatively small. In this case, roughness changes the phase of the wavefront independently in many small regions (of size L2) within the footprint of the transducer on the part’s surface. Since the signal is based on the addition of random changes in the phase from each of these regions, the amplitude of the inspector’s observed signal will be close to the average signal. For the second example, suppose that the correlation length is relatively large. In this case, there will be relatively few regions of size L2 within the footprint of the transducer. Consequently, there will be large variations in the observed signal due to the statistics of small numbers.

Mehmet Bilgen, James H. Rose

### A Generalized Model of the Effects of Microstructure on Ultrasonic Backscattering and Flaw Detection

The influence of microstructure on ultrasonic inspection is well known. Familiar examples include the attenuation of ultrasound due to scattering from grain boundaries and the anisotropies in velocity that are associated with preferred grain orientation. Less commonly discussed are the creation of backscattered noise, which can mask flaw signals, and the modification of transducer radiation patterns, e.g. the modulation of the phase fronts in a beam, which can cause fluctuations in signals reflected from surfaces [1]. The latter influence the measurement of attenuation as well as the strength of signals reflected from flaws. The goal of this work is to develop a unified basis for understanding these phenomena, as can be used in the analysis of the performance of ultrasonic flaw detection systems. Of interest are correlations of noise in time as well as the variance of noise signals (about their mean of zero) and reflected signals (about a non-zero mean).

R. Bruce Thompson

### An Experimental Investigation of Ultrasonic “Grain Noise” in Titanium -6AL-4V

Ultrasonic testing is used for the detection of flaws in critical parts, such as forgings in gas turbines[3]. Benign features within the microstructure, such as grain boundaries, also reflect part of the ultrasonic pulse. These signals will generally be of a lower amplitude than flaw signals, since they represent a softer discontinuity in material properties. Figure 1 shows a simplified distribution of such ‘grain noise’ signals, and a possible distribution of signals resulting from small flaws. Two difficulties arise due to the presence of grain noise: 1.The smallest flaw signals cannot be observed since they are masked by the grain noise. This defines the limiting resolution of the scan.2.The largest noise signals are mistaken for flaws, leading to ‘false calls’ and the rejection of sound material.

S. A. M. Foister, S. G. McKenzie, R. C. Chivers

### Survey of Ultrasonic Grain Noise Characteristics in Jet Engine Titanium

In ultrasonic inspections of titanium billets and forgings, grain noise echoes are routinely observed. These arise from the scattering of the incident sound beam by the metal microstructure, and can limit the detection of small or subtle defects. We report on a survey of grain noise characteristics in fourteen billet and forging specimens supplied by aircraft engine manufacturers. All specimens were examined in a similar manner using a 5-MHz focussed transducer, with pulse/echo noise measurements made through three orthogonal sides of each specimen. Emphasis is placed on describing two related probability density functions (PDF’s) which characterize aspects of the backscattered noise seen in a scanning experiment. The first PDF describes the RF noise voltages seen at a fixed observation time t; the second describes the gated peak-to-peak noise voltages seen for time gates of various durations. The PDF for the RF noise voltages is expected to be Gaussian if a large number (>10) of grains contribute appreciably to the noise at time t [1], but non-Gaussian behavior is seen in some specimens. The use of K-distributions to describe the non-Gaussian cases is examined. This work is in support of efforts described in a companion article [2] to develop methods for predicting gated peak noise (GPN) distributions.

I. Yalda, F. J. Margetan, K. Y. Han, R. B. Thompson

### Ultrasonic Noise and the Volume of the Ultrasonic Pulse

The ultrasonic inspection of large grained engineering materials such as titanium alloys is often limited in finding low reflectivity targets due to reflections from grain boundaries. These unwanted signals are often referred to as grain noise. Recently, experimental work was performed showing the effects of ultrasonic scan parameters on grain noise level and signal to noise ratio. [1,2] Techniques such as using broadband ultrasonic pulses, subsurface focused beams, higher frequencies and smaller beam diameters were shown to dramatically improve SNR for both air-backed and low reflectivity targets in Ti6–4. There is one common element in all these items which reduce the ultrasonic noise level when compared to that of the target signal: a reduced ultrasonic beam volume. This paper will outline an experimental technique for measuring the volume of an ultrasonic pulse and demonstrate with experimental data that the level of ultrasonic grain noise in titanium alloys is proportional to that volume.

P. J. Howard, R. S. Gilmore

### Determination of Grain-Size Distributions from Ultrasonic Attenuation

The overall research goal of this project is the nondestructive measurement of grain-size distribution parameters using ultrasonic attenuation. Ultrasonic attenuation α is dependent upon the sound wavelength (λ), the size of the grains (D), and in many cases elastic constants and sound velocities of the material. Assuming that multiple scattering can be ignored, the expression for the wavelength dependence of the attenuation is 1$$\alpha (\lambda ) = \int\limits_0^\infty {N(D)\,\alpha (\lambda, D)\,} \,dD$$ where N(D) is the grain-size distribution. In previous work [1] the sizes were assumed to be distributed following a power-law with exponent γ: 2$$N(D) = K{D^{ - \gamma }},0 < D < \infty$$ Different justifications for this assumption have been provided [1]. After substituting the power-law expression for the grain-size distribution, the attenuation was shown to be a power-law: 3$$\alpha (\lambda ) = K'\,{\lambda^{ - \gamma }}$$

Aran Anderson, Denise Nicoletti

### Predicting Gated-Peak Grain Noise Distributions for Ultrasonic Inspections of Metals

In ultrasonic pulse/echo inspections of metal components, defect detection can be limited by backscattered “grain noise” from the metal microstructure. The absolute level of grain noise observed in a given inspection depends on the metal microstructure and on details of the inspection system, such as the focal properties of the transducer, the spectral content of the incident sonic pulse, and the receiver amplification settings. In earlier work [1–3], we presented models which account for both measurement system and microstructural effects, and which predict certain aspects of the backscattered noise in weakly-scattering materials. For example, one of the predicted quantities is the “rms noise level”, illustrated in Fig. 1 and defined as the root-mean-squared average of the RF noise voltages seen at a fixed observation time when the transducer is scanned above the specimen. The absolute rms noise level as a function of time (or penetration depth) can be predicted from knowledge of the transducer diameter and focal length, a “reference echo” from a flat surface, and certain material properties of the specimen including its density, velocity, attenuation coefficient, and Figure-of-Merit (FOM).

F. J. Margetan, Isaac Yalda, R. B. Thompson

### The Statistical Distribution of Grain Noise in Ultrasonic Images

Ultrasonic imaging technologies are rapidly being transitioned to the production environment. An example of this is occurring in the aerospace industry, where digital data acquisition and imaging are being used to improve the ultrasonic inspection of large grained alloys. [1] The availability of digital data and ever increasing computing power opens the door for more sophisticated data analysis techniques than have been used in the past. Such potential techniques include the Wiener filter to improve resolution, dynamic thresholding to improve detection, signal-to-noise (SNR) based material acceptance criteria, and the estimation of the probability of detection (POD) of a given inspection. [2–5] An element critical to the success of all these techniques is an accurate estimate the distribution of the ultrasonic reflections from grain boundaries which are commonly referred to as grain noise. This paper presents a technique to estimate the parameters of closed-form statistical distributions from grain noise data and analyzes the quality of the fit of several distributions to the grain noise found in ultrasonic images of titanium alloys.

P. J. Howard, R. H. Burkel, R. S. Gilmore

### Ultrasonic Attenuation Measurements in Jet-Engine Titanium Alloys

In the inspection of titanium material intended for use in aircraft engines, a number of unusual phenomena are observed, including significant fluctuations of the amplitude and phase of back-surface echoes and of the amplitudes of pulse-echo signals from nominally identical flaws[1]. Practical implications include a broadening of the probability of detection curves and difficulties in determining the ultrasonic attenuation, a parameter used in interpreting flaw response data. Incorrect determination of attenuation can lead to errors in distance-gain corrections and hence in estimates of the magnitude of the flaw response. In this paper, we report experiments designed to elucidate the mechanisms responsible for these signal fluctuations.

P. D. Panetta, F. J. Margetan, I. Yalda, R. B. Thompson

### Analysis of Mechanical Damage Detection in Gas Pipeline Inspection

In the context of gas pipeline inspection, mechanical damage means dents or gouges in the pipe wall, usually caused by equipment operating near the pipeline. Such defects are the principal cause of pipeline failure, which makes their early detection and characterization a major concern of the industry.

R. E. Beissner, G. L. Burkhardt, E. A. Creek, A. E. Crouch, T. Grant

### Metal Loss Characterization in 55-Gallon Drum Steel by the Magnetic Flux Leakage Method

Estimates indicate the number of 55-gallon carbon steel drums, storing various waste forms, total in the millions and are distributed in localized waste storage sites all across the US. Other nations are expected to have similar situations as well. Figure 1 shows steel drums in a burial trench at Hanford. Each Government site working on radioactive weapons production store their drums differently but each site is beginning to experience problems with leaking drums.

R. Hockey, D. Riechers, D. Duncan

### Estimation of Grinding Burn Damage Using Barkhausen Technique

Mechanical properties of materials are significantly affected by changes in the surface microstructure [1]. Improper manufacturing process conditions for structural components can adversely affect the microstructure and residual stresses in the material and lead to early fatigue failure.

A. Parakka, D. C. Jiles, H. Gupta, S. Jalics

### Time Resolved Infrared Radiometry for Subsurface Interface Imaging

While most thermal NDE methods concentrate on defect detection and imaging, time-resolved infrared radiometry (TRIR) [1,2,3] has also been used successfully to determine material parameters such as thermal diffusivity and thickness. This has allowed information about material structure such as presence of corrosion, porosity or voids to be obtained. In the TRIR technique, the surface temperature of a sample is monitored via infrared emission during application of a heating pulse from an optical, microwave or induction source. Smaller heating intensities result with this technique as compared to the more common short pulse methods [4,5]. Furthermore, the early time behavior in the temperature-time response allows a self calibration to be performed for each pixel in the image, thus removing emissivity and heating beam intensity variations. The development of infrared focalplane arrays with their full field imaging capabilities at high speeds and current image processing equipment allows the TRIR algorithms which have so far have been used only on single point measurements to be applied to full images.

R. Osiander, J. W. M. Spicer, J. C. Murphy

### Photothermal and Photo Acoustic Investigation of Zirconia Coating Delaminations

Plasma sprayed ZrO2 coatings on ferritic steel with artificial adhesion defects were investigated using multiple photothermal and photoacoustic methods. The delaminations were produced by preventing the sand blasting of sections of the surface prior to applying the coating. Case study comparisons between an IR-camera/ flash lamp system, line scanning IR-imaging, pulse photoacoustic imaging, and thermal wave interferometry were performed. The methods offer a varying level of contrast for the defects depending on their relative sensitivity, resolution, and operating principle.

