Application of Imaging Techniques to Mechanics of Materials and Structures, Volume 4

This paper deals with the use of full-field measurement techniques in experimental solid mechanics. The main techniques used in practice are briefly described and the different types of applications are classified according to their link with modelling. Finally, one these applications: identification of constitutive parameters from full-field measurements is developed in more detail.

A hybrid framework for inverse analysis of crack-tip cohesive zone model was developed to determine cohesive zone laws from full-field measurement of crack-tip fields by combining analytical, experimental and numerical approaches. The framework is based on the analytical solution method developed to extract cohesive-zone laws from elastic far-fields by using eigenfunction expansion of cohesive crack-tip fields and path-independent integrals. Electronic Speckle Patten Interferometry (ESPI) was used to provide crack-tip deformation fields as input data for the inverse analysis. To overcome ill-conditioning of the inverse problem, a global noise reduction algorithm was developed by implementing a PDE-constrained error minimization problem. The analytical, experimental and numerical approaches were combined to extract cohesive zone laws of fracture processes in glassy polymers, so called crazing. The results demonstrated that the inverse analysis framework provided a systematic and rigorous method to obtain cohesive-zone laws from experimental measurements, so that more realistic cohesive zone modeling can be achieved to predict fracture processes in various engineering materials and interfaces.

The identification of cracks stress intensity factors (SIFs) in elastic solids from full-field in-plane displacement field measurements is addressed. Because it is known that significant three dimensional effects alter the values of plane stress computed SIFs, and that the overall geometry of the crack front plays also a role, the problem of SIF’s identification is tackled here in the full three-dimensional framework. First is derived a data completion method in elasticity enabling the determination of the elastic displacement and stress fields inside the solid, or in the part where the behavior remain elastic, from surface displacements including the case of only in-plane displacement fields measurements over a traction-free surface. Then usual numerical methods for the computation of SIF or energy release rates are used. Numerical applications in three-dimensional elasticityfor true 3D geometry and loading are presented.

In the 2009 SEM Conference in Albuquerque, a measurement technique that combines the digital image correlation and the fringe projection was proposed by the authors to evaluate the 3D displacement of a specimen during a tensile test: the DIC is used to measure the in-plane displacement on both faces of the specimen while the fringe projection and phase shifting is used to reconstruct the shape with a high spatial resolution. In this work, the mentioned experimental technique was employed to characterize the plastic behavior of an aluminum alloy. First a 3D mesh was utilized to regularize the measured data and to get the strain field inside the specimen, in such way the full strain history of the test was reconstructed until failure. Then the Virtual Fields Method was adopted to identify the parameters of an anisotropic plasticity model. A comparison with FEM was also made to assess the correctness of the identified parameters.

Digital Image Correlation (DIC) is a technique that has a wide range of applications. At the High Explosives Applications Facility (HEAF), Lawrence Livermore National Laboratory, we are using DIC to perform a variety of strain measurements on plastic bonded explosives (PBXs). Because of the nature of these highly filled polymer composite materials, some conventional strain measurement devices, for example strain gages, cannot be used to measure strain reliably. While there are some drawbacks to DIC, there are also many advantages to this non-contact, optically based measurement system.

We have been successful in using DIC to measure strain in high strain rate experiments involving explosively driven plates. However, in this paper we will focus on our application of DIC to low strain rate measurements. Included in the work to be discussed will be DIC measurements of thermal expansion, Poisson’s ratio, strain derived from axial-torsion loading, and tensile creep. For each measurement type we provide some details on the additional hardware required and on the requisite modifications to our existing equipment. Also, in each case we will assess the effectiveness of our approach and, where applicable, compare results to those obtained using more traditional measurement techniques.

In this project, in cooperation with Airbus-France, an innovative failure approach to predict the rupture of composite structures subjected to complex 3D state of stresses has been proposed [1]. It was first necessary to understand the failure mechanisms occurring in L-angle specimens and to determine intrinsic out-of-plane strengths. The different out-of-plane failure mechanisms in composite laminated structures were identified by an experimental test campaign conducted on composite specimens subjected to different 3D states of stresses with different stacking sequences and thicknesses. This experimental test campaign has been performed on T700GC/M21 material and can be decomposed in three main batches of tests: Four Points Bending (FPB) tests on L-angle specimens, InterLaminar Shear Strength (ILSS) tests on plain laminated coupons, Unfolding Tests (UT) on L-angle specimens to validate the proposed mesoscopic 3D failure criterion. In each case, advanced measurement techniques was used (such as 3D Digital Image Correlation) in addition to standard investigation (strain gauge, LVDT measurements and micrographic analysis) in order to capture the different damage and failure mechanisms occurring during 3D loadings and to validate the FE computation. Due to the complexity of the problem, this study has been performed with a strong connection between experimental data and modeling with finite element analysis.

Image decomposition techniques such as geometric moment descriptors, Fourier descriptors, wavelet descriptors etc. have been used commercially in the fields of biometrics for finger print, iris matching and face recognition for almost over a decade. Initial test results suggest that these techniques can be used to assess the type and extent of damage in composite panels. The assessment can be performed by comparing the key geometric features in the full-field displacement, strain or stress maps of damaged components with those in the corresponding maps of undamaged components. In this study two shape descriptors viz. Zernike moments and Fourier descriptors will be used to represent full-field maximum principal strain data obtained from digital image correlation for composite specimens with different levels of damage. The advantages and short-comings of these shape descriptors will be discussed together with the possibility of combining Fourier decomposition with Zernike moments to provide a simple index of damage.

This paper presents a study of Finite Element Model Matching, based on the use of displacement field measurements. The wealth of information provided by full-field measurement techniques is extremely useful in experimental mechanics, because it is without contact. This study is a three-dimensional mechanical problem. An investigation of the secondary bending on aeronautical structures is presented, especially on the single shear lap joint. First we introduce the optical method and the results of field measurements applied to a tensile test on single-shear lap joint specimen. The experimental technique to determine the displacement field is presented. Afterwards we outline the use of Finite Element Modelling to achieve a correlation between numerical fields and their experimental counterparts. This study is applied to one type of joint; numerical and experimental example is presented: metal/metal lap joints, with one fastener (LGPL type). We describe the matching process. Finally, a parametric study is presented to show the effect of fastener preload clamping and fit clearance between plate hole and shank fastener on the secondary bending.

