Review of Progress in Quantitative Nondestructive Evaluation

Scattering cross-sections are calculated for a crack in three-dimensional elastic solids. The crack opening displacements are evaluated first by the boundary element methods. Then the scattering amplitudes for the crack are derived from the far-field representations of the scattered fields. In the final step to calculate the scattering cross-sections from scattering amplitudes, two methods are compared. One is the method based on the definition and here the scattering cross-section is calculated from the integration of the differential cross-sections over the solid angle. The other is the method based on the elastodynamic counterpart of the optical theorem. It is verified that the results obtained from the elastodynamic optical theorem are accurate enough to evaluate the scattering cross-section for the crack in elastic solids.

The use of computer simulations is becoming an increasing popular strategy for designing ultrasonic inspections. There are many benefits of accurate simulations, the most important one being their cost effectiveness. In many cases, before inspection procedures are finalized, it is possible to simulate the competing inspection plans, and to use the outputs of simulation trials to choose the best plan. This strategy is particularly useful when there is limited accessibility to the components that need to be inspected, as in the extreme case when the inspection procedure requires that operating equipment be removed from service. In such cases, it is best to be fully prepared before taking the inspection equipment to the test site and computer simulations can play an important role in such preparation, often at a significantly reduced cost with respect to traditional methods.

Recently, the development of a novel ultrasonic inspection technique that detects radial fatigue cracks on the far side of so-called “weep” holes in thin airframe stiffeners was reported [1]. These cracks tend to be located on the upper part of the weep hole (at 12 o’clock position) therefore are not readily detectable by conventional ultrasonic inspection techniques from the lower skin of the wing. The new technique utilizes circumferential creeping waves propagating around the inner surface of the hole to perform the inspection. However, the wet wing has to be emptied and dried out before inspection because even a small amount of fluid fuel trapped in these rather small (approximately 6–7 mm in diameter) holes would strongly affect the propagation of circumferential creeping waves. We have searched the literature for published results on circumferential creeping wave propagation around fluid-filled cylindrical cavities in elastic media. Surprisingly, although the analytical solution of this canonical problem can be readily constructed from existing building blocks, very little was found in terms of numerical results that could be used to gain better understanding of the phenomenon. This motivated us to attack the problem by numerically solving the dispersion equation and constructing the corresponding dispersion and attenuation curves for a specific case of interest, namely, for that of a water-filled cylindrical hole in aluminum.

Wave propagation through particulate composites has received considerable attention in recent years. The dispersive wave propagation through particulate composites with both random and periodic distributions has been studied theoretically [1] and experimentally [2–7]. The response of a layered composite with a finite number of layers can be predicted using the acoustic complex-valued transfer functions for a single layer [8]. The effect of the in-plane structure of an inclusion layer and resonance of individual particles on the wave propagation phenomena have been studied [9]. For a single layer of inclusions, it was shown that the arrangement of the inclusions has a significant effect on a wave propagating normal to the layer. The objective of this work is to study further the effect of the in-plane structure of an inclusion layer and acoustical properties of individual particles on the wave propagation phenomena.

Composites are prevalent in high technology devices such as aircraft, computers, automobiles and communications systems. They improve brittleness and provide a lower density which enhances mechanical strength. Electron and light manipulating composites will be used more and more in the future. It is necessary to have a capability of inspecting composites, both to assure production quality and as a baseline for later NDE. In this paper, we present a study using wavelet, inverse neutron optics and the grazing angle neutron spectrometer, GANS, at the Missouri University Research Reactor, MURR.

Concern has been expressed about the capabilities of performing non destructive evaluation (NDE) of flaws located near to the outer surface in nuclear pressurized water reactor (PWR) vessels. The ultrasonic examination of PWR is accomplished from the inside with ultrasonic focused transducers working in the pulse echo mode. By recording the echoes as a function of time, the Ascan representation may be obtained. Many ultrasonic flaw detectors used for NDE are based on the simple Ascan concept involving measuring a time interval called “time of flight”. By combining the Ascan concept with synchronized transducer scanning, one can produce Bscan images that are two dimensional descriptions of the flaw interaction with the ultrasonic field.

Material property measurement is an important area of basic and applied research and can be defined as an inverse problem in which knowledge of the system input and output can be used to determine material properties.

On a mesoscale the pore structure in natural rocks is strongly inhomogeneous. With an increase of the scale size one may find that the pore structure has preferred orientation (texture) which leads to anisotropy of permeability and tortuosity. In this paper the Biot theory was applied to an orthotropic fluid saturated porous medium. Such a medium supports four different wave types: fast quasilongitudinal, two quasishear and slow quasilongitudinal waves. The properties of the orthotropic frame are described by the nine independent elastic constants of the dry frame. Pore structure characteristics such as tortuosity, permeability and shape factor become direction-dependent and in the coordinate system collinear with the acoustical axes each of these parameters is represented by a second-rank tensor with the only non-zero elements on the diagonal. Our results show that the velocities of quasilongitudinal and two quasishear waves depend mostly on the properties of the frame and are not sensitive to the permeability and tortuosity directly (the frame stiffnesses, permeability and tortuosity are indirectly related due to dependence on pore structure; however they can formally be considered as independent parameters). Thus by measuring these velocities one can determine the frame elastic constants. The slow wave velocity, on the contrary, depends mostly on the pore geometry. Its angular dependence in a water- or air-saturated solid allows us to recover the components of the permeability and tortuosity tensors. This approach opens new possibilities for determination of such characteristics of porous materials as preferred pore orientation and tortuosity which have been previously inaccessible experimentally and thus to retrieve information about the pore structure.

Nonspecular reflection of bounded beams has attracted interest since the work of Schoch [1] and Bertoni and Tamir [2] gave an accurate description of the nonspecular reflection of well collimated Gaussian beams from half spaces at or near the Rayleigh critical angle. Investigations on plates and half spaces concentrated on planar interfaces [3,4], while our previous work extended to the interaction of divergent Gaussian beams with planar surfaces (half space and plate) and of collimated beams with curved surfaces (solid cylinders and shells) [5,6].

The large difference in the ultrasonic velocity between the solid and the liquid phases of most semiconducting materials results in reflection/refraction of ultrasound at solid-liquid interfaces and an interest in using laser ultrasonics for sensing solid-liquid interfaces during single crystal growth. Using a ray tracing analysis, a set of measured ultrasonic time of flight (TOF) projection data can yield the ray paths connecting the source to the receiver, which can subsequently be used to reconstruct the solid-liquid interface. In previous work [1] 2-D wave propagation in cylindrical single crystal solid-liquid bodies was used to explore the feasibility of using ultrasound to characterize solid-liquid interfaces during vertical Bridgman growth of semiconductor materials. Detailed study of ray paths, wavefronts and TOF for ultrasound propagating in both transverse and diametral planes of liquid-solid single crystal (Ge) bodies was reported. Numerical simulations indicated that the magnitude and direction of the group velocity, the solid:liquid velocity ratio and the curvature of the interface together controlled the ray bending behavior and thus determined the ultrasonic data across the interface. Knowledge of ray paths at the interface enabled reconstruction of the interface using a small set of ultrasonic TOF’s.

Plane compressional wave propagation in a fluid is significantly affected by the shear viscosity of the fluid [1]. Several different theories have been developed [2,3,4] to understand effects of viscosity on wave propagation characteristics. Presence of concurring phenomena such as thermal conductive losses and molecular relaxations [5] has frurther complicated the study of wave propagation. In liquids however, the effect of thermal conductivity is not comparable to the viscous losses [1]. In such cases, it has been possible to associate viscosity with the wave propagation characteristics.

Linear viscoelasticity theory offers a minimal framework within which to construct a consistent, linear and causal model of mechanical wave dispersion. The term dispersion is used here to imply temporal wave spreading and amplitude reduction due to absorptive material properties rather than due to geometrical wave spreading. Numerical modeling of wave propagation in absorptive media has been the subject of recent research in such areas as material property measurement [1] [2], seismology [3] [4] [5] and medical ultrasound [6] [7]. Previously, wave attenuation has been included in transient finite element formulations via a constant damping matrix [8] or functionally in terms of a power law relation [9]. The formulation presented here is based on representing the viscoelastic shear and bulk moduli of the medium as either a discrete or continuous spectrum of decaying exponentials [10]. As a first test of the correctness of the viscoelastic finite element formulation, the finite element results for a simple hypothetical medium are compared with an equivalent Laplace-Hankel transform domain solution.

The use of guided ultrasonic waves to rapidly interrogate large structures is a topic that is currently receiving considerable attention. The purpose of this paper, and the companion paper by Alers [1], is to briefly review some past experience that may not be readily available to current researchers since many of the results were not presented in archival publications. The work described in this paper was conducted in the context of exploring applications of electromagnetic-acoustic transducers (EMATs) [2,3] as a part of the NDE effort at the Rockwell International Science Center in the period 1970–1980. In addition to the author, others playing key roles in various parts of this effort included G. A. Alers, R. K. Elsley, C. M. Fortunko, M.W. Mahoney and C. F. Vasile. The companion paper by Alers includes subsequent developments at the private company, Magnasonics, Inc. as well as more recent work at the National Institute of Standards and Technology. Although EMAT’s were used in all of this work as the sensors to excite and detect the guided ultrasonic modes, the basic ideas apply to the use of guided modes excited by any kind of sensor to scan structures.

A collection of fundamental experimental results on edge waves propagating along the edge of free and immersed elastic wedges and plates were presented at the Special Session entitled “Long-Range NDE Using Guided Waves/Extended Overviews” chaired by Professor S. Rokhlin. This paper summarizes the experimental results discussed on the effect of wedge apex truncation and water loading on the phase velocity of the slowest antisymmetric flexural wedge mode. The remaining results that were presented on plate edge flexural waves and edge waves along elastic wedges with range-dependent apex angle are described elsewhere [1–4].

Guided waves scattered from defects in solid material might contain significant information about those defects. The major goal of non-destructive evaluation is to interpret the ultrasonic signatures from these defects. Scattering of Lamb waves from a normal, surface breaking defect is considered in this study. Reflection and transmission factors of the scattered modes are used to reconstruct the size of the flaws. Scattering Lamb waves from a crack have been investigated in [1–7]. Resonance phenomena for a crack parallel to the plate surface is considered in [5,6]. In this study, the boundary element method combined with the normal mode technique [1,8] is used to study the relationship between crack size and the parameters of the incident mode. Irregular behavior of the reflection coefficient for special parameters of the incident mode is used for size classification.

The evaluation of adhesive bonds involves the detection of the continuity of the adhesive and also its effective elastic properties. There are several advantages in employing guided waves for interface testing: the intensification of the measured effects along the path of the wave propagation, the possibility of testing very thin adhesive layers, only one dimensional scanning is necessary and they are less sensitive to the variations in the properties of the adherends relative to the method of ordinary bulk ultrasonic wave. The impinging guided ultrasonic wave is scattered by flaws embedded inside the adhesive layer. By examining the characteristics of the scattered field from the flaw, it is possible to extract useful information about the flaw location and its dimension. This study presents a computational solution to the ultrasonic scattering of guided waves, in an adhesive layer, by a cylindrical flaw. The scattering field contains information about the integrity of the adhesive layer. We consider the case of an adherend-adhesive-adherend sandwich. The adhesion strength is assumed to be good and uniform.

Many airplanes, both military and commercial, have exceeded their original design service lives. For such aging structures, the presence of hidden damage can severely limit their performance. It is important to monitor the progress of damage to ensure the safety and integrality of these structures. Typical damage can be several types of corrosion and fatigue cracking. Lap-joints trap moisture and air and are initiation sites for corrosion. These sites are potential locations for wide spread fatigue damage. Generally, corrosion can cause material loss and thickness reduction, which can be detected by non-destructive ultrasonic methods. Conventional methods require point-by-point inspection, which is a time-consuming process. Guided waves, which direct wave energy in the plate, carry information about the material in their path and offer a possible more efficient tool for non-destructive inspection of material loss or thickness reduction. Using a pitch-catch technique, in which one transducer sends a guided wave in a plate structure and a second transducer picks up the signal at a different position, guided waves can be launched and detected to inspect plate-like structures, line by line, thus increasing the inspection efficiency by an order of magnitude. Also, when guided waves pass through a region with material loss, some energy would be reflected back. By studying the characteristics of the reflected waves, information about material loss can be obtained.

Grinding the surface of ceramic components inevitably introduces micro-damage in their near-surface region. Surface damage appears in the form of surface roughness, residual stresses, and distributions of microcracks. Microcracks generated by surface grinding are of three kinds: lateral, median, and radial cracks. Lateral cracks run parallel to the surface, and are responsible for material removal. Median and radial cracks extend normally to the surface into the bulk, with the former running parallel to the grinding direction, and the latter extending normally to it. Median cracks are the deepest of the two, and are responsible for degradation of the surface fracture properties. A method for the non-destructive characterization of surface crack distributions in brittle materials is, therefore, much needed.

The interaction of ultrasound with rough surfaces is being actively investigated, from both a theoretical and experimental standpoint. The problem is important to several areas, including ocean acoustics [1,2] and dielectric waveguides, such as optical fibers. In either of these problems the fields can be assumed to be represented by a single scalar potential. The problem we address here has previously received very little attention, and concerns the propagation of guided elastic waves in a planar solid waveguide having randomly rough surfaces with compressional and shear potentials that are coupled at each interface. The rough surface is intended to model incipient corrosion in aluminum aircraft structure.

Nonspecular reflection effects of a bounded ultrasonic beam incident from a liquid onto an elastic structure have been the subject of a great deal of interest during last decades for material characterization [1–10]. It refers to phenomena where the reflected beam has an intensity profile different from that of the incident beam, including a lateral beam displacement, one or several minimum intensity area and a trailing field (Fig. 1). This phenomena occurs when the incident beam is phase-matched to one of the leaky waves supported by the structure. Numerous theoretical studies, based on the calculation of the reflection coefficient, have successfully explained the nonspecular reflection profile of a bounded beam incident at a critical angle [1, 3, 6–10]. In this paper, we present a mode theory for analyzing these nonspecular reflection effects. This approach, which gives a good physical insight, has been recently used to study the excitation of Lamb waves by the a bounded beam [11].

The application of guided waves in NDT can be hampered by the lack of readily available dispersion curves for complex structures. To overcome this hindrance, we have developed a general purpose program that can create dispersion curves for a very wide range of systems and then effectively communicate the information contained within those curves. The program uses the global matrix method to handle multi-layered Cartesian and cylindrical systems. The solution routines cover both leaky and non-leaky cases and remain robust for systems which are known to be difficult, such as large frequency-thicknesses and thin layers embedded in much thicker layers. Elastic and visco-elastic isotropic materials are fully supported; anisotropic materials are also covered, but are currently limited to the elastic, non-leaky, Cartesian case.

Leaky Lamb Wave (LLW) has drawn interest from the research society for its application in the nondestructive evaluation of plate-like structures. Starting from isotropic plate[1] to plates with anisotropic materials such as fiberreinforced composites[2], the LLW problem increase complexity in a very fast manner. With the dielectric effect included in the LLW problem, a piezoelectric plate immersed in a dielectric fluid can be extended from the framework setup by the NDE LLW study. Among these studies, Nayfeh et. al.[3] investigated the influence of piezoelectricity on Lamb wave propagation. However only the mechanical effect of the fluid is treated in their study, leaving the dielectric effect with two extreme assumptions: either short-circuit or open-circuit condition. Later, Yang and Chimenti[4] modeled the piezoelectric LLW problem with fluid dielectric effect included and conducted extensive experiments[5]. The influence of the fluid conductivity on the wave propagation in a piezoelectric plate is the major interest in the current research. Theoretical modeling as well as experimental measurements are presented in this report.

Eddy current methods are a widely used technique in the nondestructive inspection of aircraft structures and parts. The method consists of inducing eddy-currents in the material being tested using a probe coil. The magnetic field produced by these eddy-currents opposes that of the probe coils (Lenz’s law) and the net effect is a reduced magnetic flux linking the coil. The presence of defects in the material under test disturbs the distribution of eddy-currents which in turn disturbs the net field. This change in the field is detected as a change in the impedance of the coil. The changes in coil impedance measured as the probe scans the specimen contitutes an eddy current defect signal.

In [1] we developed a volume-integral eddy-current model that is applicable to steam generator tubing. The model is now implemented in VIC-3D1, and in this paper we present some results computed with it.

Eddy current testing is a widely used nondestructive evaluation (NDE) technique in which flaw information is extracted from the impedance change of a coil placed above a metal testpiece. Typical applications of eddy current NDE are the inspection of heat-exchanger tubes in steam generators of nuclear power plants and detection of hidden corrosion in the lap-splices of aircraft skins. To obtain quantitative information about flaw size and shape, we would like to have a forward model which is able to predict the impedance change of a coil for different flaws in the test geometry. Analytical solutions exist for simple test geometry and flaws with good symmetry properties. However, for flaws with irregular shapes in 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 [1], the boundary element method [2], and the volume integral method [3–5]. Those numerical models can be used in a wide range of applications with moderately complex geometry. However, numerical models are inherently computational intensive and thus are not suitable for applications in which modeling speed has the first priority. One application of a fast forward model is to build fast eddy current simulators which can be used for educational purpose. Another application of the fast forward model is in the solution of the nonlinear inverse problem in which a large number of forward solutions must be computed.

The ability to model complex probes and simulate eddy current examinations is critical to our eddy current inspection system design efforts. The increasing demand for specialized probes creates the need to solve more complex problems than in the past. In this paper, we present details of an effort to improve our modeling capabilities by combining several techniques in order to provide greater flexibility in probe design. The result is a fast and efficient eddy current exam simulation capability for designing probes or probe arrays with complex geometry and material.

For the past several years, we have been developing an eddy current model, using the boundary element method (BEM). Last year, in particular, a BEM algorithm based on the Hertz potential approach was found and shown to be effective in dealing with complex part geometry, while keeping the computational resource requirement to a minimum [1–3]. This paper concerns a further extension of the model to include cracks.

Ferromagnetic steels usually have a high value of relative permeability which means that, for frequencies commonly used in eddy-current non-destructive evaluation, the electromagnetic skin-depth is typically much smaller than the depth of a crack. This behavior allows the use of approximate, thin-skin theories to describe the interaction between induced eddy-currents and surface-breaking cracks in steel. Thin-skin approximations are also appropriate for use with non-magnetic materials at sufficiently high frequency. Auld et al [1] treated the high-frequency, surface-crack problem in aluminium using a magnetic scalar potential formulation. The surface impedance boundary condition was applied to the field on the crack faces to obtain an expression for the impedance change in the coil due to the defect in terms of known field components. This approach was followed in a more recent treatment of the long, surface crack by Burke [2].

Eddy currents are used to inspect metals for small near-surface cracks and other defects. The eddy-current signal can be calculated quantitatively, at the cost of some effort, for non-magnetic metals at room temperature. Surprisingly, the same cannot be said for ferromagnetic metals [1]. Neither a quantitative nor a qualitative understanding exists for the change in the impedance when an air-core coil is placed next to an otherwise unspecified ferromagnetic metal. Nickel and iron are the ferromagnetic metals most commonly used in commercial applications. The authors are conducting an experimental/theoretical program aimed at developing a fundamental understanding of the swept-frequency impedance of coils placed next to thick plates of these elements. We start with commercially pure nickel.

Eddy current testing is commonly used for steam generator tubes inspection. The capabilities of detection have to be increased in order to detect smaller defects particularly on the outer surface.

Eddy current testing (ECT) is a type of non-destructive testing and effective for detecting surface cracks or flaws in conducting materials. A typical application of the ECT is non-destructive testing for heat exchanger tubes of steam generators (SG) of chemical plants and nuclear power plants. Higher accuracy is needed for external very small cracks from the practical point of view. Recently there are some requests not only for the location of cracks but also for the characterization of crack shape and the type of cracks. In order to improve the accuracy of the ECT it is considered to be important to use numerical analysis and to optimize the method of testing and the shape of probe coils. Since the electromagnetic phenomena in the ECT are 3D in nature, 3D numerical analysis is required in order to know eddy current distribution in the conductor and to improve the ECT technique. Therefore one of recent research interests of the ECT is the development of more effective and accurate 3D eddy current analysis.

Nuclear power plants supply about 80% of the total production of electricity in France. Non-Destructive Evaluation (NDE) is of prime importance in verifying the soundness of components such as the steam generator (SG), casted elbows, core, etc. In order to facilitate diagnostic, now mainly performed through signal processing and human interpretations, we attempt to image damaged internal structures of these components through data inversion. This NDE inverse approach, similar to medical imaging, was successfully applied to radiographic NDE of casted elbows. From a few number of projections (less than 10) and after rough localization of defects, our software SIROCCO3D is able to reconstruct 3D images of flaws using ART and markovian inverse algorithms [1].

Eddy current techniques have been widely used in the NDE inspection of aircraft engine components. Depending on the flaw characteristics and specimen composition, various EC probe designs have been employed to achieve the maximum probability of detection (POD). Traditionally, the effectiveness of a probe design for a given inspection is determined experimentally. In particular, parameters such as probe types, operating frequency, scan spacing, etc. are evaluated experimentally in terms of POD. It is obvious that this is a costly way of defining inspection parameters. A more cost-effective alternative is to evaluate the test parameters through the use of numerical simulation. This can be done by casting the entire EC inspection process in terms of a numerical model governed by a set of integral equations. By computing the solutions to the integral equations, outputs in the form of impedance changes due to flaws can be used to generate the POD. Previously, we have introduced a modified version of the Hertzian magnetic potential approach for eddy current probe design [1]–[3]. In those papers, it was shown that the formulation can be used to solve problems with arbitrary geometries including geometrical singularities such as edges and corners. In the present paper, we have modified the boundary integral equations (BIEs) formulation for computing the impedance change in the presence of ideal tight cracks. Some unique features of this model include the allowance for arbitrarily shaped air core probes and test components that include singular geometries.

Since the depth of surface-breaking cracks often determines the remaining life of a part, the characterization of surface-breaking cracks is a key problem in nondestructive evaluation. Eddy currents have been used for this purpose [1]. However, traditional eddy current methods have poor success when the dimensions of the crack are small compared with the inner radius of the eddy current probe. This is as expected, since the spatial resolution of an eddy current probe is comparable with its inner radius. Moulder et al [2] and Nakagawa [3] have described a new eddy-current based method of nondestructive evaluation, known as the photoinductive method. It offers greatly enhanced spatial resolution by combining the eddy current method with a laser. Images made with the photoinductive method provide a map of the surface-breaking portion of the crack and directly measure its length along the surface. In essence, the photoinductive method allows one to map the square, E · E, of the electric field, E, on the surface of the metallic part. In this paper, we develop and present a new method for estimating the depth and the shape of surface-breaking cracks in metallic parts from photoinductive data.