M. Oksanen, P. Fenici, A. Salerno, J. Varis, F. Cernuschi

### Subsurface Defect Detection in Ceramic Materials Using Optical Gating Techniques

Components made from advanced ceramics materials, because of their thermomechanical and chemical properties, have several advantages over traditional steel parts, making them well suited for use in severe operating environments. In particular, silicon nitride (Si3N4) ceramics, because of their stiffness and resistance to corrosion, are being considered for use in rolling contact elements such as bearings and contact races. In addition, when combined with rare-earth oxide sintering aids such as yttria (Y2O3), silicon nitride ceramics have high-temperature strength which makes them excellent candidates for components such as rotors and blades in advanced turbine engines.

M. Bashkansky, P. R. Battle, M. Duncan, R. Mahon, J. Reintjes

### Eddy Current Characterization of Flaws in NiCoCrAlY-Coated Nickel-Base Superalloys

Increases in performance of power generating combustion turbine engines have driven up operating temperatures and placed greater demands on turbine components. To meet these increased demands, some components such as fan blades, are fabricated from Inconel and Udimet superalloys and are coated with 0.10 to 0.15 mm of NiCoCrAlY. This coating affords the blades protection against oxidation and particle erosion, and provides a degree of thermal insulation to lower blade operating temperatures.

Brian Larson, John Moulder, Paul Zombo

### Eddy Current Measurement of Thicknesses in Multiple-Layered Film

This paper describes a system for measuring the thicknesses of a multi-layered film. Standard techniques measuring the thicknesses of plastic coatings on a metal substrate do exist [1]. However, such techniques are incapable of offering the micron level resolution required for the application at hand. This project concerns measuring the individual layer thicknesses in a three layer film. The objective of the research is to estimate the thickness of each layer. This paper describes a system designed for measuring the thicknesses of the film layers.

D. Cooley, Y. S. Sun, S. S. Udpa

### Ultrasonic Evaluation of Case Depth in Case-Carburized Steel Components

Performance of many engineering components depends to a large extent on their near surface characteristics, which in turn are affected by wear, corrosion, and fatigue in the presence of loading forces. One way to improve the engineering component’s performance is to tailor the surface properties using laser and electron beam processing, coatings, ion-implantation, and carburizing.

Surendra Singh, Rob Mitra, Dave Leeper, Rosendo Fuquen

### Surface Elastic Wave Measurements for Determination of Steel Hardness Gradients

Many researchers have used Rayleigh wave dispersion measurements to examine surface and subsurface material properties, in the ultrasonic frequency range below 50 MHz, using contact transducers in a pitch-catch mode configuration, e.g. [1–4]. Unfortunately there are many potential sources of error which can lead to poor measurement reproducibility and low accuracy. This poor reproducibility, and the often stringent requirement for measurement accuracy, have been obstacles to the more wide spread use of Rayleigh wave dispersion and time of flight measurements in industrial settings [3].

G. A. Gordon, B. R. Tittmann

### Surface Characterization of Material Using Rayleigh Velocity Measurement in the Broad-Band Mode

The surface damage of materials can be characterized by measuring the variation of the Rayleigh wave velocity. There is also a need to study the effect of the anisotropy and the gradient of the damage versus the depth. The acoustic microscopy using broad-band pulses is one of the methods which can produce interesting results in a simple and fast way. In this case, the Rayleigh velocity can be also obtained through a time of flight measurement. In order to have a sufficient separation between the different components of the reflected echoes, it is necessary to properly select the geometrical and acoustical parameters, and that requires a sufficient comprehension of the phenomena. One of the challenge of this technique is to be able to perform time measurements with the necessary accuracy. We will show also that it is possible to extract conventional V(z) curves for a given frequency.

C. Gondard, F. Tardy, MH. Noroy, L. Paradis, JC. Baboux

### Ultrasonic NDE of Sprayed Ceramic Coatings

Thermal spraying of protective coatings has been in use since 1917 when the initial application was the spraying of zinc layers onto steel structures to prevent corrosion [1]. In the 1970’s plasma-spray technology was first used with the introduction of vacuum plasma-spraying [1]. Today, gases such as argon and nitrogen (sometimes with an additional gas such as helium or hydrogen) are often used in plasma-spray guns. In plasma-spraying an electric-arc discharge heats the gas stream to high temperature (≥ 10,000 K), turning it into a plasma. The gas exits the spray-gun at high speed (≈ 200 m/s to 600 m/s) towards the material to be coated. Material powder (often carried by a second gas stream) is injected into the plasma stream, where it melts into liquid droplets. These droplets are carried onto the target surface, where they rapidly cool (≈ 106 K/s) into solid, flat splats. A layer of material can be built up by repeated spraying of the same surface area. This high-temperature process allows for the spraying of virtually any material possessing a stable molten phase, including ceramics [2]. Further technical details and the history of plasma-spraying can be found in [1–2].

J. A. Slotwinski, G. V. Blessing

### Ultrasonic Predictions of Plastic Strain Ratios in Steel Sheet

Control of texture (preferred crystallographic orientation) is an important step in ensuring that steel sheet has the desired characteristics in deep drawing operations. Traditionally, texture is measured by x-ray or neutron diffraction techniques. However, despite obtaining the most complete information, the latter require very specialized capital facilities and the former involve radiation hazards and sense only a very thin surface layer. Hence, there has been considerable interest in developing alternative techniques, suitable for field use, which provide volumetrically averaged information.

K. Forouraghi, R. B. Thompson, A. J. Anderson, N. Izworski, M. Shi, F. Reis, J. Root

### Using Rayleigh Wave Dispersion to Characterize Residual Stresses

Acoustoelasticity has been the subject of a great deal of research over the past thirty years. However, most of this work involved bulk waves. For surface waves’ activity has been limited. Rayleigh wave acoustoelasicity was first studied by Hayes and Riviin [1]. Subsequently, Iwashimizu and Kobori [2] analyzed Rayleigh wave propagation in a finitely deformed isotropic elastic material. Martin [3] investigated the relative effects of stress and preferred grain orientation. Adler [4] measured the residual stress of circumferential welds in pipe. Recently, Lee et al [5] utilized line-focus acoustic microscopy to determine local near surface stress in an isotropic material. However as a uniform strain distribution is assumed, the analysis is insensitive to variations in the stress through the thickness of the material. In 1981, M. Hirao et al [6] theoretically and experimentally studied the dispersion of Rayleigh waves for a plate in pure bending. They found that the dispersion of Rayleigh waves was prominent for relatively low frequencies and diminished as the frequency increases. In this paper, the research goal is to develop a more general technique to characterize the through thickness stress distribution. Assuming the strain to be distributed quadratically with depth, the formulas for the velocity change versus the initial static stress are derived based on the first order perturbation approach. This information can be used to reconstruct synthetic residual stress distributions from frequency dependent Rayleigh wave velocity data.

R. Kline, L. Jiang

### Acoustoelastic Measurement of Second- and Third-Order Elastic Constants in Silicon Carbide and Alumina Particulate-Reinforced Aluminum Metal Matrix Composites

Metal Matrix Composites (MMCs) can offer increased specific strength and stiffness, increased toughness, and an ability to operate at high temperatures. However, the mechanical properties of MMCs can be greatly influenced by residual stresses that result during fabrication from the differences in coefficients of thermal expansion of the constituents [1]. There are a number of non-destructive techniques for measuring residual stresses in these types of composites. The two most widely used techniques are x-ray and neutron diffraction. The x-ray diffraction technique is limited in that it has small penetration depths and beam sizes which are generally much greater than the reinforcement diameter. Although the neutron diffraction technique can provide through-thickness measurements, the availability of the specialized test equipment is a limitation, due to the necessity for using a neutron source [2]. An alternative technique for measuring residual stresses is using ultrasound.

A. Saigal, S. Krikorian, G. G. Leisk

### Effect of Shear Modes Interference in Anisotropic Materials

Ultrasonic signals transmitted through a plate by shear waves in weakly anisotropic materials are usually not separable in the time domain and thus interfere at the receiver. The interference signal bears information on material anisotropy due to texture or residual stresses. This dependence of the interference signals on material properties can be utilized practically. Blinka and Sachse [1] used interference of two shear waves to measure stress applied normally to the direction of wave propagation in an aluminum specimen. The phenomenon is also described in [2]. The change of acoustic microscopy contrast is attributed by Dreschler-Krasicka to stress induced interference [3]. Various interference phenomena in crystalline solids are described by Wolfe in [4].

A. D. Degtyar, A. I. Lavrentyev, S. I. Rokhlin

### Absolute Ultrasonic Measurements of Residual Stresses

The characteristic dependence of ultrasonic velocity on stress has for a long time been thought promising for residual stress measurements in materials. A review of theory and experimental methods can be found in [1, 2, 3]. The major problem in practical utilization of most of these techniques is the necessity of separating the effects of texture (anisotropy) and stress on the measured ultrasonic velocity. In most cases the materials under consideration have unknown anisotropy and even if the anisotropy is small its effect cannot be neglected in stress measurements.

A. I. Lavrentyev, A. D. Degtyar, S. I. Rokhlin

### Absolute Determination of Elastic Constants and Stresses from Ultrasonic Group Velocity Data

Last year we presented a paper on absolute stress determination from angular dependences of phase velocities in orthotropic materials [1]. No information about elastic constants or stresses is assumed to be known a priori. In this paper we describe the extension of this technique for determination of stresses and elastic properties from group velocity data.

A. D. Degtyar, S. I. Rokhlin

### Wave Propagation in Stressed Composites

High temperature composites have potential for significant performance improvement and weight reduction in aircraft engines and other structures. For successful application of these materials in different environments their mechanical properties, life capability and reliability must be well known. Thus NDE of composite structures becomes very important.

A. D. Degtyar, W. Huang, S. I. Rokhlin

### Determination of Angular Parallaxes between the Geometric Coordinate System and the Material Symmetry Coordinate System of Anisotropic Materials

The hypothesis of an orthorhombic symmetry and the knowledge of the material symmetry axes is usually necessary to characterize a medium by ultrasonic techniques [1–5]. However a wrong setting up of the sample or the strata’s stacking defects in industrial composite material lead to the non-superposition between the material symmetry coordinate system and the observation coordinate system. The development of a procedure to identify elasticity coefficients for materials that have no more symmetry planes, is necessary.

Christophe Aristégui, Stéphane Baste

### Effect of Texture on Ultrasonic Backscattering Coefficient in Pure Titanium Plate

Ultrasonic grain noise is one of the important factors which limit the detectability of small defects. The intensity of the grain noise may be represented by the ultrasonic backscattering coefficient which directly relates microstructure to the grain noise intensity. Various microstructures affect the backscattering coefficient, as reported in previous volumes of this series of proceedings1–3. In the present article, the effect of texture will be quantitatively discussed.