In this work, cylindrical inclusions of two different elastic moduli – stiff and compliant relative to the matrix – are examined. The former is a borosilicate glass inclusion whereas the latter is a polyurethane inclusion in an epoxy matrix. The 2D digital image correlation (DIC) technique along with high-speed photography is used to study crack growth behavior as a function of inclusion-matrix interfacial strength and the inclusion location relative to the crack under stress wave loading conditions. An ultra high-speed rotating mirror-type CCD digital camera is used to record random speckle patterns in the crack-inclusion vicinity at rates ranging from 150,000 to 300,000 frames per second. Two sets of images, one set before loading and another set during the fracture events are recorded. The crack tip deformation histories from the time of impact until complete fracture are mapped and fracture parameters are extracted. Different crack propagation patterns in terms of crack speed, crack path and stress intensity factor histories are observed followed by a macroscopic examination of fractured surfaces.

Segregation-induced microstructural banding is commonly encountered in commercial steels, yet its effect on the global mechanical behavior is disputed in the literature due to the difficulty of designing clean control experiments. This work locally compares the deformation of banded phases with unbanded regions in the same microstructure from in-situ electron micrographs analyzed with (optimized) micrographic digital image correlation. To this end, first, the employed electron micrographic digital image correlation (EMDIC) methodology is optimized in terms of its experimental parameters: specimen surface preparation settings, scanning electron microscopy imaging settings (contrast modes, magnification, resolution, and contrast-brightness), and image correlation settings (facet size and facet step size). Subsequently, the strength of the (optimized) EMDIC methodology is demonstrated on a case study on segregation-induced microstructural banding in steel, in which the influence of the banded phase and its morphology is probed by comparing the mechanical behavior of two carefully chosen, extreme cases of banded microstructures: a microstructure containing a continuous, hard band (the martensitic-ferritic system) and a microstructure containing non-continuous, softer bands (the pearlitic-ferritic system). The obtained micro-scale strain fields yield clear insight into the influence of band structure, morphology and band material properties.

The purpose of this study is to perform Digital Image Correlation (DIC) by using images obtained from Scanning Electron Microscopes (SEM). Firstly, some pictures of SEM at magnifications of 200X and 5000X are taken without moving the specimen, and DIC is performed using by these pictures. From the results, we can see that the errors of small random displacement occur at whole correlation area for 200X and 5000X images, and drift of specimen occurs for 5000X images. However, the errors of small random displacement can be removed by performing the original image processing. About drift, we measure the amount of drift. As a result, it is seen that the amount of drift increases monotonously. Therefore, we try to predict the amount of drift by the proposed original method. Then, the drift can be eliminated from the results of DIC using SEM image.

In this paper, we present results on estimating material model parameters from inverse analysis of full-field deformation data that was obtained with a prototype of a novel integrated tool consisting of a digital image correlation system and software for data analysis and parameter estimation. Such a tool is needed for characterizing the properties of new materials, and for calibrating and validating material models. The stereo microscope-based image analysis system may be used for measurements at temperatures up to 75°C, and field sizes of approximately 1 mm. The Graphical User Interface (GUI)-based parameter estimation tool integrates modules for image data analysis and inverse analysis, and incorporates features for interfacing the tool with commercial finite element (FEM) packages. The GUI, together with a micrograph of the sample, is used to select a subset of the imaged region for analysis, and for specifying sample grain boundaries needed for developing the FEM model. Data analysis includes data averaging to reduce measurement noise, and filtering to correct for rigid body translations and rotations. The inverse analysis module runs the FEM model under experimental loading conditions within its iterative loop, using the downhill simplex method for parameter estimation. The methodology was successfully validated from measurements on a superalloy sample.

Subcritical crack propagation in amorphous silica glass can be imaged during propagation with an AFM at different stages of propagation. Digital Image Correlation constitutes an attractive technique to measure the displacement fields relating different topographic images, and from it to estimate quantitatively the stress intensity factor governing the crack velocity. However, the emergence of the crack on the free surface, which is imaged, induces a significant out of plane motion whose magnitude is comparable to the surface roughness. This difficulty calls for an extension of the usual “brightness conservation” allowing for an evolution of the image texture, i.e. surface topography. The high level of AFM-image noise is tackled through integrated DIC, namely, few well chosen kinematic fields are selected to decompose the searched displacement field. The latter are considered because of their mechanical relevance to the problem at play (e.g., rigid body motion, plane stress mode I displacement field, out-of plane singular field) or to compensate for AFM artifacts. The resulting novel DIC algorithm provides directly an evaluation of the stress intensity factor,

KI

, without any further post-processing; 10% (resp. 15%) uncertainty on

KI

is achieved based on 1 × 1 μm

2

(resp. 200 × 200 nm2) images.

In the recent years, structural bonding takes an important place in assemblies techniques used for the automotive design. The next step to optimize the use of adhesive in car structures is to realize accurate finite element simulations of behaviour until failure of bonded joints. These kinds of calculations are only possible if fine behaviour and failure models are provided into the finite element software. In these works, tests on bulk adhesive specimens are realized to characterize the mechanical properties. 2D and 3D Digital Image Correlation are used to investigate the behaviour and failure of the shear and tensile specimens.

Computer models of head-brain biomechanics offer enormous potential for improved understanding and prevention of traumatic brain injury (TBI). However existing computer models remain controversial because their predictions have yet to be rigorously compared to measured biomechanical data. The nonlinear, anisotropic, viscoelastic, heterogeneous character of brain tissue, and the intricate connections between the brain and skull, all complicate modeling efforts. In order to make progress toward the goal of accurate simulation of TBI, experimental techniques to address these issues must be developed. In this paper we describe two magnetic resonance (MR) imaging techniques to characterize brain deformation, estimate brain material properties, and illuminate the boundary conditions between brain and skull.