Our objective is to design a “forward model” for steam generator tube flaw characterization, using eddy current technique. An investigation of the existing forward models was made.

In eddy current testing of heat-exchanger pipes the signal of the scanning probe is usually presented in the complex plane as a Lissajous curve. The size (amplitude) of the curve corresponds roughly to the volume of the defect. The phase is related to the depth of the defect and its location (inside or outside defects). Finally, the shape of the curve depends on the form of the defect.

There are a lot of physical and technical problems that need to calculate photon density distribution functions for the photon flow passing materials of different type. This flow can be described by a type of transport equations similar to the Boltzmann equation. Notwithstanding the huge number of equation types depending upon the particularities of the photon-matter interactions, the main properties of their trajectories are similar. Therefore it is possible to find a generalized model description for the photon trajectories which can be used to solve a large number of other problems.

During the last few years some attempts were made to modify the approaches for 3D reconstruction algorithms, based mainly upon Radon theory, in case of extreme lack of data, i.e. limited number of projections and views. The inverse Radon transform is not applicable in this case because of the insufficiently filled Radon space. Then the interpolation of the data, which are absent in the unfilled back projection space, is unattainable. In this case, some kind of a priori knowledge or structural constraints is required to restrict the permissible solutions [1–4]. The classical regularization procedure, also known as Tichonov-Miller regularization [5–7], can be applied for the stated problem as used in some CT applications, e.g. [1], where the a priori knowledge is introduced by a special functional type, which allows to reduce the required number of projections to about 100.

Tools for computer simulation are helpful in NDE manyfold. A global conception of reliability in NDE can include among other procedures performance prediction by computer simulation. POD data of simulated defects can be calculated. Computer simulation gives the possibilities to optimize testing parameters, to make feasibility analysis for special testing problems, and to support the interpretation of testing results especially in case of complex component geometry. In this way simulation tools are valuable for planning and for evaluating of the examinations.

In radiography the value of each pixel is related to the material thickness crossed by the X-Rays. Using this relationship, a defect in an object can be located and furthermore characterized by parameters such as depth, surface and volume.

Recently, a novel approach was proposed which combines advantages of both pulse (PT) and modulated (MT) infrared thermography. In a nondestructive evaluation (NDE) perspective, the specimen is pulse-heated as in PT and the mix of frequencies of the thermal waves launched into the specimen are unscrambled by performing the Fourier transform of the temperature time evolution over the field of view. Of interest is the maximum phase image with many attractive features: deeper probing, less influence to surface infrared and optical characteristics, rapid image recording (pulse heating, surface-wide inspection), possibility to inspect high thermal conductivity specimens. The proof-of-concept of this new approach called pulse phase infrared thermography (PPT) was demonstrated earlier [1].

Infrared thermography is a powerful, fast and non contact method for non destructive testing of materials, based on heat conduction. While techniques like pulse or lockin thermography are based on external heat sources, other thermographic techniques, like vibro-lockin thermography, use internal sources of heat, where loss angle effect is involved. One advantage of these latter techniques is that some material features are selectively heated, so that the interpretation of the image can be straightforward. Another advantage is that the heat is generated directly on the defect, allowing to reach greater depths. Some interesting examples of thermographic inspection using ultrasonic heating source are here illustrated.

Pulse-echo thermal wave imaging is accomplished using a pulsed heat source (usually high-power flash lamps), an infrared (IR) video camera, and image processing hardware and software, all of which is controlled by a personal computer. The system has been described in detail elsewhere. [1,2] Figure 1 shows the thermal wave imaging system in operation at the FAA’s Aging Aircraft NDI Validation Center (AANC). As can seen from Fig. 1, the imaging head is hand-held. The computer, power supplies, etc., are located some distance away at the end of a fifty-foot long cable, the other end of which can be seen attached to the imaging head in Fig. 1. This same cable also carries the power for the flashlamps and the control signals from the computer. To make an image, the imaging head is held in place for three seconds. During this time, the flashlamps are fired, and a sequence of images is acquired and transferred to the computer’s hard disk. The head can then be moved to the next area to be imaged. An area of approximately a square foot is imaged at each flash by the system, so that wide areas of the aircraft can be covered very rapidly.

Lockin thermography is a remote nondestructive testing method, suited for the inspection of large surfaces. As the method is based on heat conduction inside the material it can reveal any variation of the material thermal properties: the heating source is modulated at the desired frequency and the surface temperature is analyzed in order to obtain the amplitude signal and the phase signal in relation to the heating source. The feasibility of phase angle images eliminates the need for homogeneous surface heating which would be difficult for large areas. Inspection and quality control of the big structures of an airplane are essential to assure integrity and safety. To demonstrate the applicability of lockin thermography for on field inspections, we present the results obtained on an airplane consisting mainly of GFRP (Grob 115). Though no special treatment of the surface has been performed, we could detect hidden structures and repairs.

Earlier [1] we described an experimental procedure aimed at examining the spatial and temporal distribution of energy dissipation during crack initiation and propagation in a ductile polymer. 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. In the present paper, we will present an analysis of the energy balance in such measurements.

The common feature of all previous photothermal studies of semiconductors is that only electronically homogeneous materials have been assumed. Correspondingly, both carrier lifetime (τ) and diffusivity (D _n ) values have been usually taken to be the bulk characteristics of semiconductor and no experimental works or theoretical models have addressed the problem of τ and D _n depth profile reconstructions. For continuously inhomogeneous solids the treatment of the thermal-wave propagation problem using a Hamilton-Jacobi formalism of the thermal oscillating field has been presented [1] and the concept of the thermal harmonic oscillator (THO) has been successfully used for the thermal difiusivity profile reconstruction [2,3]. The present work is the result of the realization that an electron-hole photoexcited plasma also behaves like a carrier density diffusive wave [4], and, therefore, it can be theoretically treated as a plasma harmonic oscillator (PHO).

The mirage technique (with a single modulated heating source) [1–3] has been successfully applied to study the thermal, optical and electronic properties of solid state materials. Its advantages of being nondestructive, noncontact and high sensitivity make it a powerful and versatile tool. In this paper, we propose a new technique which is the same mirage technique, but with a dipole source. It inherits the advantages of the traditional mirage technique but overcomes some of the shortcomings. The traditional mirage technique generally gathers data by positional scanning, which, in additional to being time-consuming, introduces noise associated with the mechanical movement and makes the analysis susceptible to the nonuniformity of the sample. The nonuniformity can be unevenness in optical properties, surface roughness, or simply grain boundaries. With a dipole source, it is possible to gather data by frequency sweeping. In doing so, the new technique is free from those shortcomings connected with positional scanning. Also, the use of a dipole heating source nearly doubles the signal magnitude with the same amount of unmodulated heating beam power. We use this technique to study the thermal properties of CVD diamonds, glass and silicon samples. The results show that this technique has capabilities of measuring thermal diffusivity with both good resolution and wide range.

Previously, some of the authors demonstrated a technique for parallel vector lock-in thermal wave infrared (IR) video imaging, using a patented technique. [1–3] The initial version [2] of the technique was restricted to the frame rate frequency of 15 Hz for the single-detector IR camera used for the experimental verification. In an application for imaging cracks in Cu microbridges, an alternative version of the technique [3] succeeded in achieving IR video lock-in imaging at frequencies up to 2 kHz. However, the latter implementation was seriously complicated by the complex timing pattern through which the pixels were acquired by the IR imaging system. The complication resulted from the fact that one needs to know the timing of each pixel, because the method assumes that the IR signal corresponding to each pixel of the image can be multiplied by the sine and cosine functions of the thermal modulation frequency, computed at the time at which the pixel datum was obtained. After multiplication, the results are accumulated, pixel-by-pixel, in two image buffers, which represent the two vector components of the lock-in image. If desired, these image buffers can then be transformed into magnitude (the square root of the sum of the squares of the components) and phase (the arctangent of the ratio of the two components) image buffers.

Acoustic emission (AE) measurements are used to assess the degradation of loaded structures with time. Structures are routinely monitored over long time scales by measuring the energy of AE events at one or more locations on the surface of the structure, a rapid increase in the measured AE activity indicating the onset of failure. Thus the measurements can be said to be comparative, being only relevant to the structure under test and not widely comparable with the responses of other structures. An improvement in the applicability of AE monitoring could be gained if the measured AE activity could be directly related to the source strength. It would then be possible to define a critical source energy in a similar way to that of the critical crack length used in fracture mechanics.

Plate elements are common engineering structures and it is important to monitor damage evolution in these plates to ensure the integrity of the structure. Due to stress concentration in the vicinity of existing defects such as voids, inhomogeneities and surface cracks, microfracture events may occur locally in a plate highly stressed before final failure takes place. As a result, the stored strain energy radiates out in the form of elastic waves which carry information regarding the nature of the source. Due to the highly dispersive character of Lamb waves, their forms change as the disturbance propagates away from the source, and understanding the characteristics of these waves is a prerequisite to extracting source information from measured waveform.

In earlier papers, Green [1–4], results have been presented which show the surface response of a cross-ply fiber composite plate due to buried impulsive sources. The sources considered were line loads and line double forces. The present paper extends these results for sources which are line couples and line double couples without moment. The impetus for carrying out this extensive study is the need for a thorough understanding of the nature of the stress waves generated as a result of internal impulsive events. This in turn is a prerequisite for the application of acoustic emission techniques which are now standard methods used for the detection of faults in engineering structures. This technique has the potential for not only locating the source of the emission but also for determining the nature of the source. The pulses arising from events such as crack formation, crack growth or the relative slip of crack surfaces are completely different in character and give rise to different signals at the receivers. By recording the time history of these signals it should be possible to determine the nature of the initiating event. This has particular relevance to the monitoring of laminated composites where there exists a wide variety of possible defects such as fiber breakage, matrix cracking, fiber/matrix debonding and ply delamination, in addition to the crack related events mentioned above. The surface response due to a number of different internal point sources in a plate of isotropic elastic material has been calculated by Ceranoglu and Pao [5] and by Vasudevan and Mal [6]. An analytical methodology for predicting the acoustic emission associated with crack propagation and arrest in an isotropic material has been developed by Jacobs et. al [7] and they have compared their theoretical predictions with experimental results. For fiber composite plates and laminates, Lih and Mal [8] have calculated the response of a unidirectional laminate to surface point loads and line loads, and the same authors [9] consider distributed surface loading on both cross-ply and quasi-isotropic plates. An experimental study of the relation between the received signals and the initiating events has been carried out by Ono and Huang [10].

As defense budgets are reduced, military aircraft are being flown well beyond their expected lifetimes. An example of this is the Boeing B-52 bomber. Introduced in 1954, the B-52 is still one of the Air Force’s primary bomber aircraft, as evidenced in Desert Storm. Currently, it is expected to fly until the year 2024, well past is original design goal. Because of this extended duty, fatigue crack growth in the airframe has become a concern, and methods to detect these defects are being implemented.

A measured acoustic emission signal depends upon three distinct factors: its source; component geometry and material properties; and receiving sensor and instrumentation. Each of these three elements must be understood in order to effectively identify and characterize measured acoustic emission waveforms.

The general practice of acoustic leak location relies on two different physical phenomena for determining source location: 1) reduction in signal amplitude with increasing distance from the source (attenuation-based methods), and 2) increase in signal transit time with increasing distance from the source (time-of-flight-based methods). The work discussed here describes efforts at ISU directed at gaining first-principle understanding of the underlying physical phenomena of multi-mode dispersion in fluid filled pipes and to developing time-of-flight source location data processing for such dispersive systems. Results are presented for work detailing the characteristics of pipe propagation, as well as the effect of those characteristics on cross-correlation analysis. Theoretical and experimental results are also shown for two approaches which potentially overcome the limitations of cross-correlation techniques.

The fracture behavior of concrete is often attributed to a fracture process zone. This fracture process zone manifests itself in the the form of nonlinear stress-strain behavior, post peak strain softening, size effect, and numerous toughening mechanisms. Features of the fracture process zone include arrays of microcracks, aggregate interlocking, crack bridging, and grain boundary sliding friction. From a material modeling standpoint, properties of the fracture process zone must be known in order to accurately predict the response of the material to stress. Since the fracture process zone characteristics are critical to material properties, a better understanding of those characteristics will lead to a better understanding of overall performance.

The acoustic emission (AE) source characterization has been developed to understand the dynamic process of microfracture in composites. AE signals can be represented as the convolution integral of the source function due to microfracture of materials, the dynamic Green’s function of the media and the transfer function of the measuring system. We developed the advanced analysis system to evaluate AE signals quantitatively. Source location of each AE is determined from the signals recorded using multi-transducers. Each dynamic Green’s function of the specimen is calculated by a finite difference method. The transfer function of the measuring system is calibrated by the breaking pencil lead method. Source components are determined by the developed deconvolution algorithm. Then fracture mode of microcracking is obtained from the moment tensor. This analysis system was applied to the materials testing in glass matrix composites and the fracture process in these materials will be discussed.

Acoustic emission (AE) consists of high frequency stress waves generated by the rapid release of energy due to fracture, plastic deformation, wear or interfacial friction [1]. Acoustic emission monitoring is a very sensitive method with a wide dynamic range and can be used as a diagnostic means of continuous assessment of damage in materials and components. Acoustic emission methods can be applied to metallic components and specimens subjected to monotonic or fatigue loading. In general, acoustic emission can be used to monitor crack initiation and propagation and to locate the source of the emission.

The French Aerospace company AEROSPATIALE manufactures high pressure tanks for helium gas storage. Because these tanks are critical elements for rockets and satellites, a new approach has been developed to have a better knowledge of the structure reliability. Although numerical tools such as finite elements codes are used for the design of such structures and. quality rules are imposed to insure that the tanks manufactured are in accordance to the definition, it is conceivable that even a successful proof test could actually damage the composite and lead to a residual SF less than 2.

In response to numerous releases of hazardous substances from leaks in underground storage tanks and pipelines, the EPA requires monitoring so that leaks are detected, located and repaired as quickly as possible. Acoustic leak location offers the possibility of locating leaks which have been identified by other methods but which are not appropriate for performing location. The successful application of acoustic leak location requires that existing data analysis approaches be improved so that the smallest leaks of interest be locatable with the widest possible sensor spacing. Part of developing such approaches requires that the physical conditions which affect the amplitude, frequency, and dispersion of the leak signal as it propagates between source and sensor be better understood.

Laser generated ultrasound has been used to determine material properties and to characterize material defects [1–3]. To a large extent, the success of laser ultrasonics has been the researcher’s ability to correctly predict the temporal evolution of the displacement waveform resulting from pulsed laser irradiation. Theories that assume isotropic elastic properties work well for crystalline materials that have grain sizes that are small compared to the wavelength of the interrogating ultrasonic wave [4–5]. For single crystal samples or carbon epoxies, the elastic anisotropic nature must be taken into account. Royer and Dieulesaint [6] have shown, using a plane wave analysis, that the behavior of single crystal materials in the presence of an ultrasonic disturbance differ markedly from their isotropic counterparts. In particular for cubic and tetragonal systems, Royer and Dieulesaint [6] demonstrated that the decay rate of the Rayleigh wave disturbance varies strongly as a function of the anisotropy factor.

The laser-ultrasonic technique is a promising tool in the field of the mechanical characterization of materials [1,2]. The use of a laser is associated to three temporal convolutions related to the source parameters (the spot size, the pulse duration and the wavelength of the excitation) [3,4]. For a given wavelength, the evaluation of the mechanical properties of low-damage threshold materials in the thermoelastic regime, requires a compromise between the space distribution and the pulse duration of the excitation. An enlargement of the spot or an increase of the pulse duration introduces delays on the arriving times of the acoustic waves and consequently affects the accuracy of the velocity measurement. In the case of a non-metallic material, a third effect related to the optical penetration might be considered.

The use of pulsed lasers to produce ultrasonic waves in materials has proven to be attractive in many applications. However, one of the limitations of laser ultrasonics is the weak signal strength produced by thermoelastic sources. One way to improve signal strength is to use laser intensities that are high enough to ablate the material surface. While ablation leads to surface damage in solids, it is generally not a problem in liquids. Consequently ablation is a viable means of enhancing signal strength for laser ultrasonics applications such as high temperature materials processing involving molten metals. Experiments carried out at the Idaho National Engineering Laboratory (INEL) on liquid mercury, using the experimental setup shown in Fig. la, indicate that the signal strength can be increased two orders of magnitude through ablation [1]. Mercury is a nearly ideal material with which to study ablation in molten metals because it is liquid at room temperature and its properties are well known. The results of the experiments carried out at the INEL are shown in Fig. lb. For laser intensities below about 6 MW/cm2 the ultrasonic signal results from thermoelastic sources (rapid thermal expansion) while at intensities above about 20 MW/cm2 ablation is the dominant mechanism. A transition between rapid thermal expansion and ablation occurs between 6 MW/cm2 and 20 MW/cm2.

The use of lasers as ultrasound sources is of interest in nondestructive materials testing, primarily because lasers provide a noncontact ultrasonic source. The generation and reception of ultrasonic waves has by now been widely demonstrated, as discussed in References 1, 2, and 3. Comprehensive numerical models are needed in order to quantify the waveforms generated by laser sources in the thermoelastic regime, and as an aid in designing nondestructive materials evaluation systems. Comprehensive models have been developed, for example by McDonald [4, 5], Schliechert et al. [6], and Spicer [7]. These numerical models have been experimentally confirmed by a number of researchers, including Schliechert et. al [6] and Spicer [7].

Surface and plate waves are commonly used to nondestructively inspect the near-surface region of a solid component for cracks and other defects due to, for example, structural fatigue. One particularly attractive method of generating and detecting such ultrasonic signals is laser based ultrasonics (LBU) [1]. In particular, because it is non-contact (i.e., does not require couplant), LBU can be implemented for inspection of limited access components using optical fibers, requiring only a small cross-sectional area for access. An example can be found in the inspection of internal surfaces of an aircraft wing as shown in Figure 1 where a contact method would obviously be difficult to apply. Furthermore, in cases where extremely high sensitivity is required, bandwidth reduction can be employed by concentrating the laser generated signal into a narrow frequency band.

The proportion of composite parts used in new Air Force aircraft is increasing significantly with each airplane that is designed [1]. In addition, composite parts are also being used for critical applications, where the loss of the part could cause the loss of the plane. These parts are susceptible to delaminations, disbonds, and impact damage. As part of its mission to overhaul aircraft, the Sacramento Air Logistics Center needs to be able to efficiently validate the integrity of composite aircraft parts.

State-of-the-art integrally stiffened composite materials, manufactured for use in the next generation of commercial and military aircraft, are being increasingly used for structural components such as wings and fuselages. However, due to the complexity of the manufacturing process, small variations in the shape of integrally stiffened composite structures often occur. Thus, a prioriknowledge of the part shape often does not provide sufficient tolerance to allow an automated conventional ultrasonic inspection. Many of the advantages of laser-based ultrasonics, including its noncontacting nature and applicability to rapid scanning of contoured and integrally stiffened structures, have been described previously [1–5]. To further extend the utility of laser-based ultrasonics, enable limited access inspections and also provide an upgrade/retrofit path for existing ultrasonic scanning systems, it is desirable to reduce the size of current laser-based ultrasound (LBU) system scan heads and provide both generation and detection laser beam delivery via optical fibers. A promising approach is the use of a scanning head based on a Cassegrain optical collection system. This approach minimizes the load carrying requirements of the scanning assembly and is also well-suited for integration with fiber optics to allow the delivery and reception of the ultrasonic generation and detection laser beams via long lengths of optical fiber. This provides increased mobility of the LBU scan head and allows the ultrasonic generation and detection lasers and other sensitive equipment to be housed in a clean environment which potentially can be located hundreds of meters from the inspection area. The use of a pulsed CO₂ laser has been reported previously for generation of ultrasonic waves in composite materials [4]. However, the CO₂ laser wavelength (λ = 10.6 μm) and the high peak power laser pulses precludes the use of fiber-optic beam delivery over all but very short lengths (< 1.5 m) of specialized optical fiber. Consequently an alternative generating laser has been sought that can be transmitted efficiently over standard quartz optical fiber. An alexandrite laser, which is tunable over the 720–800 nm wavelength range, is being investigated for this application. Progress towards the implementation of a fiber-based LBU system for rapid NDE of large-area composites, and the use of an alexandrite laser for ultrasonic generation in composite materials are described below.

A laser-based system was used to study thermoelastic (TE) and ablative (AB) ultrasonic generation mechanisms in graphite/polymer composites. When laser pulses of sufficiently low energy are incident on the sample, elastic waves are generated by the thermal expansion which accompanies absorption of the laser energy. At higher amplitudes, where the laser energy is high enough to melt and vaporize material in the sample, elastic waves are also generated by an ablative process. Thermoelastic and ablative ultrasound generation are described in the literature [1]. A review article by Srinivasan [2] includes excellent photographs of ablation of pure polymers.

In the previous year, results were presented in which the viscosity of calibration liquids were determined by measuring the reflection coefficient of laser generated shear waves. [1] The shear waves were launched with a pulsed Nd: YAG laser into an aluminum wedge and detected using a piezoelectric transducer. This year results are presented from a totally noncontact system, generating the shear waves with a pulsed laser and detecting the reflected shear waves with a laser interferometer. The design of the wedge was modified so that the shear waves are incident to the solid-liquid interface at nearly normal incidence. They reflect off this interface and are incident on the surface of detection at greater than the critical angle. This allows for the largest possible out-of-plane displacement, which can then be detected with the interferometer. This type of arrangement has been used with both aluminum and graphite wedges.

The strength of concrete is controlled by three factors: strength of the matrix, strength of the aggregate, and strength of the bond between matrix and aggregate. It is well known that material interfaces play an important role in the overall mechanical behavior of bonded materials e.g concrete. Reflected and transmitted ultrasonic waves have been used to develop non-destructive ultrasonic techniques to examine the size and location of flaws and to provide measurements of the internal structure of concrete among other things. In the studies of elastic wave scattering by interfaces, it is generally assumed that the inclusion is perfectly bonded to the surrounding matrix material. It does, however, frequently happen that the bond is imperfect whereby reducing the stiffness and hence the strength of the material.