Y. K. Han, R. B. Thompson, I. Yalda, F. J. Margetan, A. J. Anderson, M. Hirao, J. Root

### Effect of Wave Types on Ultrasonic Texture Identification

The use of ultrasonic techniques to measure the material anisotropy of rolled metal sheets has been demonstrated in many studies. Electromagnetic acoustic transducer (EMAT) and laser ultrasonic (LU) methods have been favored since both are noncontact in nature and therefore potential candidates for in-situ process monitoring and control in rolling mills. Both techniques have the ability of generating various ultrasonic wave types. EMAT systems are usually based on the excitation of Lamb or SH (horizontally polarized shear) waves and are typically designed to generate narrow band waves with known wave types. LU, on the other hand, is characterized by the simultaneous generation of multiple wave types and modes, the specifics of which are dependent upon the properties of the generation laser pulse(s) (duration, temporal and spatial shape, power) and the boundary conditions of the specimen. In general wave propagation velocities are measured and then used to calculate the texture coefficients W400, W420 and W440 [1, 2].

W. Lu, S. Min, D. Hughes

### Texture Assessment in SCS-6 Fibers from Ultrasonic Dispersion Measurements

Advanced fibers used to reinforce composite materials exhibit complicated morphology. Typically, the fiber consists of a cylindrical core embedded in a cladding region followed by a distinct interface zone separating the fiber system from the matrix region. In addition, the cladding region itself often consists of subregions which can be identified as more or less distinct layers. According to the simplest micromechanical models these coaxial layers are assumed to be isotropic and homogeneous. At low frequencies when the acoustic wavelength is much larger than the radius of the fiber, such a composite fiber exhibits significant anisotropy of transversely symmetric nature manifested by higher axial stiffness relative to the radial one. This macroscopic anisotropy is caused by the coaxial structure and the possibly imperfect interfaces between the layers. The main goal of this study was to determine whether this structural anisotropy produced by the presence of microscopically isotropic and homogeneous constituents is sufficient to account for all of the macroscopic anisotropy observed in real fibers or, in addition, microscopic anisotropy caused by some texturing in the constituents themselves is needed to properly model the fiber at ultrasonic frequencies. Apparent texturing in the constituents can be caused by either real microscopic anisotropy due to preferred crystallographic orientation of grain growth during manufacturing or by additional structural anisotropy due to strong radial inhomogeneity in the material composition, e.g., increasing carbon content in the silicon carbide caladding.

Peter B. Nagy, Renee M. Kent

### Nondestructive Ultrasonic Characterization of the Orientation Distribution of Short-Fiber Composites

The ease of fabrication and relatively low cost of composites containing short glass or ceramic fibers embedded in a polymer or metal matrix has made them attractive candidates for a wide range of applications. The preferential alignment, or misalignment depending on one’s view, of the short fibers results in an overall texture of the composite and strongly influences its mechanical and physical response, for example, stiffness, conductivity, strength, and so on. The orientation of the short fibers depends strongly on the processing conditions. From a process-control viewpoint, it is imperative to determine if a composite component has adequate strength, stiffness, and so on, and it is preferable to obtain this information nondestructively. In short-fiber reinforced composites, the orientation distribution of the short fibers is the most significant variable that determines these overall properties. Thus its determination is essential.

Martin L. Dunn, Hassel Ledbetter

### Characterization of Corrosion in Aluminum Alloys Using Nuclear Magnetic Resonance

With an ever increasing emphasis on extending the life of both military and commercial aircraft, it is critical to have nondestructive evaluation (NDE) methods capable of detecting corrosion in its earliest stages of formation. The consequences of corrosion left undetected are material thinning and a subsequent reduction in strength. Conventional NDE methods such as ultrasonics, eddy current and radiography are capable of detecting the resultant exfoliation caused by corrosion; however, some material loss must occur before reliable detection can be made using the referenced methods.

D. D. Palmer, D. M. Snyderman, M. S. Conradi

### Quantitative Measurement of Metal Loss Due to Corrosion in Aluminum Aircraft Skin

The detection and characterization of corrosion is one of the many challenges in the nondestructive inspection (NDI) of aging aircraft. A number of groups are pursuing ultrasonic techniques for the detection and evaluation of corrosion in such aluminum fuselage structures such as lap splices and tear straps [1]. Under the FAA-Aging Aircraft Research Program, Patton and Hsu [2–4] at Iowa State University have developed the capability to apply high resolution, water-coupled, focused-beam ultrasonic NDI to aircraft fuselage structures in a maintenance hangar environment. The method, known as the “Dripless Bubbler” technique, is a combination of focused-beam immersion ultrasonics with a portable ultrasonic scanner. With the Dripless Bubbler, B- and C-scan images of the aircraft fuselage may be acquired using both high frequency and low frequency immersion ultrasonics typically only reserved for the laboratory. In this paper, we present results on corrosion detection in the outer aluminum skin of a fuselage lap splice section using high frequency (15 MHz nominal center frequency) immersion ultrasonics, and compare these results against those obtained from collimated-beam X-ray attenuation measurements. This comparison has been quite instructive and revealed several important considerations in the interpretation of both ultrasonic and X-ray data when applied to the quantitative measurement of metal skin thickness.

D. K. Hsu, T. C. Patton, V. Dayal, B. L. Hinzie, J. N. Gray

### Improved Measurements of Samples Simulating Corrosion in Lap-Seams of Aluminum Aircraft

AC magnetic bridges offer a technique for continuous electromagnetic scanning of surfaces and lap seams of aging aluminum aircraft to detect material losses due to corrosion. AC magnetic bridges are particularly responsive to changes in conductance because nearly all the electromagnetic field generated is in contact with the sample as compared with conventional eddy-current devices where the high reluctance of that portion of the magnetic field not in contact with the sample has a tendency to mask the small portion of the field in contact with the sample. Further, the eddy-current technique masks the effect of the electromagnetic interaction with the sample through the requirement that the changes produced by changes in the sample be detected in series with the impedance of the eddy-current coil. This latter effect requires the use of relative high frequencies in conventional eddy-current devices. Use of the ac magnetic bridge (the bridge) solves all of these problems while providing the physical convenience of conventional eddy-current scans if such a scans were effective. The frequencies used for bridge operation are usually of the order of 100 Hz to 20 kHz and offer greater sample penetration than conventional eddy-current devices which usually operate well above this range. Further, ac magnetic bridges maintain their sensitivity to material loss at values of lift off of 20 mils or greater, well above the usual values of paint coating thicknesses.

William F. Schmidt, Otto H. Zinke

### Nondestructive Evaluation of Corrosion through Insulation

Pipes and pressure vessels are often subjected to operating environments where corrosion can degrade metal components and welds that join them. A program of regular system inspections is required to ensure the safe use of these systems and to reduce the possibility of costly shut-downs. This is especially true with recent changes in laws and regulations and social responsibility caused by environmental concerns which emphasize “zero” leaks in a wide variety of processing equipment. A comprehensive preventative maintenance program is needed to maintain high reliability in processing systems. An effective inspection strategy is essential in that program.

Gary W. Carriveau, Russell Austin

### Thermal Wave Imaging of Disbonding and Corrosion on Aircraft

Thermal wave imaging is emerging as a strong competitor to conventional nondestructive aircraft inspection techniques. Its strengths are in its ability to do rapid, wide-area, contactless imaging to detect corrosion and disbonding. It readily lends itself to the inspection of both metallic and composite aircraft structures. Recently [1], we have described the evolution of thermal wave hardware, and the role of the FAA’s NDI Validation Center in that evolution. In this report, we provide illustrative thermal wave images which show corrosion and disbonding on the B737 testbed aircraft at the NDI Validation Center. By showing sequences of images at successive times after the pulse-heating of the aircraft surface, we show that the greatest detail of subsurface corrosion occurs at very early times, thus mandating the use of rapid imaging techniques. More detailed laboratory studies confirming this conclusion are provided in a separate study by some of the authors [2].

L. D. Favro, Tasdiq Ahmed, Xiaoyan Han, Li Wang, Xun Wang, P. K. Kuo, R. L. Thomas

### Eddy-Current Detection of Pitting Corrosion in Aircraft Lap-Splices

Corrosion is one of the most important limits on the life of transport aircraft. The outer skin of transports consists of thin sheets of aluminum that in many aircraft are fastened at lap-splices. The infiltration of moisture initiates corrosion between the sheets and thus internally erodes the outer covering of the plane. A substantial effort has been undertaken to find ways to detect and quantitatively measure such hidden corrosion. It has been shown that the swept-frequency or pulsed eddy-current impedance can be used to quantitatively measure uniform loss of material (over areas > 1 cm2) in both the first and second layers of the lap-splice [1, 2, 3]. Consequently, eddy-currents form the basis for a quantitative inspection system. However, corrosion does not always result in a uniform loss of material. Sometimes the loss is highly localized and is aptly described as “pitting corrosion”. Consequently, there is a need for understanding the effects of such pits on the eddy-current response.

Radhika Satveli, John C. Moulder, James H. Rose

### Depth Determination of Surface-Breaking Cracks in Concrete Slabs Using a Self-Compensating Ultrasonic Technique

For concrete bridge decks, the use of salt in cold weather areas is known to result in damage, such as freeze or thaw damage (microcracks) and surface breaking cracks. In this paper attention is directed to an ultrasonic method to scan for surface-breaking cracks and to determine the crack depth in concrete bridge decks.

A. Cheng, J. D. Achenbach

### Application of Spread-Spectrum Ultrasonic Evaluation to Concrete Structures

Spread-Spectrum Ultrasonic Evaluation (SSUE) is an emerging technology for the global testing of structures and materials [1]. It incorporates the correlation properties of pseudo-random signals into ultrasonic NDE using spread-spectrum technology. The SSUE instrument records the ultrasonic correlation signature that is representation of the aggregate acoustic state of the test sample. The measured ultrasonic correlation signature is used for the detection of changes in the acoustic state of the test object. One major advantage of the SSUE technique is that it provides a global inspection of the material or structure without scanning. Some examples of unique SSUE applications are when; 1) the test object is of complex geometry or too large for scanning or, 2) the test material is a highly attenuative material such as concrete, wood, or composites. This paper reports the progress in applying the SSUE technique to concrete. The aim of the reported research is to study the propagation of the spread-spectrum signal in concrete structures of various sizes and determine the sensitivity of the SSUE correlation signature to changes in the various acoustic properties of the concrete object. A comparison of the SSUE technique with conventional ultrasonic NDE techniques for concrete testing is done and advantages of this technique for standard concrete inspection methods are discussed.

Muhammad A. K. Afzal, Steve F. Russell, Jahangir K. Kayani, Samuel J. Wormley

### A Pulse Compression Ultrasonic Test Instrument and Its Applications

In recent years, so called “digital ultrasonic test instruments” have been commercially available. These conventional instruments employ electrical impulse excitation of ultrasonic probes. In this configuration, waveforms of ultrasonic pulses into a test object are determined only by the characteristics of probes. This means that variation of probe characteristics causes variation of evaluated results and reproducibility of evaluation is poor. Furthermore, some of the conventional instruments do not provide enough information required for quantitative nondestructive evaluation (QNDE) about a test object, since ultrasonic echo signals are A/D converted after an envelope detector in a receiver and only information of echo height and position is obtained.