MR tagging

is used to estimate displacement and strain fields in response to rigid-body acceleration of the skull, and

MR elastography

is used to visualize shear wave propagation induced by oscillatory loading at the surface of the skull.

Magnetic resonance elastography (MRE) is a novel experimental technique for estimating the dynamic shear modulus of biological tissue

in vivo

and non-invasively. Propagating acoustic frequency shear waves are launched into biologic tissue via external mechanical actuator and a conventional magnetic resonance imaging (MRI) scanner is used to acquire spatial-temporal measurements of the wave displacement field with micron precision. Local shear modulus estimates are obtained by inverting the equations governing shear wave motion. Changes in tissue pathology may be accompanied by a stark change in tissue elasticity. As a result, MRE has appeal to healthcare practitioners as a non-invasive diagnostic tool. Recently, MRE-based modulus estimates have been obtained in animal liver, brain, and heart [2-7]. Here, for the first time, MRE was used to probe the shear modulus of mouse eye vitreous humor

in vivo

and non-invasively.

This paper deals with the application of the Virtual Fields Method to the identification of the shear modulus of a gel from Magnetic Resonance Elastography data. Volume deformation fields in the cube were recorded at different times during the harmonic loading and the full harmonic response has been reconstructed using Fast Fourier Transform. Strains were then obtained by direct spatial differentiation, without any smoothing. The VFM was then applied with inertial forces balancing out elastic forces, without including the loading force which was not measured here. It has been shown that the choice of the virtual field is critical with such a spatial wave deformation field. A wide range of spatially harmonic virtual fields has been tested at different times within the loading period. The identified shear modulus has been shown to be consistent and to correlate with the value obtained from a simplified approach based on the shear wave solution. This is a feasibility study, it will be extended in the future to heterogeneous materials with a more thorough procedure to build up relevant virtual fields.

Magnetic resonance elastography (MRE) is a novel experimental technique for estimating the dynamic shear modulus of biological tissue. MRE can be performed non-invasively, in living subjects. Soft biomaterials are notoriously difficult to characterize, since they are typically nonlinear, anisotropic, viscoelastic, and heterogeneous. The ability of MRE to capture the frequency-dependent response of tissue to small amplitude deformation over a range of frequencies was investigated by careful comparison to two different dynamic mechanical tests; direct shear and unconfined compression. The mechanical properties of a standardized gelatin biomaterial were probed over various loading rates. Results confirm direct correlation between estimates of shear modulus obtained by MRE, dynamic shear, and unconfined compression, but quantitative differences between values obtained by MRE compared to direct mechanical test. These results in gelatin are consistent with reports in agar from other groups [1,2]. Differences may be due to non-idealities inherent in loading of soft, wet, material (in mechanical testing), boundary effects (in MRE), or differences in strain amplitude and strain rate.

Although crack mechanisms in bone have been intensively studied to have a better understanding of bone fracture, exact prefailure damage mechanisms about how cracks or deformations at nanoscales occur still remain unexplored due to technical limitations. In this pilot study, we apply back-directional gated spectroscopic imaging (BGSI) to examine the exact spatial extent of such damage in in-situ mechanical testing of bovine cortical bone. Our imaging approach provides a relatively large field of view, while the wavelength dependence of light elastically backscattered from bone at each pixel can capture structural alterations in a few tens of nanometers. Thus, our imaging method can simultaneously examine various length scales. Using a notched bovine cortical bone wafer, we report that an altered field of a couple of square millimeters forms at the tip of the notch during tensile loading in the transverse orientation, and this field disappears upon unloading. We conducted simple pilot simulations of optical waves in one-dimensional layered media to gain an understanding of the potential mechanisms about the spectral dependence on nanostructure alterations. Our results imply that the bone nanostructure may allow the formation of nanoscale deformation over a relatively large area to prevent microcrack formation or fracture as an energy dissipating mechanism. We further envision that BGSI may facilitate understanding how the nanostructure of bone controls bone characteristics and properties.

This study represents a preliminary activity for the biomechanical numerical modeling aimed at the prediction of the human foot behavior and the deformation under different load conditions. It also represents the starting point to develop a scientific approach for the functional mass customization aimed at the optimization of comfort in footwear. Reverse Engineering (RE) methodologies developed for building up the external shape of the human foot are presented and discussed. Aim of this work is to study the problem of the digitalization of human feet under different conditions using three technologies: shape from stereo, from silhouette and from shading. The foot is one of the most difficult human parts to reconstruct taking into account the complex surface and the high curvature. In this article the disadvantage and advantage of each technique are analyzed. In particular tests about reliability and precision of the measure are considered.

Force-sensitive molecules, called mechanophores, exhibit a chemical response to mechanical force and can be incorporated into the polymer chains. Mechanically stressing these polymers in turn can activate the mechanophore, producing an advantageous chemical response. We have previously demonstrated activation of a mechanophore called spiropyran, which undergoes a force-induced, 6-

p

electrocyclic ring-opening reaction accompanied by a color change, in linear polymers in solution via sonication and in bulk solids via tension and compression. Reliable, fully characterized transfer of macroscopic stress on a bulk solid polymer to the mechanophore remains a topic of active research. The premise for mechanical activation in linear polymers is that aligned polymer chains can better transfer mechanical energy to the mechanophore than a randomly oriented chain. We have combined photoelasticity and fluorescence measurements for the same field of view during uniaxial tension experiments of two bulk linear solid spiropyran-linked polymers, elastomeric poly(methyl acrylate) (PMA) and glassy poly(methyl methacrylate) (PMMA), in order to quantify the influence of polymer chain orientation, determined from optical birefringence, on mechanophore activation evident by color change. These experiments elucidate the critical molecular orientation and macroscopic stress level required to activate the mechanophores, which are critical for the design of systems incorporating mechanochemically active polymers.

