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Advancement of Optical Methods in Experimental Mechanics, Volume 3 of the Proceedings of the 2015SEM Annual Conference& Exposition on Experimental and Applied Mechanics, the third volume of nine from the Conference, brings together contributions to this important area of research and engineering. The collection presents early findings and case studies on a wide range of optical methods ranging from traditional photoelasticity and interferometry to more recent DIC and DVC techniques, and includes papers in the following general technical research areas:

Advanced optical interferometry

Developments in Image correlation (Digital &Volumetric )

Full Field Methods

Novel Optical Methods for Stress/Strain Analysis

Advances in Optical Methods



Chapter 1. Comprehensive Approach to Deformation Dynamics

Deformation of solids is described as wave dynamics. Dynamics in all stages of deformation from the elastic sage through fracture are formulated comprehensively based on a fundamental physical principle known as local symmetry. The field equations derived from the present formalism lead to longitudinal, compression wave solutions in the elastic regime, decaying transverse wave solutions in the plastic regime, and solitary wave solutions in the transitional stage from the elastic to plastic regime. The decay characteristic in the plastic wave reflects the irreversibility of plastic deformation. The solitary wave can be interpreted as representing the dynamic shear band known as the Lüders band or the type A Portevin–Le Chatelier band.

Sanichiro Yoshida

Chapter 2. In Situ Modal Analysis of Gears

There has been a vast amount of work in the analysis of multistage gearbox housings and the effects vibrations can have on them. This subject is of concern in the aerospace industry and is handled by Finite Element Analysis (FEA). Often experimental verification is required, particularly when a new design or material is introduced. Holographic interferometry and speckle interferometry are optical tools that provide in-situ comprehensive solutions when investigating the modal analysis of gears. Both of these experimental techniques provide an output that determines the dynamic characteristics of the gear being analyzed which can be directly connected to the FEA solution. Experimentally it is possible to obtain the dynamic displacement and strains for all the points of the area of interest and hence can be utilized to modify the FEA in such a way that the results of the FEA agree with experimental values. This type of analysis is particularly critical when it is important to determine the in-plane vibration modes of large gears, known as the “oval modes”. These resonant modes are particularly important in aerospace applications because they can cause serious damage if left unchecked. The analysis of oval resonant modes were carried out on a spur gear to show the effectiveness and accuracy of this in-situ approach.

F. M. Sciammarella, C. A. Sciammarella, L. Lamberti

Chapter 3. High-Speed Stereomicroscope Digital Image Correlation of Rupture Disc Behavior

Three-dimensional deformation of rupture discs subjected to gas-dynamic shock loading was measured using a stereomicroscope digital image correlation (DIC) system. One-dimensional blast waves generated with a small-diameter, explosively driven shock tube were used for studying the fluid-structure interactions that exist when incident onto relatively low-strength rupture discs. Prior experiments have shown that subjecting the 0.64-cm-diameter, stainless steel rupture discs to shock waves of varying strength results in a range of responses from no rupture to shear at the outer weld diameter. In this work, the outer surface of the rupture discs were prepared for DIC using 100–150 μm-sized speckles and illuminated with a Xenon flashlamp. Two synchronized Shimadzu HPV-2 cameras coupled to an Olympus microscope captured stereo-image sequences of rupture disc behavior at speeds of 1 MHz. Image correlation performed on the stereo-images resulted in spatially resolved surface deformation. The experimental facility, specifics of the DIC diagnostic technique, and the temporal deformation and velocity of the surface of a rupturing disc are presented.

Marcia A. Cooper, Michelle N. Skaggs, Phillip L. Reu

Chapter 4. An Experimental-Numerical Hybrid Approach to Analysis of Fiber-Matrix Interfacial Stresses

In this study, the single fiber composite was used to evaluate the stress transfer between a single fiber and a matrix. Single steel fiber was inserted to the epoxy resin, and it was applied the tensile load. Applied load was from 0 N to 177 N, and then the photoelastic images were taken by a digital CCD camera. On the photoelastic analysis, the stress separation was carried out using an experimental-numerical hybrid method. The boundary conditions for a local finite element model, that is, the tractions along boundaries are inversely determined from photoelastic fringes. After determining the boundary conditions for the local finite element model, the stresses can be obtained by finite element direct analysis. Using this input data and the finite element model of analysis region, not only the stress but also the strain distributions were obtained. Consequently, by using the photoelasticity data and geometric data for input data of finite element analysis, accurate data was obtained by the hybrid method for stress separation.

Takenobu Sakai, Yasunori Iihara, Satoru Yoneyama

Chapter 5. Stochastic Progressive Damage Process in Thick Composites: DIC-Based Experimental Characterization

This study is focused on development of experimental capabilities to (a) understand and (b) quantify progressive damage processes (PDP) in thick composite structures, as well as (c) generate outputs sufficient for confident validation of corresponding existing or future modeling solutions. It is proposed to apply quasi-2D statements of analysis to monitor detailed through-thickness PDP of considered materials and/or structural elements, especially, with thick lay-ups and relatively complex patterns of inter- and cross-laminar damage. Thus, the Digital Image Correlation (DIC) technique is fully applicable to quantify processes of crack initiation, growth and networking. An additional key focus of this study is the assessment of the inevitable randomness of PDP, which is often not captured in more traditional experimental implementations. It aims to understand the actual multi-step nature of stochastic PDP and develop efficient methods for their statistical characterization. Demonstration of the capabilities is considered on representative examples of generic laminated carbon fiber polymer matrix composites with different lay-ups and thicknesses. It is shown that the proposed DIC-based implementation provides convenient yet accurate assessment of the stochastic nature of PDP. Details of the implementation and statistical post-processing are discussed in detail.

Mark R. Gurvich, Patrick L. Clavette

Chapter 6. DIC Strain Analysis of FRP/Concrete Bond After Sustained Loading

Externally bonded glass and carbon fiber reinforced polymer composites are increasingly being used to upgrade, repair, and rehabilitate steel reinforced concrete structures such as beams and slabs. The effectiveness of the bonded composites depends on the integrity of the composite/concrete bond over years of exposure to sustained loads and various weather conditions. The present investigation was conceived with the aim of characterizing bond behavior over a long period of time in realistic indoor and outdoor environments, with sustained loading. Plain concrete beams were externally reinforced with wet-layup glass and carbon composites and subjected to sustained flexural loads for 6 years. Then, the beams were loaded to failure while strains on the composite were recorded using resistance strain gages and full-field digital image correlation. The results indicate that ultimate bond strength is strongly correlated with changes in the concrete strength. This finding is consistent with bond failures being mainly controlled by the concrete rather than the composite material. Glass composite specimens generally had less debonding prior to failure, along with higher strength concrete. Furthermore, it appears that the sustained loading itself had little effect on the ultimate bond strength.

Charles E. Bakis, Heather Salasky, Kivanc Artun, Maria M. Lopez, Marcus B. Whitaker, Thomas Boothby

Chapter 7. Damage Detection in CFRP Components Using DIC

Unidirectional carbon fiber reinforced polymer composites (UD CFRP) are high performance materials for structural components