S. Wadaka, T. Nagatsuka, K. Misu, T. Kimura, M. Koike, H. Ichikawa

### Defect Detection in Concrete Bridge Decks Using Radar

This paper summarizes the findings of a study on the use of Ground Penetrating Radar (GPR) for nondestructive evaluation of concrete bridge decks. GPR has been found to be a proven technique for condition assessment of bridge decks [1]. However, most GPR studies have involved testing in the field, where the conditions are complex [2]. The objective of the present study is to assess the effect of isolated anomalies on the radar waveforms and determine the capabilities and limitations of the GPR technique under controlled laboratory conditions. Also, a radar data analysis model previously developed by Halabe et al. [3] was further improved, and its accuracy of prediction was compared to the actual conditions existing in the laboratory specimens.

Udaya B. Halabe, H. L. Chen, Vasudev Bhandarkar, Zahid Sami

### A Novel Inductive System for Two-Dimensional Imaging of Reinforcing Components in Concrete Structures: From Hardware to Image Enhancement

There is now growing concern regarding the safety of many prestressed concrete structures [1]. Of primary importance is the condition of the reinforcing steel, and much research effort has been expended on the development of non-destructive systems for assessing the integrity of internal reinforcing components [2–5]. Concrete is notoriously difficult to inspect, and most civil engineers, when assessing the quality of the internal reinforcing steel, will resort to a battery of tests, since a single test in isolation will rarely provide sufficient information. The system described below is a novel re-bar imaging system, based on an inductive principle, which should eliminate the need for many ancillary tests once fully developed [6]. A sensor which responds to the area and depth of metal in its sensing region is scanned across the concrete under inspection by a system linked to a computer. The signals acquired are stored, processed and displayed as grey level images. The initial images are blurred, due to the point spread function (PSF) of the sensor. These images have been significantly enhanced using a form of processing termed digital deconvolution. This form of processing allows considerable improvements in image quality to be realized from a single scan if the PSF of the sensor is known. In this instance, the PSF has been obtained through the development of an empirical model.

P. A. Gaydecki, K. J. Glossop, F. M. Burdekin

### Ultrasonic NDT Prototype for the Inspection of Ducted Post Stressing Tendons in Concrete Beams

There is worldwide concern about concrete bridges which derive their strength from post stressed steel tendons in grouted ducts. Problems arise when grouting has not been carried out properly, since the combination of voided regions and moisture penetration can cause corrosion of the tendon and catastrophic failure of the structure [1]. Post tensioned bridges built during the middle of this century are particularly susceptible and there is a pressing need for a cost effective non destructive means of assessing the condition of the embedded ducts and tendons. In response to this problem, a track mounted ultrasonic scanning system ‘CANDI’ (Cable Analysis by Non Destructive Inspection) is currently under development in UMIST, and this work describes the CANDI prototype and results obtained.

P. J. Duncan, P. A. Gaydecki, F. M. Burdekin

### Improved Assessment of Concrete Dams Using Sonic Tomography

Reclamation evaluates at least ten existing large concrete dams each year. The primary focus of these evaluations is to determine the dam’s ability to withstand a major earthquake or flooding conditions. The dam’s survival depends on the mechanical strength developed throughout the complex, discontinuous mass of the structure.

William F. Kepler, Leonard J. Bond

### Assessment of Industrial Floor Slabs Using Nondestructive Testing Methods

An animal feed manufacturing plant features a 78-foot-long by 53-foot-wide by 136-foot-high slip-formed concrete mill structure with cast-in-place concrete beams and slabs; see Figure 1. The facility was constructed in 1976. Receiving, storage, warehouse, and sacking systems are located on the ground floor. The second floor houses an electric room, a 4-ton mixer, storage, pellet coolers, and a blender with sacking conveyor. Hammer mills, pellet mills, and a roller mill are located on the third floor. Storage bins occupy the top floors of the five-story building. A stairway is situated along one side of the building.

Hemant S. Limaye, H. Daniel Rogers, David T. Biggs, Mark C. Kanonik

### A Rolling Weight Deflectometer for Quantitative Pavement Measurements

This paper describes a new machine that has been developed for directly measuring the deflection of airfield pavements under a rolling load wheel. This rolling weight deflectometer (RWD) optically measures the maximum amplitude of the depression basin using a new type of laser sensor that operates on the principle of triangulation. Four sensors that are mounted on a beam make pavement measurements. The key to producing accurate results is the requirement that the sensors remain in a straight line. Previous attempts using the method were plagued with deflection errors caused by thermal and vibrational beam bending. The patented RWD uses a laser to monitor bending of the physical beam in realtime. The information is used with pavement sensor data to produce accurate deflection measurements in the presence of significant beam bending. The bending-compensated RWD produces a deflection measurement with an accuracy of 40 μm (0.0015 inch). The RWD consists of a horizontally transported beam, strategically placed pavement sensors, a subsystem that monitors beam bending, an odometer, and a data acquisition computer.

R. F. Johnson, P. D. Bondurant, M. H. Marvin

### A Coring Method for the Nondestructive Evaluation of Effective Prestress Force in Prestressed Concrete Structures

Prestress forces are often applied to concrete structures to create compressive stresses in regions that will be subjected to superposed tension stresses due to applied service loads. Often these prestress forces are applied using steel tendons that are imbedded in the structure and stressed in tension. The tendons react against the structure and thus subject the concrete to compression. The structure may be fully-pre-stressed, so that the precompressed regions remain in compression under applied service loads, or partially-prestressed so that the precompressed regions may experience tension stress under applied service loads. Additional details of the mechanics of pre-stressed concrete members can be found in many textbooks (e.g. Reference [1]).

Stephen Pessiki, Jinlu Wang

### Ultrasonic and Electromagnetic Wave Propagation and Inverse Scattering Applied to Concrete

Nondestructive testing in civil engineering (NDT-CE) comprises the application of ultrasonic and electromagnetic wave propagation and inverse scattering. The aims of our current research are threefold: 1.Numerical modeling of ultrasonic wave propagation and scattering in concrete with the EFIT code (EFIT: Elastodynamic Finite Integration Technique) to get a better understanding of the ultrasonic wave phenomena in concrete,2.application of the elastodynamic vector imaging scheme EL-FT-SAFT to modeled data (EL-FT-SAFT: Elastodynamic Fourier Transform Synthetic Aperture Focusing Technique) to detect delaminations in a metal duct or for thickness determination,3.application of the electromagnetic vector imaging algorithm HD-POFFIS to measurements in order to locate a metal duct in reinforced concrete (HD-POFFIS: Hertzian Dipole Physical Optics Far-Field Inverse Scattering).

R. Marklein, K. J. Langenberg, R. Bärmann, M. Brandfaß

### Long Range Bridge Girder Evaluation Using Lamb Waves

The detection of fatigue cracks in steel bridge girders has become a major problem for bridge engineers in the USA. Currently, most bridge inspection is carried out using visual methods. Visual methods are time consuming and often miss fatigue cracks hidden under paint or by other bridge members. Snooper trucks or platforms are often required so that the inspector can gain access to all parts of the bridge. This requirement adds to the expense and complexity of the inspection process and often requires that one or more traffic lanes be closed to accommodate the required equipment. In view of the above mentioned problems and that bridge inspection budgets are limited, there exists a critical need for global and semiglobal Nondestructive Evaluation (NDE) methods which can evaluate critical parts of the bridge structure rapidly and inexpensively.

Clinton Woodward, K. R. White, Antonios Parashis, Vincent Carrica

### New Work in Acoustic Leak Location in Underground Pipelines

During the last ten years, the nation has become increasingly aware of the potential for environmental damage from leaks in underground and aboveground storage tanks, and related pipelines. Federal and state regulations have been developed that mandate inspections, require regular testing and set out design standards of construction for underground storage systems. More specifically, existing federal regulations (40 CFR Parts 280 and 281, September 1988) require that underground tanks and pipelines containing petroleum products and other hazardous substances be tested for leaks on a regular basis and that once a leak has been detected, it be corrected. While this requirement appears to be relatively simple, it, in fact, presents substantial difficulty in implementation. The difficulties are of two types.

M. A. Clark, R. Roberts, L. E. Rewerts

### The Inspection of Chemical Plant Pipework Using Lamb Waves: Defect Sensitivity and Field Experience

Corrosion and pitting defects in pipework are major problems in the oil, chemical and other industries. These defects can occur at the outer or inner surface of the pipe and can lead to a serious loss of pipe wall thickness. As a high proportion of industrial pipelines are insulated, this means that even external corrosion cannot readily be detected without the removal of the insulation, which in most cases is prohibitively expensive.

David Alleyne, Michael Lowe, Peter Cawley

### Ultrasonic Inspection of an Underwater Piping System Covered with Thick Coating

The largest oil production field in Venezuela is located in the Maracaibo lake, where 900,000 barrels are extracted daily. As part of the production system there exist 20,000 kilometers of underwater pipes, which transport crude oil, natural gas, and water among thousands of production platforms. One important problem faced in the maintenance of this complex system of pipes is the wall thickness loss caused by corrosion and/or erosion from the inside of the pipes [1]. Even though the occurrence of these two phenomena have been somehow reduced in the last five years, the problem of monitoring the wall thickness still remains. In this regard, the pipes at the Maracaibo lake present a unique condition that is not share by any other piping system in the international oil industry: the pipes are coated with thick layers of polyethylene or vulcanized neoprene, depending on their location or their depth below the lake level, in order to avoid external corrosion caused by the highly aggressive waters of the lake. Although, ultrasonic underwater inspection systems have been applied in the offshore oil industry for many years to inspect platform structures and piping systems, an ultrasonic inspection procedure capable to inspect underwater piping system covered with thick coatings of polyethylene or neoprene has not yet been developed. In this work we present an ultrasonic method capable of measuring the thickness of the pipe through the protective coatings.

Valery F. Godínez Azcuaga, Jorge Salcedo, Laszlo Adler

### On-Site Ultrasonic Inspection of Highway Culverts

Thin galvanized steel culverts, 2.6 mm (0.1 in) thick, are used when constant or intense water flow from one side of the highway or road to the other has to be facilitated. The New York State Department of Transportation has long considered NDT detection of corrosion in culverts necessary for the realistic evaluation of their remaining safe life. This is a problem with immense economic consequences, because the potential of safe life extension of the vast number of culverts in New York State and their replacement cost. In this paper the development of a non-contacting NDT method for on-site inspection of culverts is presented. The culvert is accessible only on the inside and the measurement is complicated by the fact that this is in contact with water and air, and is covered with rust and dirt. It corrodes down to less than 0.5 mm (0.020 in) giving the surface an orange rind appearance. Contact or delay transducers could not be used due to the extreme conditions of the surface, which in many cases does not guarantee the parallelism necessary for the measurement. Of another concern was the indication in some critical cases of false readings which commonly result in the overestimation of the real thickness. The system developed utilizes commercial off-the-shelf ultrasonic equipment and piezoelectric transducers in a bubbler configuration. In order to optimize the transmission of the sound through the water stream and the culvert, a rubber shoe was utilized. A detailed description of the problem and of the solution is given.