The standard for sensitive detection and resolution of defects in metal components is scanned focused immersion inspection to produce C-scans. Laser ultrasound, although successfully applied to composite inspection, has previously not produced comparable results in this arena.

Laser ultrasonic generation and detection systems have been shown to be effective in the inspection and evaluation of both metals and composite materials [1–3]. Advantages of these noncontact systems include rapid scanning capability, the inspection of parts with complex geometries, and the ability for use in hostile environments. Unfortunately, laser ultrasonic systems are somewhat less sensitive than conventional contact piezoelectric systems. In order to increase the sensitivity, careful consideration must be paid to the choice of both generation and detection laser systems. Although the sensitivity of current laser ultrasonic systems has been shown to be sufficient for several applications, small improvements may allow for a more wide-spread use.

The optical detection of transient surface motion has many practical applications which include, in particular, the vibration monitoring of engineering structures (aircraft, power plants,...) and the detection of ultrasound produced by piezoelectric transducer or by pulse laser excitation. This last application where ultrasound is generated and detected by lasers, presents many advantages over conventional piezoelectric based methods. First, laser-ultrasonics is a remote sensing technique. Consequently it can be used, for example, for inspecting hot materials and products moving on a production line. Second, surfaces of complex shapes can also very easily be probed. For many applications, these advantages compensate the usually lower sensitivity of the laser-based technique compared to piezoelectric transduction.

The important benefits of quantitative nondestructive evaluation can be realized by a manufacturer if the system is fully compatible with the manufacturing environment and cost effective. In many cases an in-line real-time process control would be desirable. There have been a variety of ultrasonic inspection techniques which have successfully met the aforementioned conditions. These techniques are able to capitalize on a large body of established ultrasonic methods and signal analysis. However, there are environments and processes which cannot use these conventional procedures because they require the part under inspection to be either in physical contact with the transducer, via an epoxy, gel or fluid couplant, or maintained in close proximity to the transducers. These requirements may not be compatible with processes in which the part is at elevated temperatures, incompatible with the fluid couplant, physically remote or moving in a manner which adversely effects the required spacing. We present a robust ultrasonic technique which can work in these environments utilizing a laser-based ultrasound sensor incorporating photo-induced emf detection and a time-delay interferometer.

Acousto-optical measurement inside transparent media is a well known non invasive method for acoustic fields probing. The optical index variations induced by acoustic wave propagation in a liquid or transparent solid can be detected using several techniques: optical deflection, diffraction methods, etc ... Last year, X. Jia, G. Quentin and Laszlo Adler [1] described the ability of a interferometric detection technique to provide local and quantitative measurements of pressure fields in water as well as dilatational fields inside transparent solids. This method combines the acousto-optical effect with heterodyne interferometry. It measures the phase shift of a laser beam passing through an ultrasonic wave. The interferometric detection provides a local measurement and is sensitive not only to the amplitude but also to the phase of the acoustic wave. Finite amplitude waves in liquid, [1], bulk waves in solids [1] as well as guided waves (Rayleigh waves [2], Lamb waves [3] and Interface waves[4]) has been successfully detected using this method.

The nano — sized materials are the advanced materials developed in the eighties[1]and being called nanocrystalline materials, ultra — fine grained materials or nanophase materials. Because there are a lot of interfaces within the nano — scaled materials, the volume fraction occupied by the interface is comparable with that of particles. The particle size effect and disordering effect of interface exist in the materials. They are referred to have“gaslike” structure. So the nano — sized materials have a number of advantages excelling to the traditional materials properties. Many new phenomena have been discovered from the investigations of their optical and electric properties. However few works are related to their mechanical and ultrasonic properties.

Q-switched lasers are often used as a non-contact ultrasound source in non-destructive evaluation (NDE) of materials [1]. Q-switched lasers typically have ns pulse durations and generate broadband ultrasound waves, though longer laser pulses, of 100 microseconds or greater, have also been used [2] for NDE. These longer pulses tend to produce somewhat lower center frequencies than do Q-switched pulses, though they are still a broadband source. But it would be desirable in some NDE applications to narrow the signal bandwidth to improve the signal to noise ration (SNR), and also to have direct control over the center frequency of the generated ultrasound. In principle, this may be achieved by temporal [3,4] or spatial modulation [5,6] of the laser pulse, or both [7]. The purpose of this work was to develop a numerical model of a single, temporally modulated laser source of ultrasound in the thermoelastic regime, for isotropic metals.

In general, an interferometer may consist of coherent source split into a’ signal’ beam and a ‘reference’ beam, with irradiances I₁ and I₂, respectively. The signal beam carries information about surface/thickness of the test object. By bouncing the signal beam off of the test object (highly reflective objects) or passing it through (transparent objects) the object, the phase of the signal beam is altered according to the surface (or thickness) variations of the object.

Electronic shearographic interferometry is a nondestructive evaluation (NDE) technique in which qualitative detection of subsurface defects is readily achieved. In both industrial and laboratory environments, various full field stressing methods, including vibration, vacuum, thermal and mechanical loading, have been employed to produce characteristic deformations which can be monitored shearographically [1,2]. However, quantitative measurements of parameters such as defect depth are difficult to make with these techniques. This paper presents the results of using controlled thermal stressing with shearography in an effort to expand the quantitative capabilities of the technique. The use of controlled thermal-stressing allows a totally noncontact inspection technique with a large standoff distance to monitor the time-dependent deformations of test specimens. Typically laser power levels of tens of milliWatts are sufficient to generate measurable deformations.

The SAW corrosion sensor has many potential applications in both mechanical and electrical systems. For example, it could be used to monitor the corrosion of electrical contacts found in power systems. An early detection device would allow for preventive actions in order to avoid the high cost of down time or equipment damage that can result from electrical failures. The SAW sensor could also be used in mechanical systems, such as aircrafts, automobiles and spacecraft to continuously monitor the corrosive activity and determine when structural elements may need to be replaced.

Fiber optic sensors have emerged as important sensing devices for the detection of a broad range of physical parameters. In many applications they offer a number of advantages over traditional sensing elements, including small size, light weight, immunity to electromagnetic interference, and the ability to operate at elevated temperatures [1]. Fiber optic sensors when configured for the detection of ultrasound have potential for use in nondestructive evaluation applications. However, a drawback associated with fiber optic ultrasound sensors is their lower sensitivity when compared to traditional piezoelectric transducers. In this work, the sensitivity of an embedded fiber optic Fabry-Perot ultrasound sensor has been investigated.

Quantitative nondestructive evaluation of structures is achieved by this remote sensing technique which measures the deformation vector components of a surface in all 3 dimensions. Traditional techniques involve predominantly the application of strain gauges for the two in-plane deformation vector components. This technique unfortunately only reveals the in-plane deformation between two distinct points where the strain gauge is glued onto the surface. Interferometric techniques on the other hand are most common to measure the out-of-plane deformation component. Conventional film or thermoplastic holography is one of the most sensitive among those techniques measuring out-of-plane deformations in the micron and submicron range. One major disadvantage of this technique is the low time-resolution due to the fact that it involves the processing of photographic or thermoplastic film.

Open planar resonators like single and stacked microstrip resonators were used in the past for the measurement of dielectric constants and thicknesses of lossy and lossless dielectrics at microwave frequencies [1–3]. With a large width, the microstrip resonator effectively acts as a planar antenna in which case the fringing field is significant for the two slots at the two ends of the resonator and the resonator Q-factor is low. One of the limitations of the microstrip resonator is its spatial resolution which is determined by the size of the resonator. It is envisaged that this problem can be overcome by the use of planar slot resonators, Fig.1. Furthermore, compared to the microstrip-fed microstrip resonator, a microstrip-fed planar slot resonator would provide a better isolation between the feed and the material under test.

The construction industry has a keen interest in using a nondestructive, real-time, reliable and inexpensive technique for the in-place evaluation of the compressive strength of concrete structures. Compressive strength of concrete is usually determined by drilling a core and testing it in a laboratory. This method is relatively expensive, and it may take a few days for the results to be known. In addition, this method is destructive. Consequently, several nondestructive techniques have been developed for this purpose. These include: pulse velocity method, surface hardness, penetration, pullout, breakoff and maturity techniques [1]. The major disadvantage of these techniques is their limited accuracy, and the fact that they are not totally nondestructive.

Microwave nondestructive testing (NDT) techniques offer an alternative to other conventional NDT methods. Microwave/millimeter wave techniques (which roughly cover 0.3 to 300 GHz) are particularly useful for examination of dielectric composite materials because their low dielectric losses provide good depth of penetration of electromagnetic radiation in this band [1–4]. Conventional NDT techniques, such as high-frequency ultrasonic testing (UT), are associated with limitations, e.g., large variations in elastic properties of low-density composite materials, that make interpretation of complex UT signals difficult. Furthermore, the criticality of coupling a transducer to a sample surface limits the use of such techniques for on-line applications. High-frequency microwave (millimeter waves, 30–300 GHz) systems, when compared to their low-frequency counterparts, offer higher resolution and sensitivity to variations in dielectric properties of low-loss composites. Moreover, higher frequencies allow utilization of more compact systems, which are often important for practical applications.

Near field imaging with open ended waveguides has found increasing interest [1–3]. The basic idea is to scan across samples with a waveguide transducer as a reflection near field probe in order to characterize material properties and image defects that are much smaller than the wavelength. The reflection will form in the waveguide a standing wave where amplitude and phase depend on local intensity and phase of reflection. These effects can be demonstrated with slotted line measurements of the standing wave pattern. For example the investigations in figure 1 show the standing wave patterns for homogenous material and for a hidden hole. Depending on the material properties there is a change in phase and magnitude of the standing wave.

Microwaves have been shown to be able to detect surface breaking hairline cracks on metal specimens [1]. A microwave signal is typically fed through a rectangular waveguide probe. The incident and reflected signals in the waveguide form a standing wave, whose characteristics change depending on the relative position of a crack and the rectangular waveguide probe. Two separate electromagnetic models have been developed to mathematically predict the crack characteristic signal, i.e. the variation of the measured standing wave in the waveguide when it is scanned across a surface breaking crack. These models can be used to optimize measurement parameters. One model uses a mode matching approach, whereas the other model involves a moment solution approach. This paper presents a comparision of these two methodologies, which demonstrates the major advantages in the use of a moment solution approach. The most important result shown is that a moment solution approach is more general, eliminating the destinction between a crack being at the edge or in the middle of a rectangular waveguide probe. The convergence behavior is also studied for both methodologies. Faster convergence is observed using the moment solution approach. Finally, a moment solution approach allows for an easy expansion of the electromagnetic model to the analysis of finite cracks, and can be more readily expanded to encompass covered cracks as well.

Microwave sensors have been adapted to measure and create an image of the complex permittivity of lossy sheet materials non-destructively. The intended application is a portable probe which can map near-surface dielectric anomalies in various types of partially conductive sheet materials commonly found in aerospace equipment. Examples abound: radar-camouflage panels and textiles, coated canopies and moisture in non conducting composites. The need for such a probe originates in a number of different ways: The radar signature of low-observable panels may be altered by any one of a number of different types of damage occuring in flight or otherwise.A canopy has an encapsulated thin metal film which is not accessible to a 4 point probe for quality assurance.The surface temperature of a high-performance aircraft can rise above 300 •F in flight. Therefore, absorbed moisture in sufficient quantity can vaporize and cause structural damage and/or degraded radome performance. In general, moisture in composites can cause swelling, blistering, microcracking, reduced glass transition temperature, exacerbated repair problems, and increased weight and possible imbalance of the aircraft.

The determination of surface roughness is an important measurement in many manufacturing processes and corrosion detection schemes. These applications vary widely from the detection of surface weathering and unwanted wear of our national monuments to the detection of corrosion under insulation on the interior of aircraft skins. Besides the need for nondestructive surface inspection techniques there is also a need to monitor surface roughness during processing. One example of this is the molten alloy spray forming process where the roughness of the freshly sprayed surface is considered one of the best indicators of material end quality.

Ultra wide band (UWB) radar holography is a unique technique developed at PNNL for the U. S. Army to obtain nearfield“3-D” images and scattering characteristics of full-size vehicles in the field and at production line facilities. The extremely high-resolution imaging capability of this technique maps a vehicle’s scattering areas and identifies the“hot spots” which dominate the far-field signature to the enemy’s radar receiver. The combination of generating near-field high resolution images and RCS measurements with the same data provides a very efficient LO signature reduction tool for the Army, Air Force and Navy.

A microwave impulse radar system for nondestructive evaluation (NDE) and inverse scattering imaging has recently been developed at the University of Illinois [1,2]. This automated time-domain ultra-wideband radar system consists of a Pi cosecond Pulse Lab (PSPL) 4050B step generator, a PSPL 4050RPH remote pulse head, two PSPL 5210 impulse forming networks, a Hewlett-Packard (HP) 54120B digitizing oscilloscope mainframe, an HP 54121A 20 GHz four-channel test set, a broadband Vivaldi antenna [8] array, two ultra-wideband amplifiers and two mi crowave switches. The system is automated and controlled by a computer via the IEEE-488 bus. The performance of this impulse radar NDE system will be eval uated by collecting useful measurement data from different test targets including metallic and dielectric objects. Two iterative nonlinear image reconstruction algo rithms, the distorted Born iterative method (DBIM) [3,4] and the local shape func tion (LSF) method [5,6,7], are used to process the time-domain measurement data to reconstruct the image of the target object. Our goal is to combine the impulse radar system and inverse scattering imaging algorithms to provide an NDE system with high resolution imaging capability for civil structures.

In developing a system for high-speed, high-resolution, large-area, ultrasonic scanning of steel rubber composite material, higher resolution was required of the imaging system than was possible given transducer constraints. Constraints on spot size were imposed by the operating conditions and design of the focusing hemispherical transducer (FHT) used. To enhance the resolution of the transducer, deconvolution of the acquired signal and the transducer’s point spread function (PSF) is performed. Specifically, for a transducer pair operated in through transmission conditions, the resolution of the receiver is enhanced by a deconvolution of the receiver’s PSF. This paper presents a theoretical review of this concept with results of numeric simulation and experiments showing resolution enhancement by deconvolution of an experimentally recovered PSF.

Ultrasonic NDE images are often contaminated with speckle noise. The degradation caused by the presence of speckle noise makes it difficult to identify features of interest that are typically thin or small in nature. A variety of techniques have been proposed to date for reducing such noise. As an example, lowpass filters can be employed to reduce speckle noise. However, they tend to blur thin features and edges. Median filters are also used widely to remove impulse type noise while preserving edges in images [1]. Unfortunately, such filters perform poorly when the spatial density of the noise is high [3,6]. As an alternative, gray-scale morphological approaches involving such operations as opening, closing or combinations thereof can be applied to reduce noise in gray-scale images [1–5]. Even in this case, features that are thin or small tend to be filtered out along with the noise [6]. Prior attempts to remedy the problem have relied on the use of multi-resolution (or multi-scale) morphological filters using an array of structuring element sizes. Such algorithms tend to be overly complex and computationally expensive to implement [6].

Gas transmission pipelines are routinely inspected using a magnetizer-sensor assemblage, called a pig, which employs magnetic flux leakage (MFL) principles to generate defect signals that can be used for characterizing defects in the pipeline[1]. Previously reported work[2] demonstrated that radial basis function(RBF) networks[3–5] can be employed to characterize MFL signals in terms of defect geometry. Further development of this research work, related to three dimensional defect characterization are reported elsewhere in these proceedings. This paper presents an alternate neural network approach based on wavelet functions to predict three dimensional defect profiles from MFL indications. Wavelet basis function neural networks are comprised of a hierarchical architecture and are capable of multiresolution functional approximation. They offer a powerful alternative to RBF based signal-defect mapping techniques, in that the level of output prediction accuracy can be controlled by the number of resolutions in the network architecture. Consequently, the network itself can be employed to generate measures of confidence for its prediction. Such confidence factors may prove to be extremely useful in pipeline inspection procedures since they can form a basis for subsequent remedial measures. The feasibility of employing a wavelet basis function network for characterizing defects in pipelines is demonstrated by predicting defect profiles from experimental magnetic flux leakage signals.

Recent inservice inspection experience, round robin tests of ultrasonic inspection reliability [1] and calculations of flaw detection reliability necessary for specific nuclear power plant applications have consistently shown the need to improve the reliability of ultrasonic inspection. The need to improve ultrasonic inspection reliability is further emphasized when one reviews the pass rates for performance demonstrations specified by ASME Section XI Appendix VIII.

Interest in data fusion techniques have been growing in recent years due to the belief that a single NDE measurement may often be inadequate for providing sufficient information about the state of a test specimen. A variety of data fusion approaches have been proposed for combining results obtained by different methods, as well as different sensors, to provide comprehensive information about the material under test [1–4]. Techniques proposed to date range from blind superposition to approaches that involve the use of statistical and AI methods [5–7].

The use of laser-based ultrasonics in the testing of materials and structures offers various advantages over more traditional ultrasonic methods, but is often less sensitive when applied to real materials. Although high energy laser pulses can generate large ultrasonic displacements, nondestructive evaluation requires that the ablation regime be avoided, thus limiting the amount of optical energy which may be used. For this reason, signal processing of laser generated ultrasonic waveforms detected using laser interferometers may be required to extract the desired information from a nondestructive laser ultrasonic test. A model-based signal processing technique offers a way to enhance the signal-to-noise ratios significantly for ultrasonic waveforms obtained using laser-based systems with the generation of the ultrasound occurring in the nondestructive thermoelastic regime.

Ultrasonic weld inspections are typically performed manually, which require significant operator expertise and time. Thus, automation of ultrasonic data analysis is an important area of current research in NDE. There is a need for automated data analysis schemes capable of handling imprecise data and providing results in real time. This paper presents a combination of neural networks and fuzzy-logic to automate different aspects of ultrasonic data analysis. Neural networks automate learning, and hence are best used when the relationship between the input space and the output space is highly nonlinear or unknown. The relationship between ultrasonic A-scan signal characteristics and defect class producing the signal is not straight forward. In this work a multi-layer perceptron is used for defect classification. Results form different feature extraction schemes icluding an unique combination of time- and frequency-domain features is presented. Fuzzy-logic automates knowledge representation using a fuzzy rule base. Hence, fuzzy-logic is best applied in situations where a knowledge base exists in the form of IF-THEN rules In this paper fuzzy-logic is applied to accept/reject criteria for weld evaluation. The advantages of using fuzzy-logic over traditional Boolean tree-based algorithms, for this application, are discussed.

This work is a first step in detecting and characterizing defects in an automatic way by using artificial intelligence. Transient thermal NDE by IR thermography is the method used for such a purpose. Data are processed by Neural Networks.

Pulsed eddy current and swept-frequency techniques have both been previously applied to thickness measurement and conductivity profiling of layered metallic samples. Methods of inverting measured signals for thickness measurements and conductivity profiling, in both cases, typically involve iterating with a forward-model until the measured signal value is reached. These inversion methods are extremely time consuming due to the iterative approach. More recently, fast, feature-based methods for conductivity and thickness estimation have been reported. However, these methods involve manually constructing a comprehensive look-up table.

The Electric Current Injection (ECI) method of nondestructive evaluation is applied to materials that are electrically conductive but not magnetically permeable, as aluminum, magnesium, and titanium. It consists of detecting current-flow anomalies due to voids, nonmetallic inclusions and open cracks in the sample, through distortions introduced in the magnetic field generated by the plate [1]. We have applied an ECI method, with low dc current levels, to aluminum plates with circular voids in the millimeter range. These plates have already been measured with a SQUID magnetometer, with large lift-off distances [2], being necessary the use of image enhancement techniques, in order to improve the visual detection of the flaws [3]. In this paper a small lift-off distance was used, so it was possible to detect the associated magnetic field with a fluxgate magnetometer without further processing. It is proposed a method for the automation of magnetic signal analysis using Time-Delay Neural Networks (TDNN), which exempts the need of a trained technician, necessary to most of the usual NDE methods. In this method a TDNN is trained to dynamically scan the magnetic signals, automatically locating the voids and classifying their diameters. Next section depicts the experimental setup, and presents some of the magnetic signals obtained. After that, the neural network used to detect and classify the defects are explained, and the results of this technique are shown.

Magnetic Flux Leakage (MFL) methods are used to detect localized phenomena such as surface or sub-surface cracks in ferromagnetic materials [1]. A dc magnetic field is induced inside the sample being tested, and the distribution of the resultant lines of magnetic flux is determined by the values of magnetic permeability within the region of interest. Characteristically, the magnetic flux“leaks” out of the object in the region of a defect, allowing it to be detected using some kind of magnetic sensor.

Ultrasonic immersion methods have been used for nondestructive evaluation of material properties as well as for B- or C-scan imaging of internal microstructure and/or defects. However, the direct application of this method is difficult for dissolving or hygroscopic materials as well as rusting metals. Thin waterproof coating on such materials enables us to apply immersion methods for those materials and it is also effective for reducing reflection at the water/material interface, if the coating has an appropriate acoustic impedance.

The present research concerns automatic control of the mechanical quality of a weld on a circumferential component. A method of image processing is developed that searches for the limits of the weld melted zone for each angular position, so as to extract the evolution in terms of the main geometric characteristics of the zone (penetration depth, etc...). It is an image segmentation operation since it achieves the extraction of useful elements for interpretation.

Eddy current imaging has been used in eddy current nondestructive testing in order to identify small surface flaws in conducting materials [1–5]. However, the conventional pancake coil probes provide blurred flaw images because the eddy current in the test material induced by the probe spreads over a larger area than the coil size. The authors first applied popular deconvolution method to blurred ECT flaw images to get clear flaw images. However, it did not work well because a small drill hole signal does not correlate linearly to slit flaw signals. So the authors have devised a new ECT method utilizing a tangential coil and computerized tomography inversion technique in order to obtain clearer ECT flaw images.

The following work is part of an European collaborative project (RADICAD: BRITE/EURAM project BE 7231) whose aim is the 3D characterization of defects in mechanical parts by multi-radiography using computer aided design (CAD) models.