Electronic Speckle Pattern Interferometry (ESPI) provides a sensitive technique for measuring surface deformations. The technique involves comparison of the speckle phase angles within surface images measured before and after material deformation. This phase angle comparison requires that the speckle positions be consistent in all images. A lateral shift between images by just one pixel substantially degrades ESPI measurements, while a shift of two or more pixels typically causes complete decorrelation and compromises the measurement entirely. To prevent such lateral motions, the specimen and the optical system must be rigidly fixed. This requirement typically prevents use of the ESPI method in applications outside laboratories or where it is necessary to remove the specimen from the optical setup between ESPI measurements. Here, Digital Image Correlation (DIC) is used to track speckle motion caused by specimen displacement between ESPI measurements. The measured images can then be mathematically shifted to restore the original speckle locations, thereby recorrelating the ESPI measurements. Examples are presented where ESPI measurements are successfully made with specimen shifts over 60 pixels.

This paper reports a new technique, namely the incremental micro-hole-drilling method (I??HD), for measurement of residual stress profiles as a function of depth. Like its macroscale counterpart, it is applicable to either crystalline or amorphous materials, but at the sub-micron scale. Our method involves micro-hole milling using the focused ion beam (FIB) of a dual beam FEGSEM/FIB microscope. The surface displacements are recorded by digital image correlation of SEM images recorded during milling. The displacement fields recorded around the hole are used to reconstruct the stress profile as a function of depth. In this way residual stresses have been characterised around drilled holes of 4microns or so, enabling the profiling of the stress variation at the submicron scale to a depth of 4microns. An average −800 MPa ± 90 MPa was identified approximately 15

°

to the specimen length and -600MPa± 90 MPa perpendicular to it. The new method is used to estimate the stresses in a (peened) surface-severe-plastically-deformed (S

2

PD) Zr

50

Cu

40

Al

10

(in atomic percent, at.%) bulk metallic glass.

In this effort, a corner crack was grown from a notch in a nickel-based superalloy specimen with a shot-peened surface treatment to induce residual stresses. The crack length was less than 200 microns, and the full displacement field near the crack was determined using advanced digital image correlation. The specimen was then annealed at elevated temperatures to reduce or eliminate the residual stresses, and the full displacement field near the crack was again determined. The displacements after annealing were indeed significantly larger than those previous to annealing, demonstrating the reduction in residual stresses. Modeling has been done to determine the approximate level of residual stresses induced and then reduced through annealing. Through this method, the effect of residual stresses on short fatigue cracks can be directly studied. Further work will be discussed on the effect of temperature on residual stresses, an area of great concern in predicting fatigue lives of components. The degree to which these residual stresses are reduced under service conditions is not well understood, and the approach described here is expected to be extremely useful in determining and predicting this residual-stress reduction, leading to greatly enhanced life prediction where residual stresses are involved.

Performance and reliability of silicon based microelectromechanical systems (MEMS) and photovoltaic (PV) devices are often strongly affected by defects and residual stresses. As a result, both industry and academia need new tools that can rapidly locate and quantify defects. The grey-field photoelastic technique has been shown to improve detection and residual stress quantification of defects in wafer bonded MEMS structures. In the PV industry, the tool has proven capable of detecting high bulk stress in wafers, damage due to wafer cutting, and stresses associated with trapped cracks. In this paper, we describe the development of an infrared photoelastic tool that captures full-field residual stress images at camera framing rates. This solid state tool, which adapts technology developed for visible light residual stress inspection, reduces the data collection time for residual stress maps from several minutes to a fraction of a second. We demonstrate the tool on canonical samples including a beam in bending and disk in compression to verify stress results and identify the lower limits of detection. We then demonstrate the tool on several industrial applications.

In-situ straining experiments and residual stress evaluations by micromachining require accurate measurement of the surface displacement. These can be conveniently achieved by Digital Image Correlation (DIC). Three surface decoration techniques are presented to enhance surface deformation and residual stress measurement capabilities on micron-scale samples within a Scanning Electron Microscope – Focused Ion Beam (SEM-FIB) instrument. They involve the use of yttria-stabilized-zirconia nano particles applied chemically, nano platinum dots applied using FIB, and Focused Electron Beam (FEB) assisted deposition. The three decoration techniques create distinctive, random surface features that can be used with Digital Image Correlation to provide full field maps of surface displacements at high magnifications. A series of experiments using a FEGSEM /FIB demonstrated the effectiveness of the three surface decoration techniques for FEGSEM imaging at magnifications from 2,000× to 60,000×. The resolution of the image correlation is substantially enhanced by the surface decoration, with displacement standard deviations reduced to the 0.005 – 0.03 pixel range, depending on the patch size used. The implications for displacement measurement at the micro- and nanoscale are discussed and some examples shown.

Full field measurements (thermal and kinematics) are powerful techniques to study the thermomechanical behaviour of materials. This communication presents coupled full field measurements conducted on NiTi SMAs during tensile tests. Temperature fields are used to estimate the heat sources associated with the stress induced phase transformations. A spatio temporal synchronisation between the thermal and kinematics fields allows to express the thermal or heat source fields in the reference (undeformed) configuration. Finally, a temporal integration of the heat sources for each material point gives the evolution of the heat induced by the phase transformation. Examples of such measurements are given in this communication.

Shape memory alloys (SMA) have specific properties that are mainly due to the martensitic transformation occurring in the material when mechanical or thermal loadings are applied. To study the effect of strain rate on the transformation occurring on an NiTi SMA different tests were performed at different strain rates in the range of 0,001 /s to 15 /s. For the dynamic tensile tests, a Split Hopkinson Tensile Bar set-up was used with a fast camera recording at 45'000 fps used to measure the extension rate of the martensitic phase region in the specimen. For quasi-static tests, an additional infrared thermography measurement was used. A superimposition of DIC and IR measurements in time and space can be done during quasi-static tests and the results show that the temperature peak, as expected follows the transformation front. As a consequence of the former validation of the DIC procedure, the velocity of the transformation front at high strain rate is deduced from space-time figures. As a conclusion, in the range of strain rates investigated in this paper, no strain rate sensitivity is observed for dynamics of extension of the transformation region.