S. C. Schroeder, J. Frankel, A. Abbate

### A New Approach to Automated Labeling of Internal Features of Hardwood Logs Using CT Images

In a typical sawmill, logs enter the mill and go through a de-barking process. Following this operation they go to the headrig where a sawyer moves the log repeatedly past a saw to remove boards one at a time. As more of the log interior is exposed with each board removed, the sawyer may re-orient the log periodically to cut from the best side. Sawn boards go through subsequent operations of edging and trimming, where defects near the edges and/or ends of the boards are removed to increase each board’s grade, and therefore its value. The cant (the cubical center section of the log) remaining from initial breakdown enters a resawing operation where additional boards are cut. These are also edged and trimmed.

Daniel L. Schmoldt, Pei Li, A. Lynn Abbott

### Nondestructive Evaluation of Green Dimension Lumber Using Stress Wave and Transverse Vibration Techniques

Predicting the material properties of wood through nondestructive evaluation (NDE) techniques is a matter of considerable importance for the timber industry. NDE techniques can significantly increase the accuracy of grading of wood compared to visual grading methods. Longitudinal stress wave and transverse vibration nondestructive evaluation techniques have proven to be accurate means of evaluating the quality of wood-based products. Researchers have found strong relationships between stress wave and transverse vibration parameters (e.g., wave velocity and predicted modulus of elasticity) with the actual static bending properties (e.g., modulus of elasticity and modulus of rupture) of dry as well as green wood-based materials [1–3]. Therefore, these NDE techniques can be used to presort or grade structural wood prior to drying. Discarding wood with inferior properties prior to drying can result in significant cost savings in the drying process.

Udaya B. Halabe, Gangadhar M. Bidigalu, Hota V. S. GangaRao, Robert J. Ross

### Non Destructive Evaluation of Cracks in Porous Building Materials by Use of Crawling Spot Thermal Technique

The infrared (IR) thermography is one of possible tools for the thermal/infrared nondestructive testing (T/INDT). Commonly, it involves quasi-uniform heating of large specimen areas with the choice of heat pulse duration being influenced by specimen diffusivity α and expected defect depth l. The simple estimate of heat transit time τ* is given by the expression which became classical in the T/I NDT theory: 1$${\tau^* } = {l^2}/\alpha$$

P. G. Bison, E. Grinzato, A. Braggiotti, A. Mazzoldi, V. Vavilov

### High Performance Ultrasonic Inspection of Tubes

Eddy current examination was selected as the industrial method to be used for the inspection of PWR steam generator tubes because of both physical and operational advantages.

O. de Vareilles, J. P. Giraud, F. Lasserre

### Ultrasonic Scan Control and Analysis Using Catia Datasets

There has been a proliferation in the use of polymer composite parts in aircraft structural applications. Designers also are using more computer-aided design (CAD) tools to define and manufacture such parts. Computer-Graphics Aided Three-Dimensional Interactive Application (CATIA) software was developed by Dassault Systemes and is used at The Boeing Company to design new composite parts. A part’s construction is often complicated by material, thickness, or profile variations that are required by engineering specifications. When complex composite parts are fabricated, it is often necessary to perform nondestructive evaluation to determine a part’s interior quality. Automated ultrasonic inspection is a common method used on composite laminate and honeycomb parts. The typical system output consists of one or more C-Scan images on a computer display.

K. M. Uhl, W. E. Woodmansee

### Adaptation of a Marine Echo-Sounder to Support UST Remediation Activities at Hanford Nuclear Reservation

A tremendous challenge exists in the remediation of some 200 underground storage tanks (USTs) in the DOE complex, which are currently used to store millions of gallons of high level nuclear waste. Hanford Nuclear Reservation, in eastern Washington state, houses 177 of these tanks and is the site of initial remediation development work [1]. The wastes consist of mixed solids, often in multiple layers formed at different times from different waste streams, in contact with concentrated salt solutions. The long term disposal of these wastes is a goal of considerable importance to the public, especially since some of the waste is stored in single shelled tanks (SSTs), a number of which are assumed to be leaking. Meeting this challenge will require the completion of three subtasks: make operational a long term geologic repostory for glass logs formed from vitrified waste, develop a process which can be approved in the U.S. for performing the vitrification, safely transfer the wastes between tanks as needed to prepare for long term storage.

C. Marmaras, M. A. Clark, D. M. Martin

### Ultrasonic Width and Thickness Monitoring System in a Lamination Process

The ultrasonic monitoring system described in this work was installed in a copper plate mill which use to have a series of mechanical thickness and width gauging devices. Those devices were taken out of use due to lack of maintenance. The operators empirically learnt to adjust mill controls in order to obtain copper strip within client specifications. This operation lead to a strong dependency on the operator’s skills, along with limitations in the quality of the final product. With the purpose of evaluating the factibility of automizing the process, an ultrasonic thickness and width gauging monitoring system was designed. The knowledge of these two parameters is an essential part in the quality control of the process of fabrication of copper strip.

M. A. Marcial, J. J. Velázquez

### Application of Dynamic Adaptive Focusing System to Ultrasonic Non Destructive Testing

To obtain the high performances requested by the safety authorities for ultrasonic inspections, in term of sizing, characterization and on-line monitoring, it is needed to vary the refracted angles, focusing depths and the size of the ultrasonic beams rapidly and over a wide range. This is not possible with conventional techniques which allow only fixed focusing. The F.A.U.S.T. (Focusing Adaptive UltraSonic Tomography) system, which is described in this paper, is an open and powerful tool designed to overcome the difficult problems in flaw assessment and monitoring.

S. Mahaut, G. Cattiaux, O. Roy, H. Acounis, G. Pincemaille

### Generation of Axial Shear Acoustic Resonance by Magnetostrictively Coupled Emat

In this paper, we present a technique to generate the axial shear acoustic resonance in a cylindrical rod and pipe by a magnetostrictively coupled EMAT. The technique is based on the electromagnetic acoustic resonance (EMAR) [1–7]. The EMAR is a combination of the ultrasonic resonance technique and a noncontacting EMAT. The heavily and coherently overlapping echoes at a resonant state improves the signal-to-noise ratio to a large extent and compensates enough for the weak coupling efficiency with the EMATs. The EMAR method was first proposed by Filimonov et al. [1] and Nikiforenko et al. [2]. In recent years, several researchers have developed this technique for the wide range of the noncontact inspection and materials characterization [3–7].

H. Ogi, M. Hirao, K. Minoura

### Nuclear Waste Drum Characterization with Mobile Tomographic Assay and NDE

Waste Inspection Tomography (WIT) provides mobile semi-trailer mounted nondestructive examination (NDE) and assay (NDA) for nuclear waste drum characterization. See Figures 1 and 2. WIT uses various computed tomography (CT) methods for both NDE and NDA of nuclear waste drums. Low level waste (LLW) transuranic (TRU), and mixed radioactive waste can be inspected and characterized without opening the drums.

Richard T. Bernardi, David Entwistle, Jerry Swinford

### Pace: An Advanced Structure for Handling Multitechnique NDT Data

The analysis of NDT data is a problem of growing importance and complexity, encountered in many industrial fields, such as electricity production, oil industry, aeronautics, infrastructure inspection or metal production. This is why dedicated systems have begun to appear in the previous years to handle this particular part of a nondestructive inspection, located after the data acquisition and leading to the diagnosis.

M. Mayos, J. L. Lesne, O. Vailhen, B. Nouailhas, S. E. Moubarik, E. Noël, D. François, C. Soors, F. Guisnel

### A Thermal Imaging System for Crack Growth Quantification in Thermo-Mechanical Fatigue Specimens

High temperature materials intended for turbine engine or advanced airframe applications must undergo extensive thermo-mechanical fatigue (TMF) testing to predict their behavior in extreme operating environments. Figure 1 provides an illustration of a typical TMF test machine where hydraulic load cylinders impart cyclic tensile loads while quartz heat lamps and nitrogen cooling jets are used to thermally cycle the test specimens over temperatures ranging from -20°C to 1000°C. The purpose of these tests is to determine when crack initiation occurs and then to measure the rate at which fatigue cracks grow. The combination of high specimen temperature, access limitations imposed by the test chamber, and test specimen surface oxidation make the desired crack length measurements difficult. Although traditional measurement techniques such as the electric potential difference (EPD) method are effective for single, relatively straight cracks, they are unable to identify the presence of multiple cracks or accurately measure the length of abnormal crack morphologies such as bifurcated or sawtooth [1].

G. White, A. Mueller, G. Torrington

### Fatigue Monitoring System Progress

The Advanced Large Structural System (ATLSS) center at Lehigh University has been actively involved in the development of a long term fatigue monitoring system, with the system almost being completely realized now. The fatigue monitoring system consists of several wireless sensor units mounted on the structure under observation and a central control office. The sensor unit takes as an input a stress signal from a strain gage, processes the signal in real time using the rainflow counting algorithm and stores a stress histogram in its internal memory [1–3]. The central control office will periodically download the data from each sensor and analyze it to determine if a structural member under observation is in immediate danger of a fatigue crack. The goal of the fatigue monitoring system is to use VLSI technology to develop a complete cost effective system by merging the sensor, data acquisition system and real time processing units into one.

Scott A. Segan, Fan Ling, Qinghong Cao, Weiping Li, Ben T. Yen

### Raising the Reliability of NDE by Combination and Standardisation of NDT-Data Using the Trappist System

The TRAPPIST (Transfer, Processing and Interpretation of 3D NDT-Data in a Standard Environment)-system is a software package and data handling philosophy developed during a three years European project. It is dedicated to handle and transfer, to reconstruct, visualize and process 1D, 2D, 3D NDE and CAD-data.

C. Nockemann, S. Heine, K. Johannsen, A. Schumm, O. Vailhen, B. Nouailhas

### Methodology for Estimating Nondestructive Evaluation Capability

This paper outlines a proposed methodology for using combinations of physical modeling of an inspection process along with laboratory and production data to estimate Nondestructive Evaluation (NDE) capability. The physical/statistical prediction model will be used to predict Probability of Detection (POD), Probability of False Alarm (PFA) and Receiver Operating Characteristic (ROC) function curves. These output functions are used to quantify the NDE capability. The particular focus of this work is on the use of ultrasonic methods for detecting hard-alpha and other subsurface flaws in titanium using gated peak detection. This is a uniquely challenging problem since the inspection must detect very complex subsurface flaws with significant “material” noise. However, the underlying framework of the methodology should be general enough to apply to other NDE methods.

William Q. Meeker, R. Bruce Thompson, Chien-Ping Chiou, Shuen-Lin Jeng, William T. Tucker

### Probability of Detection for Applied Ultrasonic Inspection

Acquisition and analysis of inspection capability data for volumetric NDE techniques, such as ultrasonic (UT) or X-ray inspection, set several challenges beyond those faced in quantifying surface techniques such as penetrant (PT) or eddy-current (ET) inspection. For the latter, well-established methodologies have been developed [1,2] and extensively applied. Typically, for both PT and ET, use is made of “synthetic” flaws, which appear to have properties close to those of the naturally-occurring flaws that they represent. Synthetic flaw characteristics such as length, orientation and location may be controlled, and can be confirmed by direct non-destructive measurements.