Piezoelectric transducers are the basic tool for ultrasonic NDE applications and are commercially available in a variety of sizes. shapes and frequencies. However. the information that is typically provided for these transducers by vendors is of limited value. This is especially true when it comes to testing anisotropic materials. The wave propagation effects such as beam skewing. splitting and distortion. when taken into experimental consideration for anisotropic media. requires knowledge of the spatial radiation field distribution of the transmitting transducer. In this paper, a previously developed methodology to map the radiation fields for piezoelectric transducers [ 1 ] is applied to imaging (mapping) three dimensional radiation fields generated by transducers of various shapes and frequencies placed in contact with anisotropic propagation media. The processed data files are calculated for transversely isotropic materials via the Generalized-Point-Source-Synthesis (GPSS) method [2,3]. The material is regarded to be homogeneous since the ultrasonic wavelength is larger than the grain dimensions. A computationally efficient version of GPSS is used for modeling the quasi-longitudinal (quasi-pressure) wave [4]. Emphasis is placed on these waves, since they are of particular interest in nondestructive ultrasonic testing of austenitic weldments. The synthetic data is transferred to an imaging workstation. where the 3-D transducer fields are displayed using commercially available graphic packages. The fields of circular transducers of various frequencies are shown for different austenitic weld material configurations. For reference purposes, fields in an isotropic base material are also shown using the same transducers. In closing, the implications of such 3-D mappings are discussed.

In modeling ultrasonic immersion inspections involving complex geometries, one of the more difficult tasks is to predict the effect that a curved surface has on a beam of sound as it propagates from the fluid into the solid. In this paper, we will consider a hierarchy of models for this problem (see Fig. 1).

In ultrasonic nondestructive testing (NDT), configurations of immersion techniques where transducers radiate through non-planar interfaces are often encountered, e.g., pipe inspection where the probe can be scanned either inside or outside the pipe. When local radii of curvature are far larger than typical wavepaths in the coupling fluid and into the piece, field predictions can often be made assuming a plane interface. For smaller radii, such an approximation is not valid.

Models of ultrasonic phenomena based on integral representations involve evaluation of integrals of the form (1)$$\int_D {A\left( s \right)\exp \left( {if\left( s \right)} \right)ds} $$ where A(s), f(s) are amplitude and phase factors, respectively, s is of arbitrary dimension, and D is either finite or infinite. Formulations yielding such forms include Fourier transform integrals and reciprocal boundary integrals employing Green functions. Various techniques for evaluation of such integrals have been developed and employed over the past few decades in research studying ultrasonic phenomena. In past work, emphasis was often placed on performing one-time computations to gain understanding of physical phenomena, thus issues of computational efficiency were of secondary interest. In the on-going NIST-funded initiative yielding the work reported here, emphasis is on the incorporation of such computation into ultrasonic measurement simulation software for general distribution within the engineering community, to be accessed as a routine engineering tool. In such usage, computational efficiency becomes an issue of very practical concern. For this reason, computational methods employed in evaluating expressions of form eq.(l) are being carefully re-considered. Work here reports on a new, efficient algorithm for such purposes.

Focused ultrasonic transducers can be useful in material characterization and tissue property measurement because of the improved signal to noise ratio as compared to flat transducers. To fully understand the transducer output, it is useful to know the system transfer function or impulse response of a focused transducer first acting as a transmitter and then receiving the signal reflected off a rigid plane. The resulting received pressure of a pulse-echo system can also be characterized in terms of a diffraction correction factor. The determination of the impulse response or diffraction correction factor is well understood for flat disk transducers, but is more complex for focused transducers.

Conventional ultrasonic or eddy current inspection of structures requires a probe to be scanned over the whole area to be tested. This is extremely time consuming, and hence costly, when large areas such as aircraft wings or pressure vessels are to be covered. A further disadvantage of ultrasonic inspection is that a coupling fluid between the transducer and the structure is generally required. The most reliable coupling method is to use immersion, the testpiece being fully immersed in a water bath. However, with large structures this is frequently not practical and they are often tested using jet probe assemblies, the ultrasound being propagated down jets of water directed at the structure. However, this is generally only practical at the manufacture stage and field inspection is often carried out manually using contact transducers, coupling being achieved by applying gel to the surface of the structure. Many structures also have critical areas which are difficult to inspect because access is impeded. For example, spars and other stiffeners in aircraft pose problems because once the aircraft is built they are covered by the fuselage or wing skin.

Inspection of welded plate and pipe assemblies with ultrasonic angle beam shear wave transducers is an important and common application of ultrasonic NDE. With the advent of the Thompson and Gray measurement model [1], many practical ultrasonic testing configurations such as angle beam inspections can now be analytically modeled. An important component of the measurement model for any UT testing configuration is the calculation of the incident wavefields radiated by the transmitting transducer, commonly known as the transducer beam model.

The high-strength alloys used in the aerospace industry today are inspected ultrasonically for material anomalies which may have resulted from the manufacturing process. These materials, such as titanium alloys, often have a large macro-grain structure which limits the sensitivity of the ultrasonic inspection to material anomalies. As a result, there has been much work directed towards developing inspection techniques which minimize the level of the reflections from the macro-grains and enhance the reflections from the material anomalies. In particular, the affect of transducers parameters such as transducer bandwidth, focus, and frequency on the signal-to-noise ratio of synthetic anomalies in titanium alloys has been investigated [1–3]. This investigation showed that the level of the grain noise relative to a known calibration target decreases in Ti-A16-V4 (Ti6-4) and Ti-A15-Sn2-Zr2-Mo4-Cr4 (Ti17) and the signal-to-noise ratio of synthetic anomalies with planar geometries in Ti6-4 increases as the volume of the ultrasonic pulse in the material decreases. This paper will extend these results to planar synthetic anomalies in Ti17 and non-planar synthetic anomalies in Ti6-4. In addition, a transducer design methodology for high sensitivity inspection based on managing the size of the ultrasonic pulse volume is presented and the implications of using this method for production inspections are described.

For diagnostic acoustic methods piezoelectric ceramic circular disks are typically used as active transducer elements. To calculate the response of such transducers, simplified numerical calculation methods, having limited applicability, are widely used. In the present work the strict analytical methods of elasticity theory have been used to calculate the three-dimensional vibrations of circular piezoelectric disks having a general dependence on both diameter and thickness. The approach we employed is based on the eigenfunction method of dynamic electric elasticity developed in [1].

Shear wave ultrasonic transducers for contact mode testing are commercially available. These packaged transducers usually contain a circular piezoelectric element and the mounting is such that the polarization direction of the shear vibration is defined by a line passing through the center of the transducer and the electrical connector. For measure ments in isotropic materials, the polarization direction is not important. For quantitative shear wave measurements in anisotropic materials such as composites1–3, it is important to know the polarization direction of the transducer with a good precision. Unfortunately, commercial shear wave transducers are usually not very accurate in their mounting; it is not uncommon to have +/- 10° of error in their polarization direction and much greater errors have also been encountered.

For ultrasonic quantitative NDE, broad-band transducers with reproducible characteristics are needed. To design this type of a probe, one has to carefully match the electrical, mechanical and piezo-electric parameters of all its components. The most important point for a broad-band probe performance is the matching of a piezo-element with a damper. The main goal of this work was a study of physical aspects of the design of broad band ultrasonic transducers with reproducible parameters taking into account possible variations of geometrical, piezo-electrical and acoustical parameters of all the components. The analysis is based on the known equivalent electrical circuit model.

From a non-destructive evaluation point of view, Lamb waves are a highly attractive means of inspecting a large area of a structure from a single point. Interdigital PVDF transducers have been used previously in signal processing applications [1] to generate acoustic waves in piezoelectric substrates. This paper in conjunction with that of Monkhouse et al [2] aims to provide an overview of the work accomplished so far at Imperial College in the use of interdigital PVDF transducers to transmit and receive Lamb waves in certain structures for non-destructive evaluation purposes. Interdigital PVDF transducers may be permanently bonded to either flat of curved surfaces and this attribute together with their low cost means that they are potentially suitable for“smart structure” applications.

In the ultrasonic community there is a growing use of models to simulate inspection processes [1,2]. One necessary input to such models is knowledge of the transducer’s radiation pattern. The radiation pattern of a commercial transducer often approximates that of an ideal, focused, piston transducer with appropriately chosen parameters (element dimensions and focal lengths), and these“ideal probe” parameters often serve as model inputs. In this paper we demonstrate beam mapping methods for determining these parameters. For probes with circularly symmetric beam cross-sections, an axial scan of the beam suffices. For more general probes, C-scan data are acquired to map out the beam cross section at several different waterpaths. In each case, the transducer parameters are determined by adjusting their values to minimize the discrepancy between the measured and model amplitudes. A technique for handling misaligned data is also described. Measured and fitted fields are compared for a variety of transducers (spherically, cylindrically, and bi-cylindrically focused) including one with a presumably damaged element. In addition, the axial scan and C-scan methods are compared for one circular, spherically-focused transducer.

Large areas of composite primary structure are now to be found both on civil and military aircraft throughout the world and the inspection of these structures contributes significantly to overall operating costs. Therefore methods to reduce the inspection time, whilst maintaining an acceptable minimum defect detection capability, are required in order to optimize the potential cost saving benefits offered by using that carbon fiber composite material.

There is a real and increasing need to monitor the presence and growth of cracks in key regions of plant within the nuclear, gas and chemical industries [1]. A rugged, permanently attached sensor, capable of operating at temperatures in excess of 400° C, would have substantial benefits, since it would reduce the down-time costs incurred through the forced inspection of known or suspected flaws. By carefully measuring crack growth in-situ, under plant operating conditions, remedial action can be taken, either during planned outages of the plant, or when the crack depth exceeds critical limits.

Increasing rocket engine performance is closely linked to as best a knowledge of the propellant combustion evolution as possible. Another important point is to estimate the protection material quantity to place between the combustion chamber and the propeller structure. After many methods such as « Strand Burner [1]», ONERA chose ultrasonic techniques whose most interesting feature is their non-intrusivity [2].

This paper reports on development of simulation software for eddy current (EC) inspections.

The inspection of aircraft engine components using eddy current (EC) techniques has played a vital role in the nondestructive evaluation industry. The objective of the inspection is to determine the structural integrity of the components in a noninvasive manner. The effectiveness of an EC probe design is often evaluated in terms of the probability of detection (POD) of flaws. Many factors need to be considered when estimating the POD. These factors include scan and index spacings, operating frequencies, and flaw morphologies. Traditionally, the EC probe design cycle is iteratively performed experimentally until one either meets or exceeds the minimum required POD. Undoubtedly, this is a time consuming and expensive process since a new probe has to be constructed and tested every time the design is changed. A more sensible approach is to numerically simulate the functionality of probes so design improvements can be done iteratively using a computer under a CAD environment. The numerical probe design model is developed using the boundary integral equation (BIE) approach. By solving the BIEs numerically using the boundary element method (BEM), electromagnetic fields produced by the EC probes can be easily obtained through simple numerical integration.

General Electric developed a new eddy current probe technology in the early ‘90’s to address critical NDE needs in the aerospace industry. The technology utilizes lasers to trace out precise, multiple turn coils on a flexible substrate. The result is an eddy current probe that is capable of conforming to complex geometries and inspecting with a very high detection sensitivity. To cover large areas quickly, arrays of these coils were also fabricated and are currently in use with great success at GE inspection facilities. The newly developed probes, however, raised some unique questions and problems that needed to be addressed in order to determine the“best” probe configuration. In this paper we summarize these issues and through a combination of experimental and finite element results, we show how the design of the probe is“optimized” for various applications. Further details on the development of the technology are provided in a companion paper in these proceedings[1].

Considerable inspection efforts are required to achieve quality requirements for mechanical parts during manufacturing and maintenance operational safety for aging power stations, fleets of civil and military aircraft, trains and offshore structures. Problems arising from parts made of metallic materials are mainly due to corrosion and fatigue crack propagation. Currently many of these inspections are carried out using Eddy Current methods. The detection and the quantification (sizing) of defects in metallic materials with Eddy Current (EC) techniques are based on the performances of the EC equipment and probes. The EC probes, which get all the information from the materials to be inspected, need to be well characterized to ensure the quality of the Eddy Current inspection. Their characterization has to take into account their geometry, their electrical and electromagnetic characteristics as well as their behavior in relation to materials and defects. The usual method for characterizing an Eddy Current probe is to measure its response to reference blocks and reference defects in terms of the resulting impedance or induced voltage in the receiving coil. Few papers [1,2] describe the characterization of the electromagnetic field generated by EC probes. The knowledge of this electromagnetic field is very important for a better understanding of the field repartition and its influence on defects but also to compare probes between them and to follow their evolution. This measurement system could also be a good method to validate electromagnetic model and to design EC probes. We describe in this paper an instrumentation for a direct measurement of the electromagnetic field of EC probes in emitting in air and in transmission through materials.

The conventional ways to evaluate the detectability of ECT probes are usually based on some type of Maxwell equations with use of the FEM or BEM method[1]. Though these approachs can give relatively accurate eddy current field and corresponding impedance, the numerical calculation needs a lot of computer memory and CPU time. This causes them a drawback in ECT probe optimization procedures. Moreover, probe optimization is not only related to the best choice of some parameters of a given probe configuration, but the configuration parameters such as the number and arrangement of exciting and pick-up coils also need to be adjusted and modified. Since the numerical method can not give us a clear image to connect the eddy current to the exciting field, the discovery of a new excellent probe is difficult if this approach is not improved.

Since its introduction several years ago, the self-nulling eddy current probe [1–3] technology has been one of the focal points of the aging aircraft related R&D effort. Numerous application areas have broadened the scope of the probe which has also helped in better understanding the underlying principle. As the technology matures, however, deeper understanding on the various details related to the self nulling effect is needed to overcome difficulties associated with the current field tests and expand its application areas. A particular problem to be addressed is in differentiating the effect of small, shallow surface cracks from that of probe wobble during automated data acquisition operation.

The detection of defects that are located deep in thick walled ( >12 mm ) aluminum plates is of interest to both the aircraft and space industries. Conventional eddy current (EC) techniques are limited to the inspection of surface and subsurface anomalies. Newly developed high sensitivity magnetic sensors, such as magnetoresistive elements and superconducting quantum interface devices (SQUIDs) have enhanced the EC technique’s capability. Such sensors can be used to detect flaws that are located deep in aluminum plates. However, inspection of a defect located 12 mm to 25 mm below the surface of an aluminum plate is beyond the ability of conventional single frequency EC techniques.

The Self-Nulling Eddy Current Probe has been the focus of much research during the past several years [1–7]. Developed under NASA’s Airframe Structural Integrity Program, past research has focused on applying the Self-Nulling Probe technology to the inspection of damage to thin aluminum airframe skins. As a result of this work prototype fatigue crack detectors, single and multi-layer thickness gauges, and a system for the detection of cracks under installed fasteners have been developed[l–2,5–7]. The probe has also been successful at detecting surface flaws in thick bulk materials, for which a commercial instrument has been produced and marketed by Kramer Branson, Inc.-This paper will explore the fatigue crack detection mechanism of the Self-Nulling Probe for shallow flaws in thick materials as compared to that of through cracks in thin skins. The resulting change in the performance of the Self-Nulling Probe will then be detailed, and proposed modifications to optimize the performance of the probe for the detection of shallow fatigue cracks enumerated.

Detection of subsurface cracks around fasteners and hidden corrosion in multi-layer aluminum structure is a critical requirement for aging aircraft inspection. Of particular concern is the Navy P-3 aircraft which is currently undergoing refurbishment In this aircraft, multi-layer structure with thicknesses up to 0.3 inch must be inspected nondestructively to identify areas in need of repair. In order to meet productivity requirements, eddy current testing (ECT) using the McDonnell Douglas MAUS III system will be performed.

Recently, much effort has been focused on the development of arrays for the non destructive testing (NDT) of metal structures in offshore, nuclear and aerospace industry, eg: [1–2]. The main feature of these arrays is electronic scanning which reduces (or eliminates) mechanical noise and makes rapid scan of large areas possible. Some arrays can also provide opportunity for direct signal processing of data. This paper is concerned with a linear array using the non-uniform field ac field measurement (ACFM) technique which is also known as the surface magnetic field measurement (SMFM) method [3].

Superconductive quantum interference devices (SQUIDS), patterned in copper oxide superconductor, offer new technology for eddy current evaluation of materials using pulsed currents. Their high sensitivity at low frequency1 enables penetration of 15 mm or so of aluminum, through multiple layers, to identify millimeter fatigue cracks and material loss of a few percent from corrosion in underlayers[1][2]. Pulsed eddy currents[3] provide a three dimensional view of defects in sublayers and enable tomographic sectioning of multilayer structures. Together with SQUIDS, they would give an ELECTROMAGNETIC MICROSCOPE for examining fatigue damage and corrosion in underlayers of structures.

Crack detection in riveted fuselages, sizing of corrosion areas and testing CFRP structures for heat damage initiated our working group step a little deeper into the problems of scanning with eddy current (EC). EC and especially scanning is mainly a method for in service inspections. Practical, light weight, effordable, easy to use solutions are required.

In Non-Destructive Evaluation (NDE), eddy current techniques are commonly used for the detection of hidden material defects in metallic structures. Conventionally, one works with an excitation coil generating a field at a distinct frequency. The eddy currents are deviated by materials flaws and the resulting distorted field is sensed by a secondary coil. Because of the law of induction, this technique has its limitations in the low frequency range. This leads to a decrease of the Probability of flaw Detection (POD) in larger depths.

Traditionally the remote field eddy current (RFEC) phenomenon has been applied to the inspection of ferromagnetic tubes in heat exchangers, boilers etc. The RFEC probe has an exciter and sensor coil spaced such that most of the magnetic field received by the sensor is due to the field that has diffused through the pipe wall. The phase difference between the exciter and sensor signals is indicative of the defect dimensions. This same idea has been applied to the design of a new RFEC probe for inspecting flat ferromagnetic structures [1–3] and thick aluminum plates [4].

Recently, the development and the research of high performance new ECT probe have been conducted aiming at high precision ECT flaw testing [1–3]. The authors devised a new ECT probe named the Hoshi Probe using uniform rotating direction eddy current. The Hoshi Probe has the following features: 1) self-nulling, 2) self-differential, 3) lift-off noise free, and 4) different signal generation depending on the direction and the position of defect. The authors reported on these basic characteristics of the Hoshi Probe [4].

Of the electromagnetic sensors currently under investigation for nondestructive evaluation (NDE), the superconducting quantum interference device (SQUID) arguably has the greatest potential. The characteristics [1] which make it suitable for eddy current NDE are: high sensitivity even in large ambient fields (detection of sub-nT signals); operation from very low frequencies (a few Hz or less) to very high frequencies (potentially MHz) permitting detection of surface and subsurface flaws; and high spatial resolution. Spatial resolution is related to the physical size of the device, which is often less than 1 mm square, even when the need to maintain its other properties is taken into account. This often allows the SQUID to be treated theoretically and practically as an ideal point sensor. However, it must be operated in a cryogenic environment: low temperature superconductor (LTS) SQUIDs need liquid helium and liquid nitrogen (LN₂) is needed even for high temperature superconductor (HTS) SQUIDs. This makes it difficult to reduce the specimen-to-sensor stand-off below approximately 1 mm.

Eddy-current technique based on high temperature SQUID gradiometer is a promising tool for nondestructive evaluation of material defects which result from the machining process or ageing of electrically conductive structures. We present here the outline of a new project supported by the italian Istituto Nazionale per la Fisica della Materia (INFM), concerning the realization of such a prototype NDE instrumentation. We discuss both the NDE measurement set-up (in particular, the design of the cryostat and thermal shielding, and the x-y sample translation system) and the design of a YBCO SQUID gradiometer based on bi-crystal technology.

Measuring elastic constants (C_ij) of composite materials from ultrasonic velocities is a very well known technique1,2, using the propagation of bulk modes generated at the fluid-solid interface. There is still a question about the validity of these measurements at lower frequencies or for static stress fields. These materials made with viscoelastic matrix are anisotropic and viscoelastic. Hence, the attenuation is also anisotropic. In an absorbing medium, the propagation of waves is dispersive: the phase velocity depends on the frequency. Attenuation and velocity are linked together through the Kramers-Kronig relations that are deduced from the principle of causality3.

The orthorhombic symmetry is considered as general enough to describe the anisotropy of most of the composite materials. The measurement of the anisotropic elasticity constants by ultrasonic techniques usually begins with an assumption that the axes of symmetry are known [1–2]: coincidence of the symmetry axes with the observation axes associated with the thin plate sample is assumed.

To date, most high performance composite materials have been fabricated in the form of relatively thin panels of uniform thickness, parallel front and back surfaces and plies parallel to these surfaces. For such panels, with only moderate curvature, analyses assuming plane wave propagation can treat the plies as being piecewise flat with curvature effects neglected. However, as composites become thicker with nonparallel surfaces and nonparallel plies, these simplified analyses must be abandoned. Depending on the extent of the nonuniformity in ply orientation, ply curvature may introduce a considerable amount of ray bending in the propagation of acoustic waves through composites with complex geometries and ply orientations. Accordingly, a mathematical framework is needed to treat this problem so that efficient, cost-effective inspection procedures may be designed and optimized.

The objectives in this study were to determine the through-thickness elastic constant of fiber-epoxy/metal composites with ultrasonic measurements and determine the elastic constants of the constituent layers. The specimens that we studied consist of alternating layers of aluminum (2024 T3) and fiber-epoxy [1], all of which are 200 to 300 μm thick. As Figure 1 indicates the outer two layers of the composite are aluminum, so that there is always one additional layer of aluminum in the composite. The fiber-epoxy layers consist of a unidirectional layup of aramid fibers for the aramid-epoxy/aluminum specimens. For the glass-epoxylaluminum specimen, the fiber-epoxy layers consist of two sublayers of unidirectional fibers where the fibers of one sublayer lie at 90° with respect to the other sublayer. Because the fibers of both sublayers are perpendicular to the transmitting transducer, the ultrasound perceives the two sublayers as a single layer.

Laser generation and detection of ultrasound has the obvious advantage of requiring no mechanical contact with the materials under investigation. Detection systems based on confocal Fabry-Perot interferometers can be used on surfaces that are rough, moving, and at elevated temperatures. This work describes the use of a laser-ultrasonic system for investigation of surface acoustic waves (SAWs) in composite laminates. SAWs in thick samples, also called Rayleigh waves, have displacement components both parallel and perpendicular to the surface. These components decay exponentially with distance from the surface, thus the wave motion is confined to a layer with thickness equal to about one wavelength. Because the wave propagation is mainly dependent upon material properties (including defects) near the surface, SAWs are an excellent tool for testing materials near the surface. Lamb waves are the counterparts of Rayleigh waves in thin samples with two free surfaces. A detailed discussion of Rayleigh and Lamb waves can be found in the treatise by Viktorov [1].