Conventional structural cables (wire ropes) are composed of steel wires helically wound into strands that are then wound around a core. Cables made from shape memory alloy (SMA) wires are a new structural element with promising properties for a broad range of applications. Among the many potential advantages of this form are increased bending flexibility for spooling/packaging, better fatigue performance, energy absorption and damping, reduced thermal lag, redundancy, and significant design flexibility. Currently there are few studies of SMA cables in the literature. This paper describes exploratory thermomechanical experiments that were performed on two commercially available cable designs.

This paper deals with the analysis of the mechanical response of an aluminium multi-crystal specimen subjected to a tensile load. Two different techniques are used for this purpose: the grid method to obtain strain maps and infrared thermography to deduce heat source distributions from temperature fields. Significant differences between the grains in terms of mechanical response are observed. Experimental results are compared to numerical results obtained with a suitable FE package. A good agreement is observed apart from very localized phenomena occurring near the boundaries of the grains which are not predicted by the FE model

Strain localization phenomena occur in many materials in different forms, such as PLC & Lüders’ bands or Shape Memory Alloy stress-driven martensitic phase transformation. The development of kinematic and thermal full-field measurement techniques, like Digital Image Correlation (DIC) and Infra-Red (IR) thermography helps understanding these tricky phenomena and allows for the identi??cation of such material behaviors. When such kinematic and thermal measurements are coupled, they even offer a full thermo-mechanical characterisation. Unfortunately the space and time association of both ??elds remains a major di??culty (antagonist surface texture requirements, imaging devices having di??erent pixel number and acquisition rate...). To get round this problem, the present paper introduces a much simpler experimental approach: a novel extended DIC technique applied to a single set of IR images gives access to both displacement and temperature ??elds decomposed over the same discretization. A first application is performed, where the strain localization due to the phase transformation of a NiTi SMA and its associated thermal dissipation are jointly measured.

The purpose of this paper is to investigate the thermo-mechanical behavior of clutch facing materials using image processing techniques. Infrared thermography and digital image correlation were used to analyze the mechanical behavior of clutch facing material under tensile loads. For specimens under tension loads, Infrared thermography clearly showed the thermo-elastic coupling as well as thermal dissipations due to the microcracking state of damaged clutch facing specimens. During an up to failure tensile test, localized thermal effects were observed at the macro-crack location before it appeared. Strain fields at the surface of specimens under tensile loadings were determined using the digital image correlation (DIC) technique. These experiments showed that the strain fields early became inhomogeneous. Strains concentrated in multiple localization zones which highlighted the role of the fibers in transmitting the internal forces.

Digital Shearography (DS) and Electronic Speckle Pattern Interferometry (ESPI) are laser based optical interference techniques used amongst other to inspect materials and manufactured components for defects. DS captures the rate of surface displacement and ESPI the displacement of an object in response to an applied stress. There are a number of ways to stress the object during the inspection process, the most common being the use of thermal heating or vacuum and pressure chambers. These forms cause whole field object responses, which generate the equivalent fringe patterns. Within these fringe patterns fringe anomalies revealing the presence of a defect are sometimes masked due to the object’s whole field displacement. As an alternative stressing technique, this paper investigates the use of vibration excitation methods. The intention with this approach is that the friction produced in an object’s defect area generates localised thermal gradients which in turn should be revealed in the produced fringe pattern. This paper describes the above mentioned inspection techniques and examines the results obtained using Digital Shearography and ESPI when applied to selected samples. In particular the results obtained using vibration excitation are compared with results obtained using thermal stressing techniques, which ultimately attempts to determine the suitability of vibration excitation methods to inspect objects for defects.

This research incorporates an experimental study of stress-induced martensitic phase transformation in the shape memory alloy Nickel-Titanium. The rich local thermo-mechanical interactions that underlie the solid-to-solid state phase transformation between the cubic austenite phase and the monoclinic martensite phase are examined via Digital Image Correlation (to obtain local strain fields) and infrared imaging (to obtain corresponding local thermal fields). Although other methods have been used to explore phase transformation in shape memory alloys, this methodology is unique in providing a quantitative estimate of the strain inside the area of martensitic transformation, as well as direct correlations of local strain and temperature fields. Using this combined methodology, we are able to quantify the complex local interactions between released/absorbed latent heat and the extent of transformation, and explore the characteristics of the phase fronts (velocity, width, etc) and the evolution of martensitic volume fraction. There is also evidence of a remarkable cyclic strain memory on the microscale.

The determination of strain fields based on displacement ??elds obtained via DIC is subjected to several errors that originate from various sources. In this contribution, we focus on a triplet of these when substantial plastic deformation of the specimen is probed. First, attention is paid to errors that can be directly attributed to the derivation of the strain ??elds, e.g. the strain-window size and the strain-window interpolation order. Next, we focus on errors that arise from different implementations of the DIC technique. In particular, we investigate the in??uence of the shape function, the interpolation order and the subset size on the derived strains. A dynamic shape function criterion is developed that increases the transformation order according to the degree of heterogeneity. It is shown that the impact of the subset size on the derived strains persists, despite the fact that it is embodied in the noise of the displacement ??elds and should largely evaporate during the smoothing procedure. Finally, we study the impact of a non-perpendicular alignment of the camera on a planar specimen. To this purpose, we make a mutual comparison of 2D results obtained via a perpendicular and a non-perpendicular CCD, and a 3D evaluation of the stereo setup. In addition, we estimate the impact of a recti??cation of the images obtained via the non-perpendicular alignment.