R. H. Burkel, D. J. Sturges, W. T. Tucker, R. S. Gilmore

### Iowa Demonstration Laboratory for NDE Applications Project Review

The Iowa Demonstration Laboratory (IDL) for NDE Applications is a State sponsored outreach program of the Center for Nondestructive Evaluation at Iowa State University. It serves as a NDE resource to Iowa manufacturers, particularly the small to medium size firms that make up most of Iowa’s economic base. The program’s basic mission is to educate Iowa companies on the use of NDE techniques and assist with implementation of the technology to improve the quality of products, control costs, and limit liability.

Brian Larson, Lisa Brasche, Dave Utrata

### A New Technique for Surface Acoustic Wave Speed Measurement in Laterally Inhomogeneous Materials

The technique of measuring surface acoustic wave (SAW) speed of a solid by an acoustic microscope has been known to the nondestructive evaluation (NDE) community for last twenty five years. A number of research papers [1–17] have been published on this subject. The technique is based on generating the V(z) curve, also known as the acoustic material signature of the solid under inspection. The V(z) curve is generated by moving the microscope lens vertically, normal to the plane of the solid and recording the reflected signal. The voltage produced by the reflected signal oscillates between a series of maxima and minima as the lens-specimen distance is changed. This is because the phases of the two rays, the central ray and the nonspecularly reflected critical ray which are received by the receiver, vary as the lens-specimen distance is altered. The constructive interference between the two rays gives maxima or peaks and the destructive interference creates minima or dips in the V(z) curve. The spacing between two successive minima is related to the SAW speed of the specimen and thus the SAW speed is measured. This technique works very well for a laterally homogeneous specimen. However, for a laterally nonhomogeneous material predicting the material property variation by the V(z) technique becomes very cumbersome. Because in this case V(z) curves are to be experimentally generated at different locations of the specimen to predict the nonmonotonic SAW speed variation in the specimen.

K. I. Maslov, T. Kundu, T. Ghosh

### Interpretation of Surface Wave Signals following Multiple Specular Reflections in an Acoustic Microscope

Acoustic microscopes are powerful tools in determining the velocity of a leaky surface or Rayleigh wave. In the most common implementation, the velocity is inferred from a narrow band measurement of the variation of signal with lift-off [1]. However we are concerned with a time-domain implementation [2], in which the velocity is found from the time difference between the front surface reflection and the trailing leaky Rayleigh wave signal. This signal is created by a complex wave path involving propagation of a longitudinal wave through the coupling fluid, conversion to a Rayleigh wave, and reradiation back to the transducer. However, when the velocity of the Rayleigh wave is too low, or the aperture of the lens is too small, this second signal is not observed because of the absence of energy at the incident critical angle [3]. Nevertheless, in this case, a trailing signal is sometimes seen after the second or third specular reflection. In this paper, we describe a series of experiments in which such signals are seen. Based on a combination of theory and experiment, we establish the origins of these signals and identify situations in which they extend the capability of a given lens.

J. C. Johnson, R. B. Thompson

### Development of an Apertured-Confocal Acoustic Microscope

To interrogate the internal structure of a part using conventional ultrasonic testing, pulsed elastic waves are induced in the part and reflect off internal discontinuities. The amplitude and time of flight of the reflected wave provide information about the discontinuity. There is a direct correlation between the location of a reflection and the round trip time. Reflections emanating from deep in the part require more time to return to the ultrasonic probe than do shallow reflections. This makes possible the use of time based “windows” or “gates” to acquire data corresponding to a specific depth of interest. Although complex and expensive electronic circuitry is required, under most circumstances time based data acquisition works well. However, there are occasions in ultrasonic testing where time based data acquisition is inadequate and where true spatial resolution is desired.

K. E. Lulay

### Acoustic Microscopy of Advanced Aerospace Materials

Ultrasonic Scanning Acoustic Microscopy (SAM) is useful for material elastic property quantification, surface and subsurface crack initiation detection and growth estimation, and fiber-matrix interfacial damage assessment [1–5]. The advantage of the method is that the imaging technique can provide the crack sizing information while helping in the detection of interface degradation and early crack initiation so that their growth can be monitored during interrupted fatigue tests. The scanning acoustic microscope technique has been applied in the past to metal matrix composites subjected to both room temperature and elevated temperature fatigue cycling in addition to thermomechanical fatigue (in-phase and out-of-phase) conditions. In this paper, we present results obtained from an in-house developed acoustic microscope operating in the frequency range of 25–200 MHz. Results of characterization of many aerospace materials such as metal matrix and ceramic matrix composites and titanium alloys are provided to demonstrate the versatility of the system.

Richard W. Martin, Mark J. Ruddell, Jeffrey A. Fox, Theodore E. Matikas, Prasanna Karpur

### A Digital Signal Processing for Measuring Leaky Surface Wave Velocity with C-Scan Acoustic Microscope

Acoustic microscopes operated with burst waves over 100MHz have been used for measuring leaky Rayleigh wave velocity, VR, within very localized region with the V(z) curve [1]. At these high frequency, the finer spatial resolution is obtained. However, at these frequency, the Rayleigh wave runs within very shallow subsurface, less than 30μm for common metals. This is a serious drawback to apply the V(z) approach for bulk materials, which are used commonly in engineering and industrial fields.

Koichiro Kawashima, Ikuya Fujii, Takeshi Sato, Motohiro Okade

### Recent Developments with the Dripless Bubbler Ultrasonic Scanner

In keeping with the requirements of the air carrier maintenance community, we have developed a closed-cycle, water-coupled, ultrasonic method using a focused-beam broadband pulse for the nondestructive inspection (NDI) of adhesively bonded aluminum and composite aircraft fuselage structures. This approach, known as the “Dripless Bubbler” technique, is a combination of focused-beam immersion ultrasonics with portable ultrasonic scanners[1–4]. The Dripless Bubbler retains the high quality associated with focused-beam immersion ultrasonics, eliminates uncontained couplant water; and more importantly, it provides the capability to ultrasonically scan the exterior of an aircraft over surface protrusions such as button-head rivets and lap splice edges.

Thadd C. Patton, David K. Hsu

### A Guided Wave Resonance Matching Technique for in Situ Monitoring of Powder Injection Molded Products

Advances have been made in manufacturing high performance metallic and non-metallic components via powder injection molding (PEM) approaches [1,2,3]. Considerable progress has been made in bringing ΡIM technology into an era where complex shapes (Figure 1) can be achieved with changes in cross-sectional area by an order of magnitude [4,5]. This has been accomplished by novel approaches in instrumentation, feedback control, and process condition optimization [6,7].

Joseph L. Rose, Suresh M. Menon, Robert Yi, Randall M. German

### Surface Monitoring by Combination of an Optical Sensor and an Ultrasonic Scanning System

Nondestructive testing is a vital tool in the effort to ensure the quality of mechanically highly-loaded aero-engine parts. Nondestructive testing methods, such as eddy current and dye penetrant, are used to verify the surface condition of components, this condition greatly affecting operational safety and service life. Yet while these methods will readily detect small cracks, they fail to show the gradual changes in surface topology commonly associated with service wear.

J. Bamberg, A. Gindorf, L. Steinhauser

### Measurement of Viscosity in Liquids Using Reflection Coefficient: Phase Difference Method

Measurement of viscosity of fluids is a critical parameter in determining the state of the fluid (ie. edible products), and the state of the forming solid (ie. molten metals and glasses). Experiments to measure viscosity using ultrasound, have been carried out since as early as 1951 [1]. Ultrasound has potentially offered a non-invasive, in-line method of property and process monitoring [2,3]. Early research has demonstrated that viscosity measurement can be accomplished by ultrasound using different linear and nonlinear techniques [4]. This paper is devoted to furthering the technique called shear reflectance method [5].

Vimal Shah, Krishnan Balasubramaniam, R. Daniel Costley, Jagdish Singh

### Synthetic Aperture Focusing Techniques in the Near Field of a Focused Transducer

Synthetic aperture focusing techniques (SAFT) represent a special class of beam steering algorithms [1] that are used to improve the resolution and signal-to-noise ratio (SNR) of ultrasonic images. It has been traditionally performed using focused transducers, with the test material located in a region where the ultrasonic beam diverges beyond the focal point [2,3]. This is to ensure that the transducer collects enough data relating to the defect. Operating in the far-field of the transducer where the spatial field variations are slow minimizes the error.

W. Masri, M. Mina, S. S. Udpa, L. Udpa, T. Xue, W. Lord

### Impact of Quantization Noise on the Quality of Ultrasonic Signal Deconvolution

In materials characterization, a material’s transfer function is calculated from experimental data. Sometimes, the transfer function of the measurement device, such as an ultrasonic transducer, must be known for this calculation. One of the common methods of obtaining a material transfer function is to measure an elastic waveform before and after it propagates through a material. Then, the transfer function is calculated by deconvolving the two waveforms. We illustrate this process in Fig. 1, where two transducers are placed with a section of plate material between them. The material transfer function is measured by deconvolving the signal at the first transducer from the signal at the second identical transducer. Ideally, this deconvolution would remove the transducer effects. Likewise, if the transducers were placed at the same location (d=0), the transfer function of one transducer could be calculated using a reference transducer.

M. C. Renken, C. M. Fortunko

### Joint Time-Frequency Processing of Ultrasonic Signals

The backscattered signal information in ultrasonic testing is an effective method for characterization and detection of defects in materials. The complexity of materials like composites, mixed metals, multilayered fiber-reinforced composites and materials with complex structures used in nuclear power plants and spacecraft add new challenges towards the characterization and defect detection. Due to frequency dependent scattering and attenuation the backscattered ultrasonic signal is nonstationary in character. Therefore, joint time-frequency (t-f) representations of such signals are more revealing than the standard Fourier analysis.

M. A. Malik, X. M. Jin, J. Saniie

### Choice of Coded Waveform and Correlation Filter for Self-Noise Suppression in Ultrasonic Correlation Systems

Various ultrasonic correlation systems have been suggested in the past [1–5] in order to improve the flaw detection capability of ultrasonic NDE systems. These systems use a coded waveform for transmission, and the received signal is processed through a correlation filter. Although, these systems provide considerable improvement in the flaw detection capability, their performance is limited by what is called “self-noise” of the system. This paper discusses the self-noise limitation of conventional ultrasonic correlation systems and presents various approaches for self-noise suppression. Theoretically predicted performance has been verified using detailed computer simulations. Finally, comparison of the performance and practicability of each approach is discussed.