A portion of the development effort for high temperature composite materials is dedicated to the assessment of nondestructive evaluation (NDE) technologies for detecting flaws in these materials [1,2]. To illustrate the importance of defect detection and characterization, figure l(a) shows the results of a delamination sensitivity analysis on a CMC material in consideration for use as a hot section material in advanced aircraft engines. The study indicates that as the size of delaminations increases from 3×3 mm to 25×25 mm, the hot surface temperature increases up to 50 percent making the material unusable for hot section application. Recent technological advancements in infrared camera technology and computer power have made thermographic imaging systems worth evaluating as a nondestructive evaluation tool for advanced composites. Thermography offers the advantages of real-time inspection, no contact with sample, non-ionizing radiation, complex-shape inspection capability, variable field of view size, and portability. The objective of this study was to evaluate the ability of a thermographic imaging technique for detecting flat-bottom hole defects of various diameters and depths in 4 composite systems of interest as high-temperature structural materials. The technique used in this study utilized high intensity flash lamps to heat the sample located on the same side of the detecting infrared camera. The composite systems were (fiber/matrix): silicon carbide/calcia-alumina-silica (SiC/ CAS) CMC, silicon carbide/silicon carbide (SiC/SiC) CMC, silicon carbide/titanium alloy (SiC/Ti) MMC, and graphite/polyimide PMC. The holes ranged from 1 to 13 mm in diameter and 0.1 to 2.5 mm in depth in samples approximately 2 to 3 mm thick. Ultrasonic and radiographie images of the samples were obtained and compared with the thermographic images.

SiC fibers in metal matrix composites may be damaged, if the fibers are in the top layer and they would be impacted by foreign objects. To avoid such a damage, sandwiched composites are often used, in which the top and bottom layers are protective metal and the middle is composite. Structural designers should know the anisotropic elastic constants of the composite to make structural analyses.

Processing conditions such as those in the gravity cast and extruded metal-matrix composites (MMC) introduce a degree of anisotropy that must be taken into consideration in applications requiring high performance isotropic mechanical properties. The presence of anisotropy in composites can be viewed as the difference in the elastic modulus from one orientation to another, and the elastic constants can be determined from ultrasonic velocities and material densities. Contributions to anisotropy in MMC’s include the preferred alignment of crystalites in the matrix (texture) and the distribution and orientation of reinforcement particles. Models involving the effect of partial orientation and distribution of the reinforcement particles and texture were described by Sayers [1], and Spies and Salama [2] respectively. In this work, we use the former model to measure anisotropy in MMC’s.

In the past decades, the incorporation of ceramic reinforcement in metal-matrix composites (MMC’s) brought about considerable improvements in elastic modulus, strength, wear resistance, structural efficiency, reliability and control of physical properties (e.g. density and coefficient of thermal expansion) thereby providing for improved mechanical performance in comparison to the unreinforced matrix [1–4]. S-N curves for materials such as steels are available elsewhere [5–6] whereas are limited for MMC’s. Studies on the elastic constants behavior for MMC’s as a function of temperature, volume fractions of reinforcement and applied stresses had already been conducted [7–8]. However, the fatigue behavior of elastic constants in MMC’s is not well understood. Further, the trend now is aimed at nondestructive evaluation (NDE) of materials which in the past years gained significant attention over the conventional destructive tests since the former is capable of determining the usefulness, serviceability or quality of a part or material without limiting its usefulness, which is not possible in the latter’s case [9–10]. In view of the above discussion, a result of the study on the fatigue behavior and ultrasonic characterization of monolithic aluminum and aluminum MMC’s will be discussed here.

Metal Matrix Composites (MMCs) can have many attractive properties including high specific modulus and strength, high temperature performance, and the ability to be tailored to specific applications. However, due to the mismatch in the coefficients of thermal expansion between the reinforcement and matrix, residual stresses can develop due to processing or in-service conditions.

Mechanical fastening or riveting of metallic doublers is traditionally used for repairing cracked structures, multi-site or wide-spread damage and any other common flaws detected during inspection of aging aircraft. This method of repair can severely reduce the initiation fatigue lifetime and the damage tolerance of the structure [1]. Several researchers for example [2–6] have demonstrated the effectiveness of bonded composite doublers as a method of repairing aircraft structures. Bonded boron/epoxy repair patches for aging aircraft appear to offer many advantages over traditional methods of repair. They are thin and light weight and can be aerodynamically tailored to reduce drag force. They are conformable to contoured surfaces, access is needed only from one side of the repair area, and they can be used in some areas where riveting is difficult. Structural reinforcement without the addition of rivets provides for a more uniform stress distribution and eliminates ancillary damage which can be caused by rivets. In addition they provide excellent corrosion resistance and are easier to inspect using NDE techniques [7].

for ultrasonic inspection of austenitic welds and cladded components mostly horizontally polarized shear waves and quasi-longitudinal (qP) waves are applied. Depending on the testing situation the one or the other wave type offers certain benefits. To explain experimentally observed phenomena and to predict how qP-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 qP-waves propagating in ideally fiber-textured austenitic weld material. Based on a mild anisotropy approximation given previously by Tverdokhlebov and Rose [1], a direct relationship between wave propagation directions and spatial coordinates has been obtained. This relationship has been applied to the Generalized Point-Source-Synthesis method (GPSS) to model radiation, propagation and scattering effects [2,3]. The GPSS-code thus improved 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 evaluations are presented for both continuous wave and time-dependent rf-impulse modeling for austenitic weld metal specimens, covering field profiles for a normal transducer and for a 32-element phased array probe. For the latter, wavefront images are also shown. The differences in the results obtained with this accelerated modeling code as compared with the exact code are of no practical importance. Further evaluations are shown in [4], where the calculation of three-dimensional transducer fields is considered.

In recent years, field welding has been applied to the butt joints of flange plates and web plates on girder bridges in Japan. Up until now, radiographic testing has been applied for inspections of welded joints. Considering the advantages of ultrasonic testing, such as being less time consuming and dangerous, ultrasonic testing will be used instead of radiographic testing.

Ultrasonic testings of bimetallic welds often show the presence of false calls. In pulseecho mode at oblique incidence, these false calls are detected close to the backwall. They are generally distributed along lines parallel to the buterrings. The phenomenon is disturbing because it limits the efficiency of ultrasonic testing in the area of the backwall: false calls can be misinterpreted for echoes from defects or they can mask the presence of real defects. If a defect is detected, they also limit the possibility of characterization.

Because of the awkward and somewhat irregular shape of the weldment, conventional methods [1] could not be adapted to the nondestructive measurement of GTAW edge weld penetration on clamshell-style catalytic converters and a special inspection system based on the electric current deflection method was developed. DC or low-frequency AC electric resistance measurements, also known as the Potential Drop Method (PDM), are well-developed for plate thickness assessment and crack detection [2–6]. The operating principle of these methods is that, under certain arrangement of the electrodes, the defect or crack in a conducting specimen will cause a measurable increase in resistance between given points compared to the situation without the defect or crack. In recent years, this simple contact technique was largely obscured by more sophisticated noncontacting eddy-current techniques especially in industrial applications. In this article, we demonstrate the distinct advantages of the Potential Drop Method through the example of GTAW edge welds where the awkward shape of the specimens and the required large penetration depth render the eddy-current method less feasible.

The Iowa Demonstration Laboratory (IDL) is an outreach arm of the Center for NDE that works primarily with in-state clients. Over a course of time, various clients have approached the IDL with a similar query: how do we nondestructively test spot welds? In our work with these clients, we encountered anecdotal information about the success or failure of various means of evaluating spot welds, and sometimes conflicting interpretations of the ease of use of competing techniques. Additionally, published articles seldom contrast the efficacy of different test methods used on the same samples. It appears that a consensus to interpreting weld conditions does not exist, presumably due to a range in effectiveness of various techniques over a range of possible welded materials. This project, in effect, will attempt to determine the best inspection method over a wide variety of welded materials, and, ideally, generate guidelines on how best to implement the various techniques.

The widespread application of diffusion bonding has been hindered, in part, by concerns over kissing bonds. Kissing bonds are generally considered to be conditions where a bond has little or no strength and the concern is that such conditions might escape detection. At Rohr we differentiate between an intimate contact disbond (which has no bond between the surfaces but is detectable by careful ultrasonic testing) and a kissing bond (which also has no bond between the surfaces but is not detectable using current ultrasonic technology). These definitions will be used throughout.

The aggressive environment encountered by the high speed civil transport (supersonic) aircraft (HSCT) places severe requirements on the types of materials used in its construction. The state-of- the-art materials available to the commercial aerospace industry will not meet these severe environmental requirements. New materials have been evaluated that will meet these severe environmental requirements. One such material is the super plastic formed/diffusion bonded (SPF/DB) titanium. Structures with this material have been fabricated to be used on the HSCT aircraft. Because the HSCT is a commercial program, the FAA requires that nondestructive evaluation techniques must be developed for the inspection of these structures.

When real engineering surfaces touch, contact occurs between the asperities of the surface roughness. For this reason the true area of contact between components can be significantly less than the apparent contact area and the stresses at the asperities are considerably higher than the average (nominal) contact pressure. Measurement of the degree of contact between solids is important in a number of applications such as the design of contacting elements (e.g. gears and bearings) [1] and the detection of ‘kissing’ bonds [2].

Adhesive bonding of aluminum holds some possible advantages over the traditional methods of construction in the aerospace industry. However, there is uncertainty about the mechanism of adhesive bonding and the changes occurring during environmental attack which reduce the bond strength. An improved understanding will aid in the development of better techniques for non-destructive testing (NDT) of joints by determining what properties should be measured and will lead to the development of better adhesive systems.

The work described here is an ultrasonics based experimental study which aims to address the lack of a reliable technique for detecting strength loss in adhesive joints after exposure to hot wet environments. This is manifested as a change in the failure mode of an adhesive system from a cohesive failure in the as-made condition, that is failure through the adhesive, to an adhesive failure, failure between the adhesive and adherend, after exposure to a hot, wet environment. This work has been concerned with the bonding of aluminum using two part epoxy adhesive. The reason for the change in failure mode is thought to lie in changes in the oxide layer which is present between the aluminum and the epoxy. The oxide layer generally has a porous structure into which epoxy can penetrate, forming a micro-composite layer, referred to as the interlayer. It is the detection of changes in this interlayer which present the biggest problem to current N.D.T. techniques for adhesive joints [1]. This is largely a problem of size, the interlayer being typically no larger than a few microns thick, sandwiched between several hundred microns of epoxy and several millimetres of aluminum. It is the need to detect changes in such a thin layer through such a thick layer which presents the biggest problem.

Great progress is being made on the use of axisymmetric ultrasonic guided waves in tubing for defect detection, classification and sizing. Benefits of excellent penetration and sensitivity to certain defects are possible. However, ultrasonic guided wave propagation and reflection in tubing is still not completely understood. For more comprehensive knowledge of ultrasonic guided waves in tubing, a study on non-axisymmetric modes cannot be avoided. A number of non-axisymmetric modes close to the axisymmetric modes on a dispersion diagram often makes it difficult to generate a single axisymmetric mode in tubing. Also, non-axisymmetric reflections occur due to non-axisymmetric defects, even for axisymmetric impingement. The purpose of this study is to explore the use of non-axisymmetric ultrasonic guided waves for both faster and simpler inspection as well as defect sizing analysis. As a result, it is hoped to optimize detection and screening via distance, incident angle, and frequency tuning.

The corrosion of pipework is a major problem for the oil and gas and petro-chemical industries. Large facilities operate hundreds of kilometres of pipes which may carry corrosive substances. General wall-thinning and localised pitting corrosion can occur both from the inside and the outside of pipe walls. A high proportion of these pipes are insulated, so that even the external defects cannot be detected by conventional NDE techniques without the expense of removing of the insulation.

Corrosion in pipework is a major problem, particularly in the oil, gas, chemical and petro-chemical industries. Since 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. There is therefore an urgent need for the development of a quick, reliable method for the detection of corrosion under insulation (CUI).

Ultrasonics has proven to be an effective method for detecting a variety of defects in gas transmission pipes including cracks, wall thinning and corrosion pits. The use of Lamb waves for the detection of defects and in situ process monitoring applications has been successfully pursued for many years [1–6]. The use of a laser-based ultrasound (LBU) inspection technique to detecti defects is attractive because of the potential for rapid inspection of large areas and because it is noncontact with large standoff distances. Owing to its noncontacting and remote nature, the LBU technique is being investigated as an alternative technology to piezoelectric transducers or electromagnetic acoustic transducers (EMATs) for the rapid nondestructive inspection of pipelines. Currently, the preferred methods for introducing ultrasonic waves into the pipe are by using a piezoelectric transducer in a liquid-filled wheel or an EMAT. In field use, the wheel or the EMAT is attached to a moveable platform (known as a pig), which travels along the length of the transmission line. The wheel must maintain contact with the pipe wall during the inspection. Although the EMAT is a noncontact sensor, it must be operated close to the pipe’s surface. The contact and near-contact requirements can result in a loss of data when pipe irregularities such as dents or joints between sections cause the wheel or the EMAT to lift off from the surface of the pipe. The liquid-filled wheel uses longitudinal waves that propagate into the wall of the pipe. For a complete inspection of the pipe’s circumference, many wheels must be used. The EMAT generates a Lamb wave in the wall of the pipe that can be directed either circumferentially or axially along the pipe. Although the LBU technique also uses Lamb waves, unlike EMAT systems, the detection sensitivity of the LBU system does not decrease with increased separation from the part. However, a potential difficulty for LBU techniques is that Lamb waves are a family of guided waves that exist in plate-like structures, and a large number of modes of vibration may coexist in a given plate thickness. A laser that has been focused to a spot or line represents a broadband Lamb wave source in both the temporal and spatial frequency domains, which leads to the simultaneous excitation of many modes. Consequently, LBU techniques for generating Lamb waves have generally been pursued only when the lowest order symmetric or asymmetric mode was needed, probably because these modes are generated and detected with the greatest efficiency and thus offer a de facto mode selection mechanism since these modes dominate the others that may be present. We previously demonstrated [7] a mechanism for efficiently generating and selecting a single Lamb wave mode using simulated arrays. In this paper, we describe the implementation of a laser array for the generation of Lamb waves. We also present some preliminary results of a study of the characteristics of Lamb wave modes to identify suitable modes for detecting defects in pipelines. The features that are important include the generation and detection efficiency of the Lamb wave modes, the mode’s energy distribution, and the velocity dispersion of the waves.

An ultrasonic guided wave system for pipe inspection is proposed. Using guided wave experience to date on a variety of different tubing problems, feasibility experiments have already been conducted on piping under insulation in both a laboratory and chemical processing facility field environment. Several guided wave techniques are introduced, one using a broad banded variable angle beam transducer on a curved shoe, and one on a newly developed pipe comb system. Discussion on both axisymmetric and non-axisymmetric wave propagation is presented.

Mechanical damage is the single largest cause of pipeline failures for gas transmission pipelines and a leading cause of failures for liquid pipelines today. Outside forces (usually construction equipment) can deform the natural cylindrical shape of a pipeline, scrape away metal and coating, and/or stress and cold work the steel changing its microstructure and altering its mechanical properties. Both the geometric deformation and the amount of residual stress, plastic deformation and cold working contribute to the severity of the defect. Having an NDE in-line inspection (ILI) technique for both detection and characterization of mechanical damage defects is important.

Stress Corrosion Cracks (SCC) are among the defects in pipelines that are least understood and most difficult to detect, but which have accounted for some of the severest ruptures in pipelines throughout the world. Pipetronix has developed a new generation of internal inspection device for detecting SCC and other cracks and crack-like defects in pipelines, the UltraScan CD. It is a completely autonomous device that travels through the pipeline, carried by the fluid, uniformly scanning the pipe wall for defects with full circumferential coverage and for lengths of several hundreds of km. The information is processed on-line to allow for storage of huge amounts of data to be processed when the tool is retrieved from the pipeline. The UltraScan CD has, up to now, successfully inspected more than 1,000 km of oil and gas pipelines. Comparison of the findings with corresponding verifications from excavations illustrate the sensitivity and reliability of the inspection method and its ability to discriminate between different types of defects found in the pipelines.

The inspection of thousands of miles of gas transmission pipelines is a formidable problem. Inspection tools based on magnetic flux leakage (MFL) and ultrasonic phenomena have been developed, and continually evaluated with respect to defect detection and characterization accuracy. The concept of probability of detection (POD) offers a measure for quantifying the capabilities of nondestructive evaluation (NDE) systems in the presence of various sources of uncertainties.

Eddy Current Testing (ECT) is used for in-service inspection of tubes in steam generators, heat exchangers and condensers in nuclear or conventional power plants as well as in chemical installations. ECT is particularly attractive because it offers both very high detectability and high scanning speeds. Direct contact with test material or a coupling medium is not necessary and the test is easily automated. For the tube inspection, the probes embedded sensor coils are usually inserted in the tubes. Therefore the detecting capability for an outer defect is low in comparison with an inner one by skin effect.

Many diseases are known to change the mechanical properties of tissue. For example, cancerous lesions tend to feel rigid when touched and infectious lesions tend to feel soft when compared to the surrounding tissue. This is why palpation is a useful diagnostic procedure.

The race-horses are prone to injuring their leg tendons, especially the superficial digital flexor tendon (SDFT). The SDFT injuries have a significant impact on the performance horse industry, accounting for an estimated 8–12% of all racing injuries. Historical data indicate that with traditional modes of therapy, there is approximately 40–50% chance the horse will return to athletic activity after significant SDFT injury. This represents a significant economic loss to racehorse owners and industry.

Our overall goal is to develop clinically applicable tissue characterization methods, based on quantitative analyses of backscattered ultrasound, which can differentiate normal from diseased heart segments. In implementing these methods there is a need to compensate for the inherent anisotropic properties of the heart that are exhibited in echocardiographic images. [1–4] Furthermore, quantitative tissue characterization methods may be able to exploit the inherent anisotropy of the myocardium to achieve assessment of cardiac properties.[5–9] The specific aims of this investigation were to measure the spectral properties of backscattered ultrasound using a clinical imaging system and to determine effects of inherent tissue anisotropy on measured spectral properties of backscattered ultrasound.

In this paper we will present initial results from an ongoing project at the University of Missouri-Columbia which focuses on prostate cancer detection using transrectal ultrasound [1,2]. The project goal is the development of effective screening procedures for the detection and grading of prostate cancer with specific objectives of: 1) cutting mortality rate by improving upon the early detection of prostate cancer; 2) providing information on the severity of the cancer as input to the treatment decision process; and 3) reducing health care costs and inconvenience to the patient by reducing the number of needle biopsies performed.

There is a growing demand in the meat industry for a quantitative method of grading beef carcasses. Commercially available beef in the United States is graded subjectively by-certified United States Department of Agriculture (USDA) inspectors. This is done by visually determining the amount and distribution of intramuscular fat (IMFAT) or marbling in the ribeye muscle. A method for estimating quality attributes (such as marbling) in live animals would help cluster feedlot cattle into outcome groups for more effective marketing.

Fiber reinforced composite materials have been used in many structural applications varying from swimming pool diving boards to advanced aerospace components. The primary advantage of composites includes a high stiffness to weight ratio which in the past has come at an increased cost. Continued improvements in the development of cost effective manufacturing methods and development of low cost fibers and resin materials have increased the use of composites in infra-structural applications such as buildings, and bridges. As fiber reinforced composites become more widely used, the need for a reliable method to nondestructively characterize the material stiffness properties and identify material defects is becoming critical for ensuring a reliable level of performance.

When an ultrasonic wave is incident on a water/solid interface generally three waves are excited in the solid: one longitudinal and two shear. The complex amplitudes of the transmitted waves depend on the material properties and the angle of incidence. At incident angles higher than the first critical angle an evanescent longitudinal wave is excited near the surface. To satisfy the boundary conditions the imaginary parts of the shear wave amplitudes become non-zero. This implies that the shear waves experience a phase shift at the water/solid interface at incident angles higher than the first critical.

Ultrasonic materials characterization is widely used to assess both properties and defects of structural components. Recently, the option of gas- or air-coupled ultrasonic testing has become a realistic possibility. In this paper we develop the application of resonant sound transmission methods through ambient air in anisotropic materials with the sound wavevector oriented in a general direction in an anisotropic laminate. Establishing and demontrating the importance of voltage contributions from rays not contained in the incident plane, for sound propagation in a non-symmetry direction, is the major result of this paper.

In this paper, we describe experiments to evaluate the use of a laser interferometer as a quantitative probe of nonlinear ultrasonic propagation. Although other detection methods for nonlinear ultrasonics have been in use for several years — most notably capacitive and piezoelectric receivers — interferometric detection provides advantages not available with these methods. The interferometer provides a direct means of absolute amplitude calibration, is noncontacting, possesses a wide bandwidth, requires less extensive sample preparation than the capacitive method, and affords excellent spatial resolution.

During the last decade, Atomic Force Microscopy (AFM) has been widely used to image the topography of various surfaces with corrugations down to the atomic scale [1,2]. Since then, development of new techniques based on AFM has been conducted to evaluate physical, chemical or mechanical surface properties [3]. We describe the use of near-field acoustic microscopy, based on AFM and hereafter referred to as Acoustic Microscopy by Atomic Force Microscopy (AFAM), as it has been developed earlier [4]. The relevance of this new scanning probe microscopy for high-resolution nondestructive testing and evaluation purposes is pointed out. It is shown that AFAM is capable of measuring elasticity on surfaces with a spatial resolution of less than 100 nm. Subsurface elastic properties and subsurface microdefect characterization can be performed by this technique. The high frequency Friction Force Microscopy (FFM) image, hereafter called Acoustic Friction Force Micropscopy (AFFM), reveals information different from the conventionally taken friction force image. We describe experimental and theoretical aspects of high-frequency atomic force and friction force microscopy.