In this investigation the vibration response of a clamped-clamped steel curved beam is studied experimentally using the non-contact, full-field 3D digital image correlation (DIC) technique. The effect on the boundary conditions of the clamping torque as well as the procedure used to fasten the beam to the testing apparatus was clearly established from measurements of the three-dimensional displacement field. Swept sine tests were performed at 1 g of base excitation and over a frequency range of 20 – 500 Hz. Resonance was qualitatively identified at frequency ranges 75 – 85 Hz, 120 – 138 Hz and 240 – 265 Hz. Swept sine tests were carried out at a rate of 0.2 Hz/s over these three frequency ranges to measure the distribution of the out-of-plane displacement of the beam mid-plane over its entire length. 1024x1024-pixel images were taken at a rate of 2000 fps at frequency 76, 126 and 253 Hz and analyzed using 3D DIC. The results show three different bending modes at these frequencies.

Digital speckle correlation techniques have been successfully proven to be an accurate displacement analysis tool for a wide range of applications. With the use of two cameras, three dimensional measurements of contours and displacements can be carried out. With rapid new developments in the field of digital imaging and computer technology, the capability of this technique is increasing, opening further applications in the fields of material testing, fracture mechanics and vibration analysis. The high resolution of the deformation measurements in space and time can accurately determines the absolute position and displacements of objects, even if they display high amplitudes and large rigid body movements. The absolute resolution depends on the field of view and is scalable. Calibration of the optical setup is a crucial point which will be discussed in detail. Examples are described including the analysis of a harmonic vibration and transient events from real industrial applications showing the interaction of large accelerations and complex rigid body motions.

Uncertainty quantification (UQ) equations have been derived for predicting matching uncertainty in twodimensional image correlation

a priori

. These equations include terms that represent the image noise and image contrast. Researchers at the University of South Carolina have extended previous 1D work to calculate matching errors in 2D. These 2D equations have been coded into a Sandia National Laboratories UQ software package to predict the uncertainty for DIC images. This paper presents those equations and the resulting error surfaces for trial speckle images. Comparison of the UQ results with experimentally subpixel-shifted images is also discussed.

We develop an Improved 3D digital volume correlatiion (DVC) technique to measure displacement and strain fields throughout the interior of a material. Our eventual goal is to perform DVC with resolution comparable to that achieved in @D DIC, with a correlation time that is commensurate with the image acquisition time .this would represent a significant improvement one the Current state-of the art available in the literature. Using an X-ray micro-CT scanner, we can resolve features at the 5 micron scale ,genarating 3D images with up ti=o 8 billion voxels. We compute twevle Degrees -of- freedom at each Generating #D images with up to * billion voxels. We compute twelve degrees-of -freedom at each correlation point and utilize tricubic spline interpolation to achieve high accuracy- For DVC the volume of data,number of Correlation points and work to solve each correlation point grow cubically. We therefore employ parallel computing to handle this tremendous increase in computational and memory requirements .We demonstrable the application of DVC using an artificial deformation of actuall PDMS samples with empedded particles forming an internal Pattern.

X-ray computed microtomography(XCMT) is increasingly being used to visualize the complete microstructure of various materials.Ons of its main advantages lies in the non-destructive way of optaining three-dimensional (3D) views of various materials. By analyzing 3D re constructured picturs, one has access,for instance,to the structure of biological or cellular naterials. and sometime to the way deform by using in Situ experiments analyzed by 3D Correlation materials, and sometimes to the way they deform by using in situ experiments analyzed by 3D correlation algorithms. In the present study, the reconstructed scans themselves are used to evaluate full displacement fields. The aim of present paper is to analyze a compression test on a stone wool sample by using XCMT performed with a lab tomograph to understand the way in which these materials deform internally .The standardized mechanical performanse of these materials refers to astrain level well above the “elastic”limit.In that casethe strain field appears to be generically non-uniform) and it is important to characterize the regions consentrations strains. and Possibly to be stress (and thus performance) limiting.full 3D strain maps and morphology are therefore extremely valuable. A finite-elemant bades=d approach to volume correlation [1] is used to measure displacement and evaluate dtrain fields during a compression test on stone wool. yhe studied material,the experimental configuration and the imaging system are Presented.the experimental results are finaly analyzed and discussed.

In this paper, we report the following important progress recently made in the basic theory and implementation of digital image correlation (DIC) for deformation and shape measurement. First, we answer a basic but confusing question to the users of DIC: what is a good speckle pattern for DIC? We present a simple local parameter, called the sum of squared subset intensity gradient, and an easy-to-compute yet effective global parameter, called mean intensity gradient, for quality assessment of the local speckle pattern within each subset and entire speckle pattern, respectively. Second, we provide an overview of various correlation criteria used in DIC for evaluating the similarity of the reference and deformed subsets, and demonstrate the equivalence of three robust and mostly widely used correlation criteria, i.e., a zero-mean normalized cross-correlation (ZNCC) criterion, a zero-mean normalized sum of squared difference (ZNSSD) criterion and a parametric zero-mean normalized sum of squared difference (PZNSSD) criterion with two additional unknown parameters, which elegantly unifies these correlation criteria for pattern matching. Finally, to overcome the limitation of the existing DIC techniques, we introduce a robust and generally applicable reliability-guided DIC technique, in which the calculation path is guided by the ZNCC coefficients of computed points, to determine the genuine full-field deformation or shape of objects containing geometrical discontinuities and discontinuous deformation.

Synchrotron X-ray tomography was used to monitor

in situ

three dimensional (3D) fatigue crack propagation in a nodular graphite cast iron. Direct image analysis allows for the retrieval of the successive positions of the crack front, and the detection of local crack retardation, while volume correlation enables for the measurement of displacement fields in the bulk of the specimen. Stress Intensity Factors (SIF) are extracted from the measured displacement fields. It is possible to link the non propagation of a crack with crack closure in COD maps or with a local value of the measured SIF range.