Jahangir K. Kayani, Steve F. Russell

### Integrity Monitoring of Pressurized Gas Cylinders Using the SSUE Technique

Spread-Spectrum Ultrasonic Evaluation (SSUE) is a newly emerging technique for the global inspection and integrity monitoring of objects such as pressurized gas cylinders and structures such as bridges and airframes. This technique is based upon the measurement of an ultrasonic correlation signature which is sensitive to the structural and material properties of the test object. Recently, we applied the technique to pressurized gas cylinders and found that it was very sensitive to small changes in the cylinder. Specifically, we looked at the ability of the SSUE method to detect artificially-induced cracks at various points on the outside of the cylinder.

Steve F. Russell, Jahangir K. Kayani, Muhammad A. K. Afzal, Samuel J. Wormley

### Nondestructive Testing Potential of a Magnetoacoustic Storage Correlator (MASC)

A simple magnetoacoustic storage correlator (MASC) was tested as a pulse compressor in an ultrasonic nondestructive testing application. The pulse compression ratio reached was 30. Higher compression ratios are readily available through choice of different device lengths and device operating regime. The system is flexible; it is not limited to any particular type of large time-bandwidth signal. The correlator reference signal is easily programmable and erasable, and can be changed at any time. This system offers real-time correlation signal processing at a repetition rate that is only limited by the temporal length of the large time-bandwidth signal.

Vladimir Ermolov, Markku Oksanen, Mauri Luukkala

### Effects of System’s Limitations on the Accuracy of Measured Ultrasonic Correlated Signal

Within a highly attenuating material, it is often difficult to identify relevant target signals due to the system’s white noise that is elevated by high gains on a conventional ultrasonic system. Ultrasonic pulse compression technique resolves such problem. The ultrasonic pulse compression technique permits an ultrasonic system to operate with long transmitted pulses for an increased detection range, but without sacrificing the depth resolution by signal correlation. Moreover, the time integral involved in the cross-correlation further suppresses the system’s white noise, and hence it produces an improved signal-to-noise ratio (SNR).

Peter Jeong, Tim Gray, Lester Schmerr

### Two Dimensional Multi-Frequency Eddy Current Data Fusion

The concept of data fusion has received significant attention recently. Work in the field has been motivated by the belief that a single NDE measurement is sometimes inadequate and does not offer sufficient information about the specimen under test [1–5]. The rationale underlying data fusion is that the use of multiple NDE methods, together with intelligent techniques for synergistically combining the data, can potentially lead to a better estimate of the desired information.

M. Mina, J. Yim, S. S. Udpa, L. Udpa, W. Lord, K. Sun

### Development and Testing of Rotating Probe Method for Airframe Rivet Inspection

In recent years much work has been published on the development, testing, and operational characteristics of the Self-Nulling Eddy Current Probe [1–4]. Although major advances have been achieved, applications of the probe in the airline industry have been limited. The rather complex geometries involved and the need to detect small flaws demand a refined inspection method. In an effort to meet these challenges two specialized techniques using the Self-Nulling Probe have been specifically designed for airframe flaw detection. In this paper a rotating probe method for the detection of small flaws under rivet heads is presented while a companion presentation details a technique for quantifying material loss in lap-splice joints [5].

Buzz Wincheski, Jim Fulton, Ron Todhunter, John Simpson

### Eddy Current Inspection of Threaded Fasteners

Anti-seize lubricant commonly used on threaded fasteners makes it difficult to prepare surfaces adequately for magnetic particle and liquid penetrant testing. In high temperature applications, the lubricant forms a tenacious residue on the threaded fastener. An alternative surface examination technique requiring minimal surface preparation would reduce inspection cost and duration. Two types of threaded fasteners, reactor pressure vessel closure studs and turbine casing studs, have been targeted for investigation. Eddy current techniques requiring minimal surface preparation and capable of detecting flaws are being developed for these two applications. Other advantages of eddy current inspection over magnetic particle testing and liquid penetrant testing include the absence of consumable materials, capability to provide permanent digital records, and the potential for automation.

Jennifer Schlegel, Robert E. Green, C. L. Friant

### Compensated High-Bandwidth Laser Ultrasonic Detector Based on Photo-Induced Emf in GaAs

The trend toward intelligent manufacturing has produced an increase in the need for sensors which can nondestructively evaluate components and processes in real-time. One commonly used nondestructive approach is ultrasonic inspection. The most common method for generating and sensing ultrasound in materials makes use of contacting piezoelectric transducers. A gel or water interface is often used to match the acoustic impedance between the sensor and part. This constraint can impose limitations on their applicability for some types of in-process industrial control or inspection, specifically, inspection of moving parts at elevated temperatures or in vacuum. While noncontact receivers have been made using capacitance or magnetic induction, often their spacing to the workpiece must be maintained within a close tolerance. The lack of a substantial standoff distance for these sensors also reduces their usefulness in some industrial inspection and process control applications. Specifically, contacting schemes and close proximity sensors are not well suited for conditions such as extreme vibrations and fast moving parts with irregular surfaces. Furthermore, it may be more cost effective, from the users perspective, to use a long standoff, remote sensing system which could be applied to a wide range of materials including metals, semiconductors and composites.

P. V. Mitchell, G. J. Dunning, S. W. McCahon, S. W. McCahon, M. B. Klein, T. R. O’Meara, D. M. Pepper

### An Application of Holographic Interferometry to Evaluate the Dynamics and Material Characteristics of Cast Compliant Structures

Holographic Interferometry techniques have been succesfully employed to characterize and test the behavior of materials in structures under stress1,2,3. These techniques are especially suited to investigating the dynamics of the material volume and the efficacy of bonds at the interfaces of different materials in bonded and cast compliant structures. Such structures are characteristic of systems in which arrays of active transducers are used to modify the surface or geometry of the structure, such as in active optical systems and acoustic sonar arrays. Knowledge of the causes and effects of structural anomalies and defects is of critical importance to insuring correct operation.

Howard Fein

### Optical Lock-in Vibration Detection Using Photorefractive Four-Wave Mixing

Many important applications for photorefractive crystals (PRCs) have been found recently by various investigators [1–3]. These applications range from volumetric information storage in optical computing to adaptive, remote detection of ultrasonic vibration in optical nondestructive evaluation [4]. In this paper, we propose the use of PRCs for lock-in detection of continuously vibrating structures.

Tom Chatters, Ken Telschow

### Automated Laser Scatter Detection of Near-Surface Defects and Machining Damage in Ceramic Components

The need to achieve higher gas firing temperatures in new stationary gas turbines has led to the requirement for stronger, longer-lasting materials capable of functioning in environments more severe than traditional materials permit. The higher operating temperatures seen in these environments (>1300 °C) would substantially increase energy efficiency and reduce emissions. Because conventional materials are not suitable, alternate materials are being considered for turbine components. Included among these are the Si3N4 ceramics. In an effort to improve and extend the working lifetime of the components, improvement in the detection of critical defects (such as cracks, voids, inclusions, or microstructural variations) in their surface and near-subsurface regions is being investigated. Although all of the defects can occur either during manufacturing or in operation, the specific type of defect most likely to occur in a given component depends on a various factors, including the material system, processing and machining parameters, component geometry, and application environment.

J. Scott Steckenrider, William A. Ellingson

### Quantitative Measurement of Chrome Loss by Optical Means

The current method for measuring chrome loss in chrome-plated barrels is by manually operated borescopes. The operator slowly translates the borescope down the barrel and notes the location and area of each patch of chrome loss. This method is time consuming and subjective. This paper describes an automated system for quantitative measurement of chrome loss in the 120-mm M1A1 tank barrel. The same principles used to determine chrome loss are also used in other systems to identify and quantify a wide variety of internal flaws in barrels or tubes. Along with quantification of internal surface flaws, the described system precisely measures the inside diameter of the tube being inspected. This capability allows chrome loss and diameter measurements to be taken during one measurement sequence and eliminates the need for two separate inspections and associated setup times.

P. D. Bondurant, M. D. Watson

### Optimization of Neural Network Parameters for Defect Characterization

Natural gas, which is one of the nation’s cheapest forms of energy, is transported to consumer sites via a vast transmission pipeline network .Safety considerations and a desire to assure uninterrupted energy supply require that the pipelines be inspected periodically. The effective detection of defects in the pipeline is vital to assure the integrity of the transmission systems. A variety of nondestructive evaluation techniques (NDE), such as ultrasonic, eddy current, and magnetic flux leakage (MFL) methods have been employed to detect defects in gas pipelines [1]. Among these methods, the MFL method represents the commonly used technique.

G. X. Xie, M. Chao, C. H. Yeoh, S. Mandayam, S. S. Udpa, L. Udpa, W. Lord

### Noncontact Measurement of Applied Static Torque

Schmidt and Zinke [1,2] have shown that the ac magnetic bridge can be used to make noncontact measurements of changes of strain in an axially loaded bar. Therefore, there seemed to be no a priori reason why the bridge could not be used to make noncontact measurements of shear stress produced by static torque. The results of such measurements are described here. A 2.54-cm cold-rolled steel bar was placed in a fixture and subjected to various applied torques. The measurements produced an initial transient signal followed by a changed static signal. The transient signal, which lasts the order of 6–8 seconds, seemed to be proportional to the rate of change of the application of torque although no quantitative measurements were made to prove this. The static signal, which was measured 30 seconds after the torque was applied, was proportional to the torque over the range of torques from 4 to 57 newton meters. If transient signals had been produced in the original linear stress tests [1,2], they would not have been noticed because of the technique of obtaining the data in those experiments.

O. H. Zinke, W. F. Schmidt

### Induction Coil Shape and Size Considerations in Magnetic Field Measurement with Spatially Fast Varying Distribution

In the context of non-destructive testing, the method of magnetometry has been used in various applications, including flaw detection, analysis of magnetic properties of materials, and corrosion study. In this method, the magnetic field distribution in the vicinity of the specimen is analyzed for obtaining unknown physical, chemical, and geometrical parameters. The source of the magnetic field may be intrinsic in the specimen such as galvanic currents due to corrosion in the specimen, or external source can be used to excite the specimen. The magnetic field measurement is performed using a magnetic field-sensitive indicator. Various magnetic field sensors have been devised with various degree of sensitivity, resolution, ease of use, etc. Hall-effect probes, magnetoresistive sensors, magneto-optic sensors, and induction coils are some of these sensors.

S. H. H. Sadeghi, B. Toosi

### Industrial Applications of Laser-Based Profilometry

Industrial applications of laser-based profilometry have rapidly evolved over the past ten years. During this time, QUEST Integrated, Inc., has been actively involved in developing systems for a wide variety of NDE and quality control applications. Employing the principle of optical triangulation, sensors have been developed for applications ranging from measuring 0.25-inch-diameter tubing to mapping a 24-inch-diameter nozzle 70 feet underwater. Three-dimensional contour maps can be rapidly and accurately generated that allow operators to obtain quantitative information regarding the condition of test articles.