It is well known that material characteristics properties such as anisotropy, grain size, damage, roughness, can affect the Rayleigh wave propagating on a sample surface. The acoustic microscopy using broad-band pulses is one of the methods which can generate Rayleigh waves in a simple way. The acoustic energy generated by a transducer in the coupling medium reaches the sample surface and is partially reflected into an axial echo and converted into a Rayleigh wave at the Rayleigh critical angle θ_R. With an impulse excitation, these two echoes are resolved in time. In this case, the Rayleigh velocity can be also obtained through a time of flight measurement. One of the challenge of this technique is to be able to perform time measurements with the necessary accuracy in order to detect shifts in the material properties.

Ultrasonic spectroscopy has for a long time been thought promising for characterization of thin layers immersed in water or embedded between two known materials (similar or dissimilar). Significant effort has been put forth by many authors.

Layered media is used extensively in the aerospace and automotive industries from paint on metal surfaces to actuators with piezoelectric layers. NDE of such materials is used for manufacturing quality control or damage inspection. A method of inversion for thickness and wavespeeds from ultrasonic data using the minimization of the difference between measured and constructed transfer function magnitudes in transmission has been developed [1,2]. Using this method, for a three-layer metal/liquid/metal specimen immersed in water it is possible to invert accurately for up to four parameters, for example, three thicknesses and one wavespeed. Kinra, Wang, Zhu, and Rawal [3] reported thickness measurements on three-layer metal-clad specimens.

Many single crystal semiconductors are grown by variants of the Bridgman technique in which a cylindrical ampoule containing a molten semiconductor is translated through a thermal gradient, resulting in directional solidification and the growth of a single crystal. During crystal growth, the shape and location of the solid-liquid interface together with the local temperature gradient control the mechanism of solidification (i.e. planar, cellular or dendritic), the likelihood of secondary grain nucleation/twin formation (i.e. loss of single crystallinity), solute (dopant) segregation, dislocation generation, etc. and thus determine the crystals’ quality [1]. For crystals grown by the vertical Bridgman (VB) technique, optimum properties are obtained with a low (∼1–5mm/hr) constant solidification velocity and a planar or near planar (slightly convex towards liquid) interface shape maintained throughout growth [2,3]. The solidification rate and the interface shape are both sensitive functions of the internal temperature gradient (both axial and radial) during solidification, which is governed by the heat flux distribution incident upon the ampoule, the latent heat release at the interface, and heat transport (by a combination of conduction, buoyancy surface tension driven convection and radiation) within the ampoule [4,5]. The solid-liquid interface’s instantaneous location, velocity and shape during crystal growth are therefore difficult to predict and to control, especially for those semiconductor materials with low thermal conductivity (i.e. CdZnTe alloys) [6]. Thus the development of ultrasonic technologies to non-invasively sense the interface location and shape throughout VB crystal growth processes has become a key step in developing a better understanding of the growth process and for enabling eventual sensor-based manufacturing.

Nondestructive evaluation techniques for aging components are becoming very important methods for identification of material degradation, life assessment and development of inspection strategies. In this frame a laser system suited for measurement of permanent deformations in power plant pipes has been designed, built-up and in-field tested.

A new approach that overcomes traditional difficulties associated with ultrasonic spectroscopy (including inspection time, resolution and signal-to-noise ratio) is presented along with examples of inspection methods made possible by the new technology. Particularly, use of the method to detect hidden corrosion, independent of material thickness, will be presented.

The presence of and/or the potential for cracks in powder metallurgy components is an industry recognized quality concern. In an industry wide survey, eliminating or controlling of cracks was found to be the second most important research priority identified [1]. The general improvement of mechanical properties was the number one issue identified. It should be noted, however, that the benefits of material properties gains only slightly exceeded that of the cracking issue in importance to the industry respondents of the surveys.

Inconel 718 is a widely used material for applications at temperatures up to 650°C. Many research groups have studied the properties of Inconel 718 at different conditions [1, 2]. It is known that, after several thousand hours at 650 °C, Inconel 718 shows roughly a 75% decrease in the Charpy V-notch impact energy [3]. However, any anisotropic characteristics of this steel have not been investigated in adequate detail. Consequently, in order to maintain the integrity of the components, it is important to clarify the orientation dependence of the mechanical properties of INCONEL 718.

Micromagnetic Barkhausen signals from magnetic material originate from the discontinuous motion of domain walls in the presence of a changing applied magnetic field. Barkhausen emissions that are detected by a surface coil as a voltage signal and come predominantly from a surface layer. The Barkhausen signal is affected by changes in material microstructure and the presence of residual stress since these affect the dynamics of domain wall motion. The selective attenuation on high frequency components of the Barkhausen signal due to eddy currents in electrically conducting materials is used to evaluate changes in material condition at different depths inside the component. Barkhausen measurements have been made on specimens subjected to different thermal exposure during surface conditioning procedures. Comparison has been made with X-ray diffraction results for assessment of residual stress, and microhardness measurements which evaluate the surface condition. The results show that the Barkhausen emissions can be utilized to evaluate changes in the surface condition of materials.

Diverse environmental, clinical, and quality assurance problems involve the evaluation of airborne particle distributions originated from materials and objects manufactured with fiber glass. This analysis is performed manually by observing scanning electron micrograph (SEM) imagery, finding an adequate observation field, recognizing the objects of interest, and measuring their geometrical properties using a reference grid or a pointing device. This task is tedious, error prone, and sensitive to observer’s bias and eye-fatigue. A dedicated digital image analysis system capable of detecting and measuring various types of fibers and other objects from SEM images under different operating conditions is therefore needed.

Relative measurements of real and imaginary reluctances were made as a function of sample thickness on both rolled and cast 6061 aluminum samples using an ac magnetic bridge. Samples were both nonannealed and annealed. Evidence was developed that the imaginary reluctance (which is shown here to respond to the conductivity of samples) responds more to the bulk properties of the samples while real reluctance apparently responds more to surface conditions such as surface residual stress.

Coil eddy-current (CEC) technology generates real and complex reactances of the search coil. These reactances are frequently plotted in a complex-impedance (C-I) plane with inductive reactance as the ordinate and resistance as the abscissa. This plane is a useful tool in nondestructive evaluation (NDE), and it is known to virtually all NDE technicians using electromagnetic fields. The complex-impedance plane is generally used to establish the operating conditions for particular eddy-current coils and for particular NDE problems. It can also be used to establish the effect of lift off and to identify flaws.

Fatigue would often cause serious damage in materials and fracture all of sudden[1]. In many studies, however, it has been investigated that the damage has gradually induced the change of material properties and led to final fracture. As a method to detect the progressive change, the ultrasonic technique has been widely applied over decades because of comparatively simple and easy instrumentation [2–6]. In these applications, the ultrasonic attenuation and velocity have been measured with contacting ultrasonic transducers. These measurements have shown that the attenuation increased linearly at first and rose rapidly at about 70–80 percent of fatigue life [4, 5]. However, it is considered the attenuation changes measured using a contacting transducers can’t exactly reflect the fatigue damage because the as-measured attenuation also includes the damping through the transducer, the couplant and the buffer, the reflection and transmission losses at the interface, and the energy leakage into the transducer.

X-ray backscatter tomography (XBT) is a relatively new radiographic NDE technology unique among x-ray methods in that it is applicable with access to only one side of an object [1, 2]. This makes x-ray tomography possible in applications where computed tomography (CT) is precluded by the object’s inaccessibility, size, or aspect ratio. In the course of developing an XBT system for the underwater inspection of naval sonar domes [3], we have investigated other applications that exploit this capability. One such application is the evaluation of porosity distribution in porous metal plates.

Phosphate-bonded silicon nitride produced by cold iso-static pressing (CIP) retains many of the desirable characteristics of hot iso-static pressed (HIP) silicon nitride including a low dielectric constant, low thermal expansion and the ability to operate in a high temperature environment, yet is considerably more cost effective due to reduced processing costs. An important difference is that unlike the HIP silicon nitride, the CIP phosphate-bonded silicon nitride cannot be produced in the fully dense state. Components that are CIP, using a US Navy patented process, at pressures of 60Ksi often have porosities between 15% and 20%. Many components where extreme high strength is not required can utilize this material even with its inherent porosity. In applications where performance engineering is involved the porosity is extremely important due to its effect on dielectric and mechanical properties. An effort has been made to characterize porosity in phosphate-bonded silicon nitride by correlating the velocity of a compressive wave in this material to a porosity value. This method allows for the determination of porosity in a small volume of the sample and can be used to quantify porosity variations in a component unlike bulk methods such as the Archimedes method which gives an average value for the entire component.

Elastic properties of high-performance ceramics is usually measured with ultrasonic bulk waves [1]. However, the bulk waves gives us only properties averaged over sample thickness. Sintered ceramics often have properties at the surface different from in the middle. Thus we need a method which distinguishes near surface property from the averaged one. Surface wave technique has been applied for nondestructive characterization of ceramics [2–4]. The depth-dependent properties of ceramics, however, has never been reported. Surface wave propagates in subsurface layer of about one wavelength. Therefore we’ll be able to estimate the depth-dependent properties of the ceramics, if we’ll use surface waves of different frequency or wavelength.

Magnetic Barkhausen technique has been used to parametrically image the effects of thermal exposure on the surface condition of industrial components. Wear and fatigue resistance of industrial components are primarily determined by the mechanical condition of the surface layer. Standard hardness measurement techniques cannot easily be used to assess the condition of components in service because they are not well adapted to in situ measurements and are essentially destructive in nature. Barkhausen signals are sensitive to changes in surface hardness because the altered microstructure affects the dynamics of domain wall motion and consequently the Barkhausen signal. This paper presents the results of Barkhausen technique used to image the surface hardness on a flat plate material.

The backscatter coefficient, η(ω), is a material dependent acoustic parameter. As such, a reliable estimate of η(ω) may provide information concerning the microstructure and material properties for various engineering materials [1–4]. Current methods for properly estimating η(ω) are limited to cases in which the single scattering assumption is valid. Many models for grain noise also rely upon the validity of the single scattering assumption [5–9]. When attempting to validate models based upon the single scattering assumption, one must first verify that the single scattering assumption is valid. Therefore, a methodology is needed to determine if the single scattering assumption is in fact valid or if multiple scattering is significant.

This paper describes research directed towards modeling ultrasonic signals from hard-alpha inclusions in titanium alloys. The modeling effort has been made difficult by the complicated morphology of such inclusions which can include voids, cracks, core and diffusion zones. Fortunately, a large portion of hard-alpha inclusions are acoustically weak scatterers in nature, and advantage can be taken of simplifications as afforded by Born approximation and some ad-hoc interface conditions. Models along these lines have been previously developed and their validations on synthetic hard-alpha inclusions of cylindrical shape at normal incidence have been reported [1]. Extensive use of these ultrasonic models were also presented in the development of a statistical methodology for estimating the probability of detection [2,3]. In current work, we extend the model capability to include arbitrary flaw orientation and oblique incidence. Model predictions are compared with experimental data collected from titanium specimens for different beam angles, focal depths, inclusion sizes and orientations. The range of the model applicabilities and their possible extensions will be presented. Morphological modeling of the three-dimensional, naturally-occurring inclusions based on stacks of two-dimensional metallographic measurements are also described.

Physical models can play several important roles in developing and evaluating ultrasonic techniques for the inspection of titanium engine components. In conventional inspection, models predicting signal-to-noise ratios can be used in system optimization. Models synthesizing time-domain wave forms can also be utilized to evaluate advanced signal detection algorithms. Finally, models can play a major role in probability of detection (POD) assessment, being used as tools to simulate (1) effects of measurement system and scan plan selection, flaw morphology and microstructure on the distribution of flaw responses; and (2) effects of measurement system microstructure on the distribution of noise.

During ultrasonic inspection for flaws in engineering materials, it is important to understand the interactions between the inspecting beam and the microstructure in which flaws are embedded. It has been found that in certain materials such interactions can have dramatic effects on the characteristics of the beam as it propagates to and from a flaw and consequently can have deleterious effects on both flaw characterization and the probability of detection. It is well known that, the microstructure can backscatter energy, creating noise which can mask small flaws. In addition, a flaw signal can be attenuated by the removal of energy from the beam by absorption and scattering. Considerable progress has been made towards developing a theoretical understanding of these phenomena. For example, backscattered grain noise has been successfully modeled by Han and Thompson [1] for duplex microstructures that commonly occur in Ti-17 and Ti-6A1-4V alloys used in the rotating components of aircraft engines. In addition, attenuation has been modeled for randomly oriented, equiaxed, cubic microstructures [2], for textured, equiaxed, cubic, stainless-steel [3], and also for elongated textured microstructures [4].

Under the sponsorship of the U.S. Air Force and the FAA Engine Titanium Consortium, models have been developed which endeavor to predict characteristics of the backscattered micro-structural noise observed in focused-probe pulse/echo inspections of titanium billets and forgings [1–5]. In this paper we briefly review the history of the model development work at Iowa State University, and then compare noise model predictions with experiment for several cases of interest.

Plasma-sprayed ceramic coatings are being developed for many applications such as thermal barriers and corrosion protection. In plasma spraying, the ceramic powder is heated into semi-molten droplets for projection onto a target surface by a high velocity gas stream. This coating is then rapidly cooled to form a solid layer. By repeating this process for multiple layers, a coating thickness from micrometers to millimeters may be built up. Inherent to this process, plasma-sprayed coatings generally possess rough surfaces and a high degree of porosity.

Coating technology is used in a variety of applications, for example to improve the wear properties or to improve the bonding properties of a surface. The properties of such surfaces depend on the mechanical properties of the coating, namely its Young’s modulus and its thickness. To be able to predict the performance of such coatings it is necessary to be able to measure these properties in a nondestructive way. Conventional ultrasound is one nondestructive method that has been used for coating measurement [1]. It has the disadvantage of needing a transducer to be placed in direct contact with the material surface, often with the use of a couplant. This is sometimes inconvenient and risks contaminating the surface. Laser ultrasound offers a complete noncontact technique that prevents any surface contamination. It also offers scanning possibilities and laser sensor systems can possess very broad frequency bandwidths [2,3].

The US Air Force needs a nondestructive inspection method to validate the integrity of thin films. Thin films are used for their unique mechanical, electrical, and optical properties. The integrity of these films depends on the thickness and adhesion of the films, and their thermal, optical, and electrical properties. A method to measure the thickness an other properties of thin films is required.

Plasma-spray coatings are widely used in industry to protect the substrate against aggressive environments or to alter the surface properties to enhance the wear resistance[1]. The coating is an aggregate of powder particles which are impacted after being heated on the substrate surface. As a result of the fast cooling rates [2], no significant diffusion occurs at the coating/substrate interface and between subsequent coating layers. Different defects in the coating layer often occur, such as delaminations between subsequent deposition layers, pores, non-melted particles, cracks due to thermal coefficient mismatch and loss of adhesion between coatings and substrates.

Eddy currents can be used to characterize the conductivity and thickness of coatings on metals. However, when the same techniques were applied to magnetic metals, some uncertainties were found. We have discovered that the broadband behavior of eddy current coils in proximity to ferromagnetic surfaces depends dramatically upon very thin surface layers. For nickel, we found a 10∼100 micrometers thick dead layer at the surface that reduces the apparent relative magnetic permeability substantially [1]. Conversely, this extreme sensitivity to surface conditions means that measurement methods can be devised that will be sensitive to very thin surface coatings, on the order of a few micrometers thick or less.

The ability to measure the thickness of paint on graphite/epoxy composite structures directly would be of benefit in the aerospace industry. The weight reduction would increase fuel mileage for the airline customer. An additional reason for reducing the thickness of paint on graphite/epoxy composites is not as obvious as the weight savings. A concern in the use of composite structures has been the effect of paint chipping after 2 to 3 years of service life. It has been postulated that the primary cause has been the application of surfacer or primer. The primer is less plastic than the top layer of paint and cracks form as the primer goes through temperature cycles. If the primer or surfacer is too thick, the crazing is more likely than with thinner applications of primer or surfacer.

Bulk and Rayleigh (or surface wave) velocities of ultrasonic waves in chromium coatings on steel were measured and an experimental correlation was found between the surface velocity and the hardness. Since qualitative coating evaluation is commonly made using hardnesses as a guideline, the possibility is herewith presented of using the sound velocities of surface ultrasonic waves for this purpose. This provides the advantage of using a nondestructive technique over one that is destructive and more qualitative.

Electroactive thin-film polymers are increasingly being used as sensors and actuators in aerospace structures [1,2]. They also have significant potential for applications in muscle mechanisms and micro-electro-mechanical systems (MEMS). In these applications, polymer films of thickness varying between 20 and 300 μm are utilized. Actuation of these polymers is attributed to piezoelectric, electrostrictive or electrostatic effects. Recent investigations suggest that polymers may produce striction which can be stronger than that delivered by electroactive ceramics. Such response may be produced by polymers with isotacticity or syndiotacticity in their molecular structure, where tacticity is the position of a pendant polymer group with a strong dipole moment that is mounted on a backbone polymeric chain.

Case hardening is a metallurgical process typically to increase the fatigue and wear resistance of steel components. The case refers to the hardened layer that is formed in this process, and the depth of the case is critical to the components’ performance. Currently, to inspect the quality of a batch of material that has gone through the hardening process, one or more of the parts are sectioned with an abrasive wheel, polished flat and the hardness profile measured using a microhardness indenter. This method of inspection can take several hours, often with the production line stopped, until the results are known. It does have the advantage of measuring directly the desired material properties but has the obvious disadvantages of costly manufacturing down-time, unnecessary scrappage of a production item and assumes that the material properties of other samples in the batch are similar to the one inspected. As a promising alternative approach, nondestructive inspection (NDI) permits a 100% inspection of the batch which is not economically viable using destructive inspection methods. Generally, NDI methods are based on inferring case depth indirectly through measuring electromagnetic or mechanical properties of the part using eddy current or ultrasonic probes [1–4]. Eddy current systems are commonly used for case depth measurements and are known to be reliable for many applications [4]. However, they lack sensitivity if the case depth is deep (e.g. greater than 5 mm in steel parts) and custom probes are required for inspection of components with different geometries.

Leaky Lamb waves have been used extensively for ultrasonic non-destructive evaluation of elastic properties: the reader is referred to papers by Dayal and Kinra [1,2], Chimenti and Martin [3], Mal et al. [4], and Chimenti and Nayfeh [5]. The principal disadvantage of this method is the high attenuation of the waves in the immersed solid plate due to continuous radiation to the surrounding fluid. As a result, their amplitudes become immeasurably small after a short distance of travel as observed by Dayal and Kinra. This provided the motivation for the present work: a study of the propagation of harmonic waves in a solid plate loaded by a fluid layer of a finite thickness. In a previous work by the authors [6], the dispersion equation for an isotropic solid/fluid bilayer was obtained. It was seen that a thin layer of fluid coupled the symmetric (5) and antisymmetric (A) modes in the solid layer and that along a branch a quasi-symmetric mode changed character to a quasi-antisymmetric mode near a region where previously the S and the A branches crossed but were uncoupled. In the present work the dispersion equation for an orthotropic solid/fluid bilayer is derived. Mode shapes are studied for a graphite-epoxy/water bilayer for the case of equal thickness of the fluid and solid layers. In this case, coupling between fluid and solid modes is observed.

Eddy current testing is currently used to determine the physical characteristics of a conductive specimen and to detect defects by measurements of electrical impedance of an eddy current probe. In this study we developed two systems of coils allowing to determine properties of conductive coatings and foils. A probe contained two plane rectangular coils connected in series and separated by a fixed distance. A coated plate or a foil was placed between the coils and the coil impedance was measured using a digital impedancemeter. The discussed probe had a large length-to-width ratio and was modeled using the simple two-conductor line model, which express solutions in terms of the integrals containing no Bessel but, only common trigonometric functions, which considerably reduces the inversion time. The method allows reproducible measurements on coated conductive sheets. Aluminum 15–45 μm layers have been measured on steel and stainless steel substrates.

The starting point for this work were damages in rolls of chill casting. Results of damage analysis have shown that the reason for these damages were cracks caused by thermally induced residual stresses [1]. The reason for these microstructural residual stresses is a mismatch in the thermal expansion coefficients between the ferrite and cementite phases. It is well known that the thermal expansion coefficients of the ferrite and cementite phases are identical approaching the Curie-temperature of cementite (T_C=210°C for pure Fe₃C), whereas they are quite different at room temperature (Figure 1) [2,3]. Below the Curie-temperature the thermal expansion of cementite is smaller than that of the ferrite phase.

Many mechanical stress situations tend to be biaxial in character in that two stresses act along axes at 90°. Examples are the stresses found in gas pipeline, oil pipeline, power plant steam pipes, and railroad wheels.

Acoustoelastic stress measurement has been established for bulk shear wave, in particular with birefringence of the wave. This method, however, evaluates only average stress over thickness of a sample. Some preliminary studies have been reported for local surface stress measurement with Rayleigh or leaky Rayleigh wave. An approach using an acoustic microscope operated with burst waves over 100MHz [1, 2] is markedly sensitive to surface roughness and anisotropy of grains due to very short wave length of the wave. The other approach using contact transducers needs long propagation distance [3, 4].

The acoustoelastic measurement of stress is a topic with a rich history and the basic principles are well known [1]. In summary, one takes advantage of various nonlinearities which govern the elastic response of a solid, including but not limited to anharmonicities in interatomic forces, which lead to a stress dependence of the ultrasonic velocity. The basic idea, then is to precisely measure the velocity and to infer stress from a relation of the form (1)$$V = Vo + K\sigma $$ where V is the measured velocity in the presence of a stress σ, Vo is the value that would have been observed in the absence of that stress, and K is known as the acoustoelastic constant.

The rare earth material based on Nd-Fe-B alloy shows remarkable magnetic properties in the energy product (BH)_max and coercive force. It is manufactured by pressing in a unidirectional magnetic field to align the easy axes, and then sintering, leading to a large magnetic and mechanical anisotropy between the normal and parallel directions. The material is first developed by Croat [1] and Sagawa [2] independently in 1984. Following them, many researchers [3–6] have studied the material. They concentrated the efforts on the investigation of the manufacturing process to have better magnetic properties as well as on the observation of the microstructure to understand the mechanism realizing the high coercive force. The previous studies showed that the material consists of the major phase of Nd₂Fe₁₄B grains and the boundary phase of the Nd-rich alloys. It is considered that their different melting points and different thermal expansion coefficients introduce microcracks during the cooling process after sintering.