Digital Image Correlation (DIC) is used to extract non-contact full-field three-dimensional displacement and inplane strains from an historic tapestries. A DIC-based approach is devised that allows the effect of RH variations on a tapestry to be quantified. A historical tapestry has been monitored in a closely controlled environment and in the natural environment. The results revealed that very small variations in RH can have significant effects on strain. An automated long term monitoring approach has been devised to allow strain data to be extracted in real time from tapestries in remote locations. The results show that DIC provides better understanding of the effect of RH fluctuations on strain which will ultimately lead to more insight into the degradation process of historical tapestries. The paper demonstrates the potential for using DIC as a condition monitoring tool

Evaluation of defects in heterogeneous materials, such as cellulose-fiber composites, can lead to methods for improving strength. Full-field displacement measurement techniques,

e.g.

, digital image correlation and electronic speckle pattern interferometry, provide useful information by which defects can be evaluated. Inverse Methods (IM) have been used to determine material properties from full-field displacement data. In homogenous materials, the resulting system of equations relating displacements with applied load and constitutive properties is overdetermined and is solved with traditional least squares methods. However, heterogeneous materials create an underdetermined system that cannot be addressed in the same way. Numerically simulated heterogeneous, orthotropic materials were evaluated in a 2-D finite element model, and the resulting nodal displacements were used as input to an IM algorithm. The algorithm determined local moduli, Ex and Ey, with errors, ranging from 9% to 20%. Errors in calculated Gxy were greater. Techniques for reducing error are provided. Simulations suggested IM can be an important tool in defect evaluation given full-field displacement measurements.

Transverse deviations from the ideal flat surface in paper, or curl, can be a serious problem in the paper industry. The manufacturing of paper materials results in the material being orthotropic and laminar. Moreover, the dominant fiber orientation in a paper sheet can vary through the thickness of the sheet. Many paper structures are produced by lamination of paper plies. In these products directions of symmetry of the elastic properties, and coefficients of moisture and temperature expansion/contraction, consequently differ through the thickness of the laminated material. When subjected to humidity changes, the top and bottom portions of a paper laminate therefore expand or contract different amounts, causing the structure to curl. Paper products that curl are difficult to convert in packaging applications and print or copy in automated feeding mechanisms, leading to large costs for companies. It can become impossible to machine process and dispense paper products (such as stamps) that have curled. The present research involves determining the moisture induced curl in [0/15] and [0/45] Whatman paper laminates using a full-field 3-D laser scanner.

Bulge tests have been widely used to characterize mechanical properties of thin films. In this technique, mechanical properties are extracted from the analytically or approximately obtained pressure-deflection response. The determination of the out-plane deflection is critical for bulge tests and to date, this has been implemented by using optical microscopy with a calibrated vertical displacement, laser interferometry and shadow moiré. All these techniques rely on reflected light from the bulged film surface, so films with relatively smooth and reflective surfaces are required. Consequently, these techniques are less applicable to transparent polymeric thin films.

Recent advances in experimental techniques (micro CT scans, automated serial sectioning, electron back-scatter diffraction, synchrotron radiation x-rays) have made it possible to characterize the full, three dimensional structure of real materials. Such new experimental techniques have created a need for software tools that can model the response of these materials under various kinds of loads. OOF (Object Oriented Finite Elements) is a desktop software application for studying the relationship between the microstructure of a material and its overall mechanical, electromagnetic, or thermal properties using finite element models based on real or simulated micrographs. OOF provides methods for segmenting images, creating meshes of complex geometries, solving PDEs using finite element models, and visualizing 3D results. We discuss the challenges involved in implementing OOF in 3D and use finite element simulations of trabecular bone as an illustrative example.

In sampling moiré method, a specimen grating pattern on an object is recorded by a digital camera. A moiré fringe pattern appears by sampling the recorded grating pattern with a constant pixel pitch. The phase analysis of the moiré patterns obtained from one grating pattern provides accurate results of displacement values of the grating. Meanwhile, it is very important to keep our life safely by predicting landslides. If the small displacement of land before the landslide is detected, it is possible to put out an alert before the disaster happens. In this paper, a measurement system for small displacement of landslide by sampling moiré method is developed.

The luminescent photoelastic coating (LPC) technique is an optical method to measure the full-field strain on 2D and 3D structural components. The maximum shear strain–or separated principal strain when oblique incidence is performed–and its corresponding principal strain direction are related to the relative changes in emission magnitude with respect to analyzer position. The amplitude of the emission change is termed optical strain response, OSR, and is a function of maximum shear strain, in the plane perpendicular to light propagation and two coating calibration parameters: the polarization efficiency, , and the coating characteristic, . Determining the coating parameters plays an important role since they affect the accuracy of the measurement. Generally, the coating parameters are calibrated

in-situ

with strain gage measurements and assumed independent of surface inclination. A better theoretical understanding of the two calibration coefficients, specifically the polarization efficiency, dependence on oblique excitation or emission inclination is necessary to improve full-field, strain-separation accuracy of the measurement. This study investigates and reports the effects of the coating parameters on OSR, and the current methods to determine the coating parameters.

The fringe projection profilometry (FPP) is one of the most widely used techniques for three-dimensional (3D) imaging and 3D shape measurements. In this paper, a FPP-based compact, portable, easy-to-implement yet robust 3D imaging and shape measurement system is explored and established. The system utilizes a series of advanced electronic devices, such as a single board computer, a credit-card sized projector, and a USB camera. It employs a number of novel techniques including ultrafast phase unwrapping with multi-frequency fringes, effective gamma correction of digital projection, arbitrary setup of system components, automatic system calibration with a least-squares inverse approach, and multi-thread parallel processing for 3D shape acquisition, reconstruction and display. The system not only provides full-field 3D information with high accuracy and fast speed, but also possesses remarkable features including but not limited to high compactness, easy implementation, and superior capability of dealing with multiple objects with complex shapes in a wide measurement range.