J. L. Doyle, M. A. Correa, P. D. Bondurant

### Minimum-Norm Least-Squares Inverse for Magnetic Flux Leakage Image Reconstruction

The main goal of this technique is to reconstruct the magnetic source distribution inside an inaccessible volume, from magnetic field measurements performed outside the volume. The approach we used models the unknown distribution as an array of magnetic dipoles with fixed positions but unknown strengths. Then magnetic field measurements can be written as a linear function of the unknown dipole strengths. The minimum-norm least-squares inverse finds the dipole strength distribution that minimizes the squared difference between the measured field and the field generated by the solution. We applied this technique successfully in situations where it would be difficult to distinguish between simulated single and multiple flaws.

A. C. Bruno

### In-Process Monitoring for Quality Assurance of Automated Composite Fabrication

Lowering manufacturing costs of composite parts is essential to their continued use on commercial aircraft. Because Quality Assurance (QA) can constitute up to 40% of the manufacturing expense [1], there is much to be gained by reducing inspection cost. A considerable amount of the QA cost can be attributed to end-item nondestructive evaluation. With the ongoing development of automated methods of composite fabrication, we have the opportunity to incorporate in-process monitoring into these systems as a means of minimizing the need for end-item inspection. One current focus within The Boeing Commercial Airplane Group (BCAG) is the use of automated fiber placement (AFP) for composite lamination. Concurrent with the development of the AFP manufacturing process has been the development of in-process inspection techniques for fiber placement. By identifying key process variables and monitoring those variables during fabrication, our goal is to produce high quality fiber placed parts while incurring lower overall inspection costs.

Kathryn A. Soucy

### A Prototype Apparatus for Determining Changes in the Electrical Conductivity of Production Run Carbon Fibers

During the production and processing of continuous lengths of carbon fibers (a tow), the conductivity along the length of the tow may vary significantly depending on the variation of the heat treatment temperature (HTT) and the resident furnace time. For PAN-type (polyacrylonitrile) fibers at HTT greater than 1000°C, a small change in HTT results in a large change in conductivity 03C3. For lower values of HTT, the corresponding change in σ can be quite small [1].

Laura L. Dulcie

### Eddy Current Determination of Semiconductor Liquid-Solid Interface Location and Shape

The vertical Bridgman method is a widely used technique for the growth of semiconductor single crystals from the melt [1–5]. In this method, a vertical multizone furnace is used to establish an optimized axial temperature gradient through which is translated an ampoule containing the liquid semiconductor. As the lower tip of the ampoule enters the furnace cold zone, single crystal solid is nucleated and a liquid-solid interface propagates upward through the ampoule. Today, the yield and quality of material grown by this technique is maximized by empirically optimizing the temperature gradient and the axial translation rate for each material system. Essentially, this involves repeated experimentation until a satisfactory material can be grown. Once obtained, temperature set points within the furnace, and furnace translation rate schedules are fixed from run to run.

Kumar P. Dharmasena, Haydn N. G. Wadley

### Magnetic Flux Leakage Inspection of Moving Steel Sheets

In the process of making drawn and ironed cans out of a steel sheet, non-metallic inclusions such as chemical oxides in the steel sheet may cause cracks. Therefore, steel sheets are inspected before they are drawn and the magnetic flux leakage testing is usually employed for the inspection [1]. When there are non-metallic inclusions in the steel sheet, magnetic fields produced by a magnetizer leak out around such defects because of the differences in material permeability. These leakage fields are detected by flux sensitive sensors such as Hall plates or magnetodiodes [2].

Young-Kil Shin

### Remote Laser-Based Ultrasonic Inspection of Weld Joints for High Volume Industrial Applications

The ability for manufacturers to implement economic techniques for process control and quality assurance can significantly contribute to their competitiveness in the world market. Consequently, many manufacturers would like to determine the quality of their products by conducting the appropriate quantitative tests at suitable stages during production. In addition, an in-line capability for real-time intelligent process-control is desirable. Many of the weld joints produced today are inspected off-line using destructive sectioning and visual examination. Due to the limited number of parts available for this type of testing, the process control is statistical in nature and 100% certification is impossible. Furthermore, visual inspection cannot directly or accurately measure many important material parameters of the weld and the parent material. A better solution to the problem would be a real-time, nondestructive, noncontact technique for weld joint inspection and feedback control of the welder.

G. J. Dunning, P. V. Mitchell, M. B. Klein, D. M. Pepper, T. R. O’Meara, Y. Owechko

### Resonance Inspection for Quality Control

Resonant Inspection (RI)* is an important new technique for non-destructive evaluation of manufactured parts. RI mechanically vibrates a part and detects defects based on changes in the pattern of vibrational resonances. RI measures the “whole body” structural response of the part rather than scanning for defects, as do most other NDE techniques. As a result it is accurate, fast, inexpensive and requires no human judgment, so it can be used for 100% inspection. However, since RI infers the presence of a defect from the resonant pattern of the whole part, up - front engineering is required to develop a detection algorithm that correlates the changes in resonance pattern to the presence of defects of interest.

J. J. Schwarz, G. W. Rhodes

### Ultrasonic Gauging for On-Line Inspection of Tubes

A Computerized Ultrasonic Gauging System (CUGS) has been developed to generate a visually useful as well as a quantitative mapping of the outer and inner surfaces of tubes during the various stages of manufacture. Measurements of the outside radius and of the thickness of the tube are performed with a resolution of 0.0025 mm (0.0001 in) as the part is rotating on a lathe with speeds of up to 1000 surface feet per minute. Typically, 500 measurements are made and displayed for each revolution of the part. The transducer is scanned along the axial dimension by the lathe carriage to which it is attached. The axial scan rate is typically 0.25 mm (0.01 inches) per revolution, resulting in very precise topographical maps of all the cylindrical surfaces of the part. The system was developed so that the run-out and the straightness of the part are calculated from the dimensional data obtained. The surface finish of the outside and inside of the part is also available in a post process, color mapped, display, and CUGS is also capable of simultaneously detecting internal flaws in the part. A fluid stream couples the ultrasound to the part, and high speed electronics is employed to perform the measurements. A description of the system is presented with experimental results.

A. Abbate, J. Frankel, Robert W. Reed

### Wall Thickness Measurements in Hot Steel Pipe Using Non-Contact Ultrasound

The production of steel seamless tubes would greatly benefit from a system that could measure the pipewall thickness at an early stage on the production line. An accurate measurement of the wall thickness could be used to improve the manufacturing process, allowing finer control of the final product dimensions. Precise control of the wall thickness is critical in order to provide a high quality product, and to minimise wastage (both in terms of rejected pipes, and the margins needed to maintain a minimum wall thickness).

D. R. Billson, C. Edwards, M. S. Rohani, S. B. Palmer

### Determination of a New Ultrasonic Indicator to Follow the Concrete Setting in Real Time

Knowing the setting of roller compacted concrete (RCC) is an essential element within the framework of the building of multi-layered structures such as dams.

V. Garnier, G. Corneloup

### Acoustic Waveguide Cure Curves for Materials Ranging from Fast Cure Resins to Slow Cure Concrete

The attenuation and transmission velocities of ultrasonic waves traveling through curing polymers are related to the three general phases of curing which can be categorized as; increasing viscosity; gelatinization (the transition from a liquid state to that of a rubbery gel), and hardening (increasing modulus). This should be expected because the acoustic wave transmission in materials depends upon their molecular structure which is related to the material viscosity, density and modulus. In 1952 Sofer and Hauser [1] passed 2.3 Mhz longitudinal ultrasonic waves through curing polymers and related attenuation and velocity measurements to the degree of cure. However, wave dispersion can be a problem with this technique and an improved approach is to use embedded acoustic waveguides to guide the ultrasonic waves through curing polymers. A step in this direction was taken by Roth and Rich [2] in 1953 who developed a 28 kHz ultrasonic technique for measuring viscosity during the polymerization of plastics. They used a thin metallic strip into which shear waves were magnetostrictively induced and when this strip was immersed in a viscous liquid, the attenuation of the shear waves traveling along the strip could be related to the liquid viscosity. This metallic strip can be considered to be an acoustic waveguide (AWG) excited and measured from one termination. Later, in 1971, Lynnworth[3] and in 1974, Papadakis [4] developed AWG techniques using magnetostrictive activation of torsional waves and applied their methods to curing polymers.

R. T. Harrold, Z. N. Sanjana, Richard Brynsvold

### The Evolution of a Nuclear Fuel Welding and Ultrasonic Inspection Machine for Improved Quality and Process Control

GE Nuclear Energy Production produces Nuclear Fuel for commercial nuclear power plants. The basic product of the GE Nuclear Fuel business is a fuel bundle made up of a square matrix (8x8), (9x9), or (10x10) of fuel rods in a spacer assembly surrounded by an outer channel. BWR nuclear power plants can have up to 700 bundles in their reactor at any one time. The fuel rod consisting of a tube loaded with UO2 pellets, and closed with welded endplugs is shown in figure 1. The integrity of the fuel rods is of great importance in order to keep radioactive materials out of the water systems. A rod failure (leak) that causes a customer to shutdown can cost hundreds of thousands of dollars.

M. T. Kiernan, J. D. Landry

### A Process Sensor for Locating the Liquid-Solid Boundary through the Mold of a Casting

Accurate process control of single-crystal and directionally-solidified castings requires knowledge of the exact location of the solidifying front. If the front advances too rapidly, single crystal growth in a preferred orientation degenerates into the formation of polycrystals. A solidification front which moves more slowly than necessary is wasteful of the casting resources. A sensing technology is being developed which determines the location of the boundary between a solidifying crystal and liquid metal. The sensing method utilizes the ordered pattern of x-rays diffracted from the solid as an absolute indicator of the liquid-crystal interface.

D. W. Fitting, W. P. Dubé, T. A. Siewert, J. Paran

### Analysis of Compressibility Origins in Monolithic Ceramic Suspensions Using X-Ray Imaging Techniques

In the past two decades, significant advances have been made in the synthesis of new types of nonclay ceramic powders (e.g., oxides, borides, carbides, nitrides), which today have an unprecedented degree of control over particle size, shape, and chemistry [1]. A particular trend has been the increased production of smaller and smaller (typically submicron) powders in order to (i) reduce sintering temperatures, (ii) strengthen ceramics by decreasing the size of Griffith’s flaws, and (iii) allow the processing of composites with individual phases distributed at smaller and smaller scales of mixing.

S. P. Huss, J. N. Gray, C. H. Schilling

### Novel X-Ray Imaging Method for Evaluating Defect Evolution in Ceramic Tapes

Ceramic tape casting is critical to the electronics industry for manufacturing a wide range of components including piezoelectric actuators, MLC capacitors, and substrates for VLSI and LSI chips [1–3]. Recent regulatory changes, led by the Environmental Protection Agency concerning hazardous chemicals used in ceramic tape-casting, have renewed interest in the development of environmentally-friendly modifications to this process. In turn, this has increased interest in developing a better, fundamental understanding of how microstructural defects form and evolve during the sequence of processing steps associated with tape casting. In order to form more reliable electronic components, there is a need to develop a better, basic understanding of how to eliminate these defects by optimizing critical processing variables.

C. W. Maranville, J. N. Gray, C. H. Schilling

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