It is well known that crystallographic texture not only modifies the elastic constants of polycrystalline aggregates at (unstressed) natural states but also affects their acoustoelastic coefficients when the aggregates are stressed. While exact knowledge about the effects of texture on acoustoelastic coefficients has hitherto remained wanting, such effects are usually assumed to be negligible and are ignored in practical applications of acoustoelasticity (cf. [1] for example). Concerning this common practice, Thompson et al. [2] have urged caution:

Care must be taken when [this] assumption is made since the influence of texture on acoustoelastic constants is stronger than its influence on elastic moduli or velocities.

The run-out report for a compressor rotor showed an almost continuous bow between the two bearings, with the maximum run-out of 0.03 mm (0.0012 in)at the inlet to the fourth stage impeller. It had been run in service, and removed for normal maintenance, when the bow was observed. The seven stage compressor rotor was made of 34 Cr Ni Mo 6 steel, which is approximately a 4340 steel. It was just over 3 m in length, with diameters of approximately 265 mm at the disk mounting areas (Fig. 1). Each of the compressor stages was mounted on the rotor at the time of the stress measurement. An evaluation of the residual stress at these locations in the shaft was performed using a nondestructive technique involving critically refracted longitudinal, L_CR, ultrasonic waves. The L_CR stress measurement data showed compressive stress on the bowed side of the rotor, supporting the conclusion that residual stress is the cause of the bow. At the time that the data were taken, the rotor was horizontal, resting on two stands at the bearing ends. The following is a brief report of the test and the results. Ref. 1 should be consulted for additional detail.

Ultrasonic wave propagation in prestressed materials has been studied extensively in the last 40 years. Most of this work was concentrated on the effect of stress on the velocities of different types of ultrasonic waves in homogeneous materials. Actually stresses affect not only wave velocities but also the boundary conditions at the interface. Many practical applications of ultrasonic stress characterization involve wave propagation through the interface between fluid and solid or two solids. In immersion measurements one needs to consider the effect of stress on wave propagation from fluid to solid. This leads to change in propagation direction and energy redistribution. Also additional modes could be excited leading to stress-induced birefringence. These are all important phenomena which require rigorous quantitative description since the stress effect in general is very small. Another important problem is ultrasonic characterization of residual stresses in composite materials [1]. It involves wave propagation through an interface between layers with different properties and stress levels.

The use of nondestructive techniques to characterize the microstructure of textured materials has been the subject of a great deal of research interest over the past fifty years [1]. While ultrasonic, radiographic and electron beam (EB) techniques have been developed for this purpose, all approaches suffer from critical shortcomings. X-ray and EB techniques are highly accurate but are limited to localized measurements of near surface orientation. Furthermore, these techniques are not useful for characterizing fiber reinforced polymer composites. Ultrasonic techniques for texture characterization have been developed, but they require some type of simplifying assumptions in order to be practically useful [2–5]. Typically, one assumes weak anisotropy or some type of symmetry (with known symmetry axes) in the textured microstructure. Here, a tensor based approach is utilized to minimize the initial assumptions necessary to obtain a useful solution.

Residual stresses in cold formed parts can be present at high levels and have detrimental effects on the performance of highly loaded parts. In particular, tensile residual stresses can have serious effects on static mechanical properties, fatigue behavior, fracture performance and stress corrosion resistance. In such parts, residual stresses may be higher than those from service loading and require characterization to ensure adequate product performance.

Oil and gas pipelines are pressure vessels with steel walls operating at up to 70% of their yield strength. They need to be inspected rigorously to avoid failure and for environmental safety reasons. Magnetic flux leakage (MFL) is the most cost effective technique for in-service corrosion inspection of buried gas pipelines [1]. In this method, the pipe wall is magnetized to near-saturation using permanent magnets. If the wall thickness is reduced by a defect, more magnetic flux leaks from the wall into the air inside and outside the pipe. This ‘leakage flux’ can be detected by a Hall probe or an induction coil [1]. The circumferential (hoop) stress generated in the pipe wall by line pressure alters the MFL signal and needs to be accounted for when sizing the defect [1]. Defects also change the local stress distribution, creating stress concentrations which may lead to plastic deformation at the defect edge. As a result, the study of stress concentration around pits of different depths and made under different external conditions is important in estimating the size of the defect. This stress concentration around a defect can be measured directly by neutron diffraction [2] and photo-elasticity measurements [3]. We also measure stress concentrations indirectly using the magnetic Barkhausen noise (MBN) technique [4].

Pipelines are pressure vessels. Their enviable safety record compares well with other transportation modes. Typical pipeline fatality rates are about 1% those of rail or air which are, in turn, about 1% of highway fatalities. Pipeline safety is first assured by rigorous inspection during pipe manufacture and line construction. All welds are inspected using radiography to detect voids and ultrasonics to sense cracks. Oil and gas transmission lines are normally buried, so in service inspection must be performed from the inside by pumping an inspection “pig” through the line. Magnetic flux leakage (MFL) pigs are the most cost effective tools for corrosion monitoring. They are propelled by differential product pressure from one compressor or pumping station to the next, which may be more than 100km away. They are self supporting, demand maximum data storage density and highest energy storage battery power supplies as well as advanced signal processing to obtain signal discrimination and data compression.

As described in previous work [1], an applied uniaxial stress σ acts in some respects like an applied magnetic field operating through the magnetostriction λ.

In civil engineering structures involving prestressed concrete, the use of posttensioned systems is very common. Post-tensioning is achieved by passing steel cables into ducts previously cast into the concrete. When the concrete has set, the cables are tensioned with hydraulic jacks and then sealed into the duct by pumping in a slurry of cement and water, called grout. The grouting mixture, being alkaline in composition, protects the steel cable from corrosion.

Bridges with reinforcement corrosion problems are now under careful inspection in Taiwan. Costly maintenance programs are underway and raising serious safety concern. There are various engineering solutions to salt-induced corrosion. Among them epoxy-coated reinforcing bars, commonly referred to as rebar, are frequently used in marine environment and other areas due to its durability, reasonable cost, and convenience. However, coated rebar has lower bond strength and is less ductile than uncoated rebar. Thus it could result in larger crack width during pull-out tests [1,2]. The bond strength between coated steel bars and covered concrete results from the adhesion at the steel-concrete boundary, the factional force, and the interlocking force provided by the raised ribs at the steel bar surface. The interlocking force is much stronger than the other two, while the factional force occurs only if the adhesion vanishes after delamination or disbonding starts.

The fracture problem in concrete has been studied in great depth over the past thirty years. There have been many experimental and analytical attempts to calculate a work of fracture and to gain an understanding of the complex mechanisms of concrete fracture. An emerging theme is the intrinsic three-dimensional nature of the damage that occurs in loaded concrete. This has led to an increase in the amount of interior studies conducted. The insides of concrete have been exposed over the years by, among others, acoustic emission techniques, ultrasonics, dye penetration and radiography [1]. Most recently, x-ray microtomography has been used in order to see into the heart of concrete failure [2].

Concrete is an extremely versatile building material. It is being used extensively as a building material for defense and civilian structures and infrastructure. In defense applications, concrete is often used as the primary structural component in facilities that are hardened against enemy attack, especially projectiles that can impact the structure with a high rate of speed and a large explosive force. The high strain and strain rate of such an event make it imperative to know the mechanical behavior of concrete at these elevated loads in order to properly design the appropriate weapons that can penetrate such structures, or, conversely for defensive purposes, design the structure to withstand and survive such an event. Similar conditions can occur in the civilian sector. Depending on the geographical location of these structures, they can be exposed to similar conditions as some of the defense facilities. For example, an earthquake is typically composed of several different types of shock waves [1]. The exact nature of the shock waves is dependent on the nature of the earthquake source.

Image analysis algorithms are often used to identify features of interest in an image and then to count or categorize them. These algorithms can also be used to determine the geometric properties and distribution of features in the image. When concrete is observed in the form of a polished cross-section, several physical characteristics of the material can be determined that have the potential to be quantified using image analysis. These characteristics include the relative quantities of its constituent materials, the air void content, and the freeze-thaw durability of the material. A microscope operator generally estimates these characteristics by making statistical measurements. The air content of the cement paste, for example, is normally measured using the ASTM C-457 test [1], where air voids in the paste are counted by the operator as the specimen traverses linearly beneath the microscope. This procedure is very time consuming and is prone to human error because of the extended viewing periods, (up to four hours), required to complete the test. It is therefore desirable to develop an image analysis procedure that will decrease the amount of time required to complete the test and reduce the inconsistencies that human error can produce. As this paper will detail, a preliminary program for making these types of evaluations has been developed which calculates a durability parameter based on the analyzed physical characteristics.

The effective life of timber bridges is often shortened by decay of timber components and failure of timber connections. Consequently, periodic inspections must be carried out to identify potential problems. Reliable methods for in-situ assessment of the strength and degradation rate in terms of strength loss over a period of time are essential for maintenance and rehabilitation of wooden bridges. A nondestructive technique such as ultrasonic measurement and testing has been found to be more accurate than the conventional practice of visual inspection for assessing the condition of wooden members. Ultrasonic measurements have shown considerable promise in determining the stiffness and strength of wood members by identifying the presence of defects such as knots and decay [1,2,3]. Experimental results have shown significant differences between the velocities of ultrasonic signals in defect-free areas and areas with knots, decays, and other localized defects [4]. Halabe et al. [5,6] has shown that frequency domain signal amplitude and wave attenuation measurements, when used in conjunction with time domain velocity measurements, can be much more accurate and reliable than simply using velocity measurements in predicting stiffness and condition of wood. Use of simple parameters such as area under FFT amplitude plots or power spectral density plots can greatly simplify comparison of various signals in the field.

Wooden pallets exceed furniture and other solid wood products as the largest single use of sawn hardwood logs in the USA. Most wooden pallets are constructed from two types of pallet parts (Figure 1): (1) stringers—the structural center members that support the pallet load and (2) deckboards—the top and bottom facing members that provide dimensional stability and product placement. There are many variants of this basic design, but most pallets contain solid wood components that are produced from lumber or from the center cant material of logs. Cant material has a high percentage of defect area and is generally not highly valuable for other solid wood products. Therefore, the pallet manufacturing industry must make use of low-quality raw materials and yet produce a product that remains in service for many trips.

Timber bridges are used on all types of rail lines including important main transcontinental routes. The large number of these structures makes it critical that the limited maintenance funds available be targeted to structures in greatest need of repair. A combined structural dynamic and ultrasonic inspection approach has been proposed for identification of suspect structural members and in-situ evaluation of suspect elements [1]. Results of preliminary field tests were presented which showed the potential for this technique. Dynamic excitation of the system is used for a global test to identify suspect elements in the bridge system. Ultrasonic inspection is used for more extensive local evaluation of the suspect elements identified in the global testing. Preliminary testing of a single bridge member using an ultrasonic technique was shown to be possible under field conditions. This work extends these concepts to a small scale laboratory investigation to test the validity of the method used. The concept of higher order elastic coefficients for strength prediction is explored. It is found that the performance of these measures is not superior to the prediction of strength by elastic coefficients. Initial ideas are presented on the apparent success of techniques used in commercial testing equipment which use purely elastic measures of the properties of wood. These elastic properties are used for the prediction of wood strength by wood scientists. Future areas of exploration are suggested.

Interest in fiber reinforced polymeric (FRP) composites for structural highway applications has generated the need for reliable techniques which may be used to measure all of the elastic constants of these materials. Mechanical techniques may only be used to measure some of the engineering constants of these anisotropic materials due to the geometry of the pultruded members. Further, mechanical tests are destructive in nature. Ultrasonic techniques are uniquely qualified for the nondestructive measurement of all of the elastic constants of these materials. This paper presents the results of three ultrasonic techniques. The first of these is an immersion technique, similar to that presented by Gieske and Allred [1]. The last two techniques were developed specifically for this research, and implement optical generation and detection of surface acoustic waves for the measurement of some of the elastic constants. The results of the various techniques are compared to each other, as well as to results from mechanical tests.

The characterization of a layered structure has been considered by many investigators. A variety of techniques has been introduced. Among these normal incidence longitudinal waves can be applied in the low frequency regime, and the transmitted and/or reflected signals can be used for the evaluation of a layered medium [1, 2]. In the technique discussed in Ref. [1], a reference signal has to be introduced in order to calibrate the evaluation. In recent work by the present authors, a self-compensating technique has been proposed to evaluate a layered structure, in that a ratio of transmission and reflection coefficients has been used [3]. Applications using a self-calibrating or self-compensating technique for the measurement of surface waves reflected or transmitted by a surface-breaking crack have been reported in Refs. [4, 5]. The major advantage of the technique is that the evaluation is independent of the characteristics of transmission and reception of ultrasound by the transducers, the attenuation in the couplant and the surface condition of the specimen. The technique is not only applicable for a contact configuration but an immersion configuration as well.

Time-resolved line-focus acoustic microscopy (TRLFAM) combines the advantages of a conventional pulse-echo system with those of the acoustic microscope. Compared to high frequency line-focus acoustic microscopy [1], this technique employs a much larger (aperture 28mm) pulsed line-focus immersion transducer at much lower center frequencies. The insonified length of the specimen is an order of magnitude larger than that of the line-focus acoustic microscope operating at 225 MHz. This has the advantage that the amplitudes and the arrival times of the directly reflected wave, the leaky surface wave as well as other possible echo arrivals, can be time-resolved with considerable accuracy when the sample is moved inside the focal region of the transducer. Moreover, since the transducer is line focused, for an anisotropic material leaky surface wave arrivals can be time resolved along different directions. In earlier papers TRLFAM has been used to determine elastic constants for both isotropic and anisotropic materials [2].

A Computerized Ultrasonic Gauging System (CUGS) has been developed to generate very precise topographical maps of the outer and inner surfaces of tubes during various stages of manufacture and life-cycle of the parts. Measurements of the tube dimensions are obtained with a resolution of 2.5 μm (10−4 inches) and accuracies of the order of 10 μm (4×10−4 inches) or better. A typical output of CUGS is an ultrasonic image, in which the horizontal and vertical axes represent the axial and angular position of the part, respectively[1]. The system is currently utilized to gauge tubes as long as 7.5 m (25 feet) resulting in the acquisition of more than 15,000,000 data points, which require up to 60 Mbytes of disk memory on a computer. A Wavelet Transform Image decomposition technique was developed for compression and processing of the ultrasonic images, resulting in a great decrease in time and resources needed to perform such operations. Waveletbased image analysis has two distinct characteristics: multiresolution and high spatial localization[2]. Multiresolution refers to the possibility of obtaining representations of the same image with different resolutions. The high spatial localization properties of the filters used for the wavelet decomposition can also be utilized for the enhancement of features such as erosion pattern without the loss of localization, a problem commonly encountered in Fourier analysis. In the application here discussed, CUGS is utilized to map the wear of the internal surface of steel tubes, before and after exposure to extreme environments involving temperature, pressure, corrosive gases and mechanical forces.

Laser generated ultrasound (LGU) is now established as an important non contact technique. The use of pulsed lasers for LGU by direct illumination of a sample surface is a well documented method [1–6]. Aside from the comparatively low repetition rates of lasers suitable for LGU, a factor that limits their use is surface damage may be caused. LGU usually occurs between two extreme regimes, the ‘ablative’ regime (intense longitudinal waves generated at the expense of surface damage) and the ‘thermoelastic’ regime (damage free but relatively inefficient source for normal incidence waves).

Most of the large variety of non-destructive methods and their associated measurement instruments for flow, multilayer and crack detection are based on two physical principles: eddy-current monitoring and ultrasonic pulse- echo measurement. Application of the first one is limited by low spatial resolution and is useful for the study of magnetic metal samples. A well developed ultrasonic pulse-echo detection technique is free of such limitations, and can be used for quality control and material testing. Almost all commercially available ultrasonic NDT devices work at ultrasonic frequencies of 0.1 to 20 MHz and thus have spatial resolutions worse than 0.3–0.5 mm. Only simple evaluations of pulse-echo characteristics are usually made, such as determinations of transit time and maximum echo amplitude. As a result, ultrasonic inspection is used only to determine whether or not a test piece is free of discontinuities, and for thickness measurements.

Interpretation of ultrasonic NDT data can be a complex task. Interpreters must be highly skilled, and in the inspection of large engineering structures a bottleneck can arise at the manual interpretation stage. Boredom and fatigue of interpreters can lead to unreliable, inconsistent results, where significant defects are not reported. There is therefore great potential for the automation of the interpretation process through computer software. If some or all of the tasks of the interpreter can be performed by computer systems, the overall process is made more reliable and the time required is reduced.

For several years, the French Atomic Energy Commission (CEA) has developed a system called CIVA for multiple-technique NDE data acquisition and processing [1]. Modeling tools for ultrasonic non-destructive testing have been developed and implemented within this system allowing direct comparison between measured and predicted results. These models are not only devoted to laboratory uses but also must be usable by ultrasonic operators without special training in simulation techniques. Therefore, emphasis has been on finding the best compromise between as accurate as possible quantitative predictions and ease, simplicity and speed, crucial requirements in the industrial context.

ULIAS is the short term for ULtrasonic Inspection Applying Simulation. ULIAS is a module based software package comprising modules for numerical modeling, imaging, data compression, signal processing, and visualization in 1-D/2-D/3-D.

This paper reports progress in on-going studies to validate UT modeling of nozzle inner radius examinations. In a previous paper [1] it was shown that geometric modeling and raytracing in the Windows-based software WARay3D predicts the geometry and location of search units needed to detect known defects in a nozzle mock-up. The present paper describes the addition of beam forming [2] and flaw response [3] modeling to WARay3D and compares predicted amplitudes with those measured in the same nozzle mock-up. Beam forming and flaw response are formulated analytically and make use of the output of geometric ray tracing, which includes flaw detection and metal path leading to a computationally efficient hybrid approach. Correlation between predicted and measured amplitude drop is presented for ultrasonic signals from corner trap inspection of innerradius flaws. Reference signals are obtained from calibration tests using corner trap at a machined flat surface.

In order to use composite materials in aeronautical turbo engines, the resistance to impact damage caused by soft body (birds) must be understood. In this work the subperforation flat-wise soft body impact resistance of two kinds of carbon fiber/PEEK systems were evaluated using a gelatin projectile. Tested systems were AS4/PEEK (APC-2/AS4, ICI-Fiberite) and AS4/PEEK+IL, which consists of APC-2 prepreg and PEEK film inserted between layers as an interleave. To investigate the effects of the stacking sequence on resistance, three lay-ups- (0/+30/0/−30)s, (0/+60/0/−60)s, and (0/+45/90/−45)s- were tested. A gas gun system was used for the impact tests, where the velocity range was between 90 and 190 m/s. The mass of the projectile was about 3 g. The projected damage areas were measured with an ultrasonic C-Scan system. The characteristics of the soft body impact damages are evaluated by comparisons of the damages caused by the aluminum and gelatin projectiles. The relative impact resistance was discussed with the data measured in the previous hard body and gelatin impact tests. The impact damage caused by the gelatin projectile was almost the same as the damage shape caused by the aluminum projectile, but the gelatin impact resulted in a smaller projected damage area. The relationship between damage area(DA) and impact energy (IE) is linear for both the gelatin impact and the aluminum bullet impact. The DA/IE of gelatin impact was less than that for the aluminum bullet impact, and the gelatin impact indicated higher impact energy thresholds. The basic deformation mode of the aluminum and the composite specimens considered were pure bending at a lowest natural frequency without node diameters or circles, but in the frequency domain, the behavior of composites was more complicated than that of aluminum plates. The effects of stacking sequence system (T200/#3900) indicated a different relationship than that for the CF/PEEK systems. The impact resistance can be assessed with the proposed non dimensional stacking parameter βT2 and the value of (DA/IE)/βT2 which is independent of both fiber angle and lamina thickness. The CF/PEEK systems indicated a good proportional correlation of (DA/IE)/βT2 between the high velocity impact to the low velocity and the gelatin impact.

Composite materials are steadily replacing traditional materials in many industries. For many carbon composite materials, particularly in aerospace applications, durability is a critical design parameter which must be accurately characterized. Lawrence Livermore National Laboratory (LLNL) and Boeing Commercial Airplane Group have established a cooperative research and development agreement (CRADA) to assist in the high speed research program at Boeing. LLNL’s expertise in fiber composites, computer modeling, mechanical testing, chemical analysis and nondestructive evaluation (NDE) will contribute to the study of advanced composite materials in commercial aerospace applications. Through thermo-mechanical experiments with periodic chemical analysis and nondestructive evaluation, the aging mechanisms in several continuous fiber polymer composites will be studied.

The high performance of the available computer technology provides the possibility to simulate the real life for ultrasonic inspections in terms of primary ultrasonic data like rf-time signals. For isotropic material codes like Generalized Point Source Synthesis (GPSS) or Elastodynamic Finite Integration Technique (EFIT) and the theoretical predictions correlate well with experimental results. Recently, the codes mentioned above have been extended to operate also in anisotropic material. In a first step the codes GPSS and EFIT have been expanded to work in materials of parallel oriented columnar grain structure with transversely isotropic symmetry. In order to verify these codes a set of experiments was carried out on weld metal pads and on welds of defined grain structure. Radiation, propagation, reflexion on boundaries and interaction of the sound field with defects for the modes “through transmission” and “pulse echo” were simulated and compared with the experiments.

Pseudo-random signal correlation techniques can improve the flaw detection capability of ultrasonic NDE systems. While the correlation-based systems provide significant improvement in the signal-to-noise ratio compared to pulsed systems, their performance is limited by the so-called “self-noise” of the system. Self-noise is a result of imperfect autocorrelation characteristics of the excitation signal. Last year, we suggested some techniques for improving the flaw detection capability of continuous-mode ultrasonic NDE systems [1]. These systems use a continuously transmitted coded waveform as an excitation signal, and the received signal is processed through a correlation filter. This year, we present another new approach and demonstrate performance results and the practicability of each approach.

Pulsed eddy current (PEC) nondestructive testing differs from conventional eddy current techniques in that the probe coil is excited by a pulse, rather than continuous excitation at a single frequency. Reviews of early work on pulsed eddy currents are given by Waidelich1 and by Renkin.2 Pulsed excitation causes the propagation of a highly attenuated traveling wave, which is governed by the diffusion equation.3 The diffusive propagation of the eddy current pulse results in spatial broadening and a delay, or travel time, proportional to the square of the distance traveled. It was realized in early work on pulsed eddy current systems that this time dependence offered certain advantages over conventional eddy currents.4 In the current study we demonstrate the ability of a prototype pulsed eddy current instrument, described elsewhere,5,6 to take advantage of this time dependence to discriminate flaws from such interfering signals as probe liftoff, air gaps, and fasteners.