Digital Phase Shifting Interferometry (PSI) is widely used in optical testing to determine the surface topography of a continuous surface. The height pro??le obtained using PSI is derived from intensity measurements made at different phases, which are seperated by a de??nite phase step. The unreliable piston travel of the peizo in response to a de??nite actuating signal is the main source of phase step and hence height errors in the height pro??le derived using PSI. The ??ve frame sequence suggested by Hariharan et. al. for intensity measurements makes PSI relatively insensitive to both positive and negative phase step errors (as large as 50% of the single desired phase step). However, we report on signi??cant errors in the PSI derived height pro??le that can result if a mathematically equivalent but experimentally different sequence than that suggested by Hariharan et. al. is employed for intensity measurements.

This research is focused on developing appropriate macro-mechanical models that account for the microstructural details unique to X- and K-cor composite sandwich panels. Digital Image Correlation (DIC) is used to elucidate on the details of the deformation fields within the sandwich structure. These details are used to enhance the models by providing critical details on the failure initiation mechanisms and the subsequent load redistribution that occurs in these structures. The effects of environmental conditions on the mechanical behavior of these composite materials are also being investigated.

Mechanical fastening is popular choice in joining composites because of the ability to transfer high loads and the ease of assembly and disassembly. However, drilling operations expose the fibers to environmental factors, and the high contact stresses between the bolt and the hole lead to localized delaminations, decreasing the joint strength. A new insert design, proposed here, seals the joint and increases the contact area between the bolt and the hole. This design is also low-cost and compatible with current machining techniques. With the novel design, a phase changing liquid is injected into the empty region between the bolt and the hole. The liquid is then allowed to cure. The tests the performed are on non-reinforced joints and joints reinforced with both machined isotropic inserts and the novel design. Initial tests in the elastic range are analyzed by using Digital Image Correlation. The joints are also tested to Ultimate failure to determine any trends. Form these tests and other preliminary considerations, it can be concluded that the novel design has tremendous potential for applications involving thick composite joints.

A method for smoothing measured displacements and computing strains utilizing a finite element method is proposed. Nodal displacement values of a finite element model are determined by fitting the interpolation functions of finite elements to measured displacement values using the method of least-squares. Then, the smoothed displacement distributions are obtained. The displacements in the region where the measurement values are not obtained or unreliable are determined by solving finite element equations. The validity is demonstrated by applying the proposed method to displacements of a plate with a hole obtained by finite element method. Results show that the displacements and the strains can be determined accurately by the proposed method. Furthermore, the strains near free boundaries can be determined easily. As strains can be evaluated easily and accurately, it is expected that the proposed method can be applied to various problems in solid mechanics.

Digital image correlation (DIC) was used as a diagnostic tool in two series of scaled explosive experiments. In this paper, we focus on the use of DIC as a tool to obtain full-field displacement measurements during high-speed events. From the displacement records we were able to obtain full-field strains, strain-rates and velocity data. The experiments discussed in this paper involved explosive charges submerged in aquarium-like structures, one side of which consisted of a 6061-T6 aluminum plate. In each experiment, the outside of the aluminum plate was patterned so that it met the requirements for use with the DIC system. Two different plate preparation techniques were used in the experimental series and both resulted in the acquisition of quality data. While both techniques were effective, each proved to have unique advantages. The details of plate preparation and a discussion of the performance of each method are included in the paper. The displacement, strain and velocity data are discussed and the output capabilities of the DIC system are demonstrated. In addition to the high-speed, transient data acquired during the deformation events, static, surface-profile measurements of the post-test, deformed plates were made using the DIC system. A discussion of the static measurements is also presented.

The development of high fidelity simulations of lead-cored bullet impacts on soft body armor, motivated by the need for improved armor designs and performance standards for law enforcement, requires accurate models for both the armor and the deformable bullet. This paper examines the ability of the Johnson-Cook constitutive model to predict the dynamic deformation of a 0.357 caliber copper-jacketed lead-cored slug subjected to a direct-impact Kolsky Bar test. A high speed 3D Digital Image Correlation (DIC) technique is used to measure the deformation history of the slug subject to an impact velocity of 15.3 m/s and an impact energy of 62.5 J, which is about 6.5 % of the impact energy required in NIJ Standard 0101.04 for testing Type II body armor with this bullet. The DIC results and force history data are compared to finite element simulations of the test. A sensitivity analysis of the material parameters is carried out to determine their relative influence on the deformation and force history response, and improved parameter values are identified and compared to the baseline values.

Dynamic tensile tests were conducted using a high speed servo-hydraulic testing machine on three types of fabric reinforced cement composites. A good correlation was found between the properties of the fabrics and the composites in high speed tensile tests. The carbon composite exhibits the highest strength, followed by the ARglass composite and then PE composite in high speed tensile tests. The cracking evolution and patterns of the composites were recorded by a high speed digital camera at a sampling rate of 10000 fps. Images showed that multiple cracking behavior was predominant for the carbon and glass fabric-cement composites, indicating good stress transfer within these systems. However, for the carbon, only the filaments at the bundle perimeter were bonded to the cement matrix as the inner filaments completely pulled out during loading. No multiple cracking was observed with PE fabric-cement composite as a single major crack was detected. This major crack opened and widened until the fabric was completely broken.

This paper deals with the analysis of an aluminium beam impacted in a three point bending configuration using a Hopkinson bar device. Full-field deformation measurements were performed using Digital Image Correlation on images captured with an ultra high speed camera (16 frames at a time resolution of 10 ??s). The performance of the deformation and strain measurements were evaluated and the measurements were then used quantitatively to analyse the very complex dynamic behaviour of the beam. It was shown that the deformation of the beam was controlled by the interaction between the striker and the flexural bending wave triggered by the initial shock. The principle of virtual work was used to reconstruct the impact force from the shear strains and to analyze how this impact force relates to the acceleration of the specimen (inertia forces) and the development of the bending stresses. The results are in good agreement with expectations. This opens up new perspectives in the quantitative use of full-field measurements to extract elasto-plastic constitutive parameters from such impact tests.