Recently, we proposed the high sensitivity ac field measurement (ACFM) technique for detecting and sizing surface cracks in metals [1–2]. This non-destructive evaluation (NDE) technique requires an inducer which possesses a field region with odd symmetry for positioning the probe. When properly orientated in this field region, a single linear probe acts as a differential probe with the added advantage of a phase contribution due to the crack. Both a rectangular coil and a rhombic wire loop can be used as inducers to provide the necessary field. In Fig.1, the positions of the probe with respect to these inducers are shown. As can be inferred from this figure, the probe is coupled to the magnetic field tangent to the metal surface.

Several commercial instruments currently are sold for the purpose of determining the strength of timber [1]. This information is needed because of the reliance of several sectors of our infrastructure on this material. The most notable uses of large timber members are in telephone poles and bridges used on railroads. In many of these applications the strength of the timber is critical to the safety and reliability of the utilities and railroads. While the use of wood treatments has extends the useful life of timber, decay remains the primary mechanism of timber bridge deterioration [2]. Decay is defined as a process which adversely alters wood properties and can be attributed to two primary causes, biotic (living) agents and physical (non-living) agents. The mechanisms of decay are complex, however those factors which are of interest in the current effort affect the strength of the wood and thus impact the integrity of the structure.

Holographic Interferometry methods have been successfully employed to characterize the materials and behavior of diverse types of structures under stress1,2,3. Specialized variations of this technology have also been applied to define dynamic and vibration related structural behavior4. Such applications of holographic technique offer some of the most effective methods of modal and dynamic analysis available. Structures and processed materials can be analyzed with very low amplitude excitation and the resultant data can be used to adjust the accuracy of mathematically derived structural models.

In an effort to enhance visualization of shock fronts associated with single explosive particle (diameter — 100μm) detonation, a Fourier holographic recording technique has been developed which relies on film non-linearities to greatly increase phaseobject visibility. The driving force behind this work is the investigation of detonation dynamics in dispersed particle explosives. These explosives, used for mine neutralization, are comprised of a fine, solid particulate dust which is dispersed as a cloud in the atmosphere over a given area. When detonation is initiated in some portion of the cloud, the ensuing detonation wave propagates throughout the entire cloud and results in an explosion, generating a tremendous pressure which serves to destroy or render useless any land mines present. Understanding the mechanism by which individual particles interact to sustain detonation in these solid dispersed particle explosives has been the research goal, and has led directly to the development of several holographic techniques.

The extent of corrosion on the world’s bridges, buildings, power line towers, and various other outdoor steel structures is a significant problem faced by today’s infrastructure. For example, in the United States there are approximately 576,000 bridges, 41% of which are deemed potentially unsafe due to their substandard condition [1]. Corrosion is the major cause of these structures being evaluated as inadequate.

Components made from advanced ceramics materials find widespread use in many industrial and military applications. However, the presence of defects in the bulk and on the surface of the ceramic parts can alter their operation and lead to a reduced lifetime or a catastrophic failure. These defects may include various inclusions, inherent powder defects, poorly distributed second phase material, as well as voids and cracks. They can be introduced at each stage of the manufacturing process. Near-surface defects are particularly critical in many applications since the stresses in this region of the ceramic component are greatest during the operation. These flaws may be intrinsic to the bulk material or can be introduced in the final stages of fabrication (e.g. machining, grinding and polishing). Additionally, in composite ceramics defects can appear as a delamination of internal layers. Because the potential market for ceramic components is so large, a considerable effort has been put into developing non-destructive evaluation (NDE) techniques to detect flaws at various stages of the manufacturing process [1–5].

Liquid penetrant inspection is one of the most widely applied nondestructive inspection processes and thus is a major source of confidence in the structural integrity of engineering systems in our society. The simplicity, broad applicability and low cost of liquid penetrant inspection enables and encourages use by workers with expertise ranging from knowledgeable and skilled, to unknowledgeable and unskilled. Although the results should not be expected to be the same, there is a natural tendency to assume equal capabilities and to assume that variations in the process will not significantly affect results. In many cases, old lessons learned must be relearned and the relearning initiative is often the result of major failure in a structure, component or system. The elimination of ozone depleting hydrocarbons has significantly changed the options for precleaning as the initial step in a penetrant inspection process. This paper is intended to re-identify the role and importance of precleaning in a penetrant process; the impact of changing established precleaning processes; alternate precleaning materials and experiences with alternates; the requirement to optimized and requalify penetrant inspection processes with alternate cleaners; and cautions on the use of silicated cleaners.

The highly anisotropic elastic properties of the plies in a composite laminate, especially those manufactured from unidirectional prepregs, interact strongly with the inplane vibration of shear ultrasonic waves propagating through its thickness1–3. The transmitted signals in a “crossed polarizer” configuration (with the transmitting and receiving transducers perpendicular to each other) were found to be particularly sensitive to ply orientation and layup sequence in a laminate4,5. This technique therefore holds good potential to be an NDE tool for detecting layup errors during the manufacturing of composite components. In such measurements, the transmitting transducer and the receiving transducer were rotated simultaneously, referred to as an azimuthal scan, while maintaining perpendicularity between them. The overall peak-to-peak amplitude of the RF waveform of the transmitted shear wave was recorded and plotted as a function of the transmitting transducer orientation. It was demonstrated experimentally that a single misoriented ply at the center of a 24-ply quasi-isotropic laminate can be detected with ease. Sensitivity to other errors in ply orientation and layup sequence have also been demonstrated.6 To continue the assessment of detection sensitivity, 48-ply graphite-epoxy laminates with and without intentional ply errors were fabricated. Azimuthal scans were performed to detect the errors and the results were quite successful, as described below.

In this paper, we describe a method to monitor changes in the diameter of through holes in thin plates. The method has the potential for applications in the industry.

Aluminum castings can contain pinholes caused by porosity from a number of sources, including the presence of gas or from shrinkage. These pinholes, although normally small in size and number, can greatly influence the properties of the casting, often resulting in the scrapping of the part. While many factors are involved in the formation of porosity in aluminum castings, such as the influences of hydrogen and aluminum oxide (A1₂O₃) on gas-porosity, this study investigates the effects of cooling rate and section thickness. These factors are studied by using a sand cast aluminum bar with different section thicknesses which undergo different cooling rates during casting [1,2].

Bench fatigue testing of the H-46 DCU rotor system components for flight qualification has provided a unique opportunity for the assessment of modal acoustic emission (MAE) as a possible health monitoring technique for crack growth detection. The DCU bench test program provided a variety of types of fatigue tests on actual helicopter dynamic system components with representative noise environments. The bench test program required to support the qualification of a production helicopter involves bench fatigue testing six specimens of each component or assembly and the DCU program had 17 components and assemblies which required qualification tests. Fatigue testing of this type exhibits some of the noise characteristics which are presumed to exist in dynamic components during flight such as bearings and pins interacting with component surfaces. Additional noises encountered during fatigue testing were from the test rig and electromagnetic interferences. Gathering time histories of both growing cracks and noise are important goals for assessment of the applicability of MAE to health monitoring.

The powder metallurgy (PM) industry is a high volume, low cost margin market. Therefore, it is essential that flaws are detected as early as possible in the manufacturing process. Previous NDE research has been directed towards finished products and has employed traditional methods such as ultrasound and eddy-current based testing techniques. However, the goal of this work is to develop a NDE method for green-state PM parts, the state prior to sintering. The material in these parts consist of large loosely packed grains of compressed powder, which are often ferrous, making most traditional methods unsatisfactory.

Single crystal CdTe and its solid solution Cd_1-x Zn_xTe alloys (0.03<x<0.05) are used as infrared transparent substrates for infrared focal plane array (IRFPA) detectors [1,2]. Cd_1−xZn_xTe substrates are normally “mined” from large polycrystalline boules grown by the unseeded directional solidification of Cd_1−xZn_xTe melts using either a vertical or horizontal Bridgman process[3–7]. The vertical Bridgman method is particularly popular for growing CdTe and its related compounds. In this process, a Cd_1−xZn_xTe ingot is produced by first melting a charge in the hot zone of the furnace and then vertically translating the furnace (with its associated axial temperature profile) relative to the stationary crucible to cause directional solidification. In spite of many experimental efforts to investigate the relationships between material purity, stoichiometry, the growth parameters (furnace temperature profiles, ampoule geometries/materials, furnace translation rates and starting position) and the resulting material characteristics (grain structure, dislocation densities, IR transmission, macro-segregation etc.), the yield of Cd_1−xZn_xTe of a quality suitable for large area (e.g. 4cm × 6cm) substrates remains low (<10%). Since much of this poor yield is directly associated with the crystal growth process (e.g. melt stoichiometry, solidification velocity, interface shape, temperature gradients, cooling rate, etc.) intensive efforts are under way to improve this technology.

This paper provides a progress report on the development of methodology to estimate Nondestructive Evaluation (NDE) capability. The methodology uses combinations of physical modeling of an inspection process, along with laboratory and production data, to estimate Nondestructive Evaluation (NDE) capability. The methodology is based on a physical/statistical prediction model and 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 physical model will explain and allow predictions for the effects of making changes to the inspection setup (e.g. probe properties and scan increment). The statistical/empirical model will quantify unexplained variability, adjust for model bias, and provide a means for obtaining corresponding uncertainty intervals. Previous work on this project was reported in Meeker et al. (1996). 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 because the inspection must detect very complex subsurface flaws in the presence of significant “material” noise. The underlying framework of the methodology should, however, be general enough to apply to other NDE methods. This paper describes recent work based on application of the new methodology to the detection of synthetic hard alpha flaws in titanium alloys.

The Federal Aviation Administration requires that transport aircraft be “damage tolerant.” That is, they must be: “evaluated to ensure that should serious fatigue, corrosion, or accidental damage occur within the operational life of the airplane, the remaining structure can withstand reasonable loads without failure or excessive structural deformation until the damage is detected. ” [1]

There is currently considerable interest in devising appropriate means for measuring how well test methods suit their avowed purpose. One of the major expressions of this interest has been through implementation of standards and guidelines issued by ISO/IEC (International Organization for Standardization/International Electrotechnical Commission) and, in Europe, CEN/CENELEC (European Committee for Standardization/European Commission for Electrotechnical Standardization). Among the goals of the ISO 9000 family of International Standards is provision of quality system guidelines [1] to complement the specific product requirements that are typically incorporated in specifications provided by “customers”. These criteria are intended to aid “suppliers” in achieving continuing improvements in product quality and customer satisfaction. Four key “facets of quality” are identified, associated with a) definition of needs for the product; b) aspects of product design that influence performance of the product; c) consistency in conforming to the product design; and d) providing life-cycle product support.

A new ultrasonic scanning system has been developed which is capable of accurate velocity measurements with high spatial resolution. This performance is achieved while using relatively low frequencies to minimize the cost of the instrument. A waveguide detector is used in place of the normal focused ultrasonic transducer. The waveguide receiver makes it possible to provide the needed spatial resolution without limitations imposed by the finite aperture of the transducer. An increase in the complexity of the signal processing required and reduced throughput of the instrument results from this approach. However, these disadvantages are amply compensated by the ability to investigate materials with high attenuation and low wave velocities. These measurements are not possible with traditional acoustic microscopes.

Laser generation of ultrasound and the subsequent detection of the ultrasonic waves using laser interferometry are areas of active research [1–6]. In earlier papers, the present authors have discussed an LBU system which employs a diffraction grating for illumination of a line-array to generate narrow-band surface waves and Lamb waves [4], and a fiberized heterodyne dual-probe laser interferometer to measure signals [3]. This paper reports progress towards the development of a robust low cost fiberized Sagnac laser interferometer suitable for field applications. Bowers first reported [7] the use of a Sagnac-type interferometer for surface acoustic wave detection, and the present authors have previously reported [8 QNDE 95] a variant of that scheme. In this paper, we present an alternative lower noise system that uses low cost, long coherence He-Ne lasers that have better intensity noise characteristics than typically used laser diodes. A scheme for elimination of a parasitic interference utilizing a frequency shifting technique has been developed. The primary advantage of the Sagnac interferometer is that it is exactly path matched and as such requires no heterodyning or static path compensation for sensor stabilization. The Sagnac interferometer described below is suitable for the measurement of ultrasonic surface waves arising from laser- or PZT-generated sources or from acoustic emissions. The laser-based ultrasonics (LBU) system can be used to detect and characterize discrete defects such as cracks.

for economic reasons, there is an increasing tendency to perform automated ultrasonic scans of near net-shaped forgings, which can have rather complex shapes, as opposed to inspection of simpler, sonic shapes, which typically have only planar inspection surfaces. A difficulty in the former approach is that the surface curvature of forgings causes the ultrasonic beam to focus or defocus within the component and, therefore, the ultrasonic sensitivity to internal defects changes as compared to inspection through a flat surface. It is certainly possible to account for this sensitivity variation by using curved calibration or reference blocks. However, a more convenient and cost effective approach is to use analytical models to predict the UT instrument gain corrections required for the curved surface inspections as compared to sensitivity levels measured from standard flat surface calibration blocks. This paper describes such models and their application to specification of ultrasonic inspection parameters for scanned ultrasonic inspection of curved forgings using both planar and focused transducers. Examples of these applications will be presented via case histories from the electric generation and aircraft engine industries.

The “Dripless Bubbler” technique[1–3] merges the spatial resolution and coupling advantages of focused-beam ultrasonic immersion probes with an apparatus for maintaining a contained water pool and a commercially available portable robotic scanner and image processing software. The focused probe resolution is necessary for the detection and characterization of corrosion on the inside of the fuselage skin, within laps and joints and disbonds or other defects in composite materials. The system scans over the aircraft skin, following the curvature of a fuselage while maintaining a transducer orientation normal to the surface. Surface irregularities including button-head rivets and lap splices are scanned over with no couplant loss.

Nondestructive testing (NDT) with surface waves is very sensitive to surface defects because the ultrasonic energy is concentrated in a small region at the surface of a specimen. The penetration depth of a surface wave is of the order of a wavelength [1].

The FAUST (Focusing Adaptive UltraSonic Tomography) system was developed at the French Atomic Energy Commission (CEA) to improve performances of ultrasonic non destructive testing in terms of adaptability to various control configurations and defect characterization. Unlike conventional techniques only allowing fixed focusing, this system can dynamically modify the characteristics of the ultrasonic beam. This system relies on optimized phased array transducers connected to a multi-channel acquisition system supplying amplitude and delay laws allowing to drive the ultrasonic beam.

The demand for information on the status of pipelines still rises. Stringent environmental protection laws, rising insurance costs and rising lifetime of existing pipelines ask for more detailed and reliable inspection results [1,2]. An ultrasonic inspection system is developed for the assessment of corrosion in fluid filled pipelines. In this paper an overview of the measurement system is given. Special attention is paid to the measurement head and the data-acquisition unit. Application of the system for the assessment of internal corrosion damage is illustrated.

Eddy current testing is a widely used nondestructive testing (NDT) method, particularly for inspecting the heat-exchanger tubes in steam generators. Due to the complex nature of the eddy current technique, the analysis of such inspection data is a difficult task which requires a huge amount of work by experienced human analysts [1]. This is time consuming and expensive. Human nature itself will cause some variance in analysis. Also, the variation of many different properties in the eddy current signal makes it very difficult to analyze. To overcome these obstacles, an eddy current automatic analysis system is needed to aid the analysts.

To enable GE Aircraft Engines to meet stringent engine inspection requirements a new eddy current inspection technology has been developed — the Eddy Current Array Probe (ECAP) [1]. The ECAP is based upon flexible micro-fabricated eddy current sensor elements that are capable of conforming to complex geometries. The technology has been used quite successfully by GE Aircraft Engines to inspect fracture critical aircraft engine components in a manufacturing environment.

During the past several years the electromagnetics laboratory at NASA Langley Research Center has focused on the Aging Aircraft Program. A major goal of this program has been the development of easy to use yet highly accurate inspection methods for the detection of flaws in airframe fuselage structures. A major breakthrough in this research came with the discovery of the Self-Nulling Probe Effect in November of 1992 [1]. It was clear that the unambiguous flaw signature of the probe could be developed into a low cost and easy to use fatigue crack detection device. Work toward this goal proceeded quickly, and a prototype hand held crack detector was introduced by mid 1993 [2]. As research into the precise flaw detection mechanism of the probe began to provide a deeper insight into the device [3–4], more sophisticated uses of the probe were conceived [5–6]. In particular, the Rotating Probe Method for the Detection of Fatigue Cracks under Airframe Rivets was beginning to be developed and tested by the end of 1993 [6], less than 1 year after the original discovery of the Self-Nulling Probe Effect.

On the basis of the approved nondestructive testing methods Barkhausen noise measurement and eddy current testing a microscope for both techniques has been built up. With this Barkhausen Noise and Eddy Current Microscope (BEMI) high resolution images of the distribution of the maximum of Barkhausen noise and the eddy current coil impedance can be obtained by scanning a sensor with a precise manipulation unit over a test specimen [1, 2]. The aim is the characterization of magnetic and mechanical properties of ferromagnetic materials (e.g. thin films). In addition to a suitable manipulation and signal processing system probes with high resolution and high sensitivity are necessary. These probes or sensors strongly affect the measured signals and the imaging characteristics. The frequency-dependent influence of the probe on the transmitted signals can be determined by the transfer function derived from a model of a ferrite ring head respecting the frequency-dependent complex permeability. The head parameters for computing the transfer function were found by measuring the head impedance at several frequencies and fitting the theoretical impedance function to the experimental results. To answer the question of the resolution of the microscope the effective width of the sensors were detected by linescans over a ferrite test specimen. In addition there will be shown examples of images obtained by the Barkhausen Noise and Eddy Current Microscope (BEMI) that indicate the range of applications.

Superconducting Quantum Interference Devices (SQUIDs) are extremely sensitive detectors for the measurement of magnetic flux. Especially the current High Temperature Superconductor (HTS) SQUIDs, which can be operated at liquid nitrogen temperature with easy cryogenic requirement, are well suited for the practical use [1]. Nowadays this HTS-SQUIDs are used for applications like the detection and localization of currents within the human heart, for nondestructructive evaluation of materials and for geological exploration.

The measurements of photoexcited excess carrier lifetime and activation energies in a semiconductor are useful in the characterization of the quality of semiconductor materials and in evaluating the performance of working semiconductor devices. The noncontact method of photothermal infrared radiometry (PTR), with both frequency-domain (PTR-FD) [1–3] and rate-window (PTR-RW) [4,5] detection configurations has been shown to be promising for remote on-line or off-line impurity/electronic defect diagnostics. A new PTR deep-level transient spectroscopy (PTR-DLTS) which combines the PTR-RW with semiconductor temperature ramping has been developed recently [6] and found to possess high spectral peak separation and spatial resolution.

Laser shearography, a form of electronic holography, provides more than 350,000 adjacent real time strain gages on the surface of the component or structure being imaged. This is performed without contact or surface preparation. This powerful tool has been successfully applied to a broad range of nondestructive testing applications, providing a rapid and precise wide area inspection. This technique provides unique capabilities and cost efficiencies over more typical inspection techniques such as x-ray and ultrasonics.

The present study is a part of the development of 3 critical components for a hybrid electric/gas turbine engine. One of these components is a radial Si₃N₄ ceramic turbine wheel that requires a nondestructive inspection prior to experimental tests. The goal of the examination is to detect, size and localize flaws 80 μm or larger in the turbine wheel whose respective diameter and density are 100 mm and 3.22g/cm3. 3D X-ray cone beam tomography was selected by Renault as a nondestructive evaluation method capable of reaching this objective. The strong X-ray attenuation of such a ceramic part and the research of small size defects have driven the design of our system.

There has been considerable interest in recent years in obtaining industrial x-ray computed tomographic (CT) images with micrometer or sub-micrometer resolution. Such systems are often referred to as micro-CT systems. These systems use a combination of high radiographic magnification and/or high-spatial-resolution detectors to collect either 1 dimensional (1D) or 2D projection data. This data is suitable for fanbeam or conebeam reconstruction, respectively.

Computed tomography (CT) is a radiographic method that provides an ideal examination technique whenever the goal is to locate and size volumetric detail in three dimensions. Because of the relatively good penetrability of X-rays, as well as the sensitivity of absorption to density and atomic number of matter, CT permits the nondestructive physical and, to a limited extent, chemical characterization of the internal structure of materials. Also, since the method is X-ray based, it applies both to metallic and nonmetallic specimens, solid and fibrous materials, and smooth and irregularly surfaced objects. X-ray CT provides quantitative, readily interpretable data and enables the inspections of structures that are not amenable to any other nondestructive evaluation technique. As a result CT has become well established as an inspection, evaluation, and analysis tool[1].

The PACE system has been designed by Electricité de France (EDF) to meet the needs of NDT data analysis encountered in the electricity production industry (but which can be extended to many industrial fields) [1]. Its main objective is to allow the exchange, the visualization and the processing of multitechnique NDT data being provided in a standard format, and their representation in a 3-dimensional environment, with respect to actual component geometries. The system has to be adapted to the requirements of multiple user profiles, ranking from a mere replay of an inspection to a more or less advanced expertise. It is also meant to take into account the difference of competence between NDT and computer science specialists, so that software developments must not be carried on by the NDT expert.

The program for Integrated Design, NDE and Manufacturing Sciences at the Center for Nondestructive Evaluation (CNDE) has as its mission to integrate NDE inspectability, materials and stress information, and overall product reliability into the designer’s computer aided design capabilities. In recent years the Center has provided design data to give design teams a way to consider the effects of certain design changes on inspectability in typical canonical family geometries of design parts. Providing software modules via the Internet is a logical extension to provide data.

A substantial portion of the hardwood lumber industry is devoted to the processing of lumber into secondary wood products (like furniture parts). Hardwood boards are typically remanufactured into smaller parts by a series of rip and cross cuts with no consideration of parts placement or the location of defective areas. These cuts yield pieces, in accordance with the manufacturers cutting bill, that are free of defective area. The entire process is labor intensive and results in a substantial loss of valuable lumber. With traditional gang/rip methods yields of 50% are considered to be very good. These losses increase with operator fatigue and inexperience.

Assessment of stresses in axially loaded bridge members is designed to evaluate and determine the load bearing capacity of the bridge. The load borne by a bridge changes over time because of rust, creep, loosening of components and changing live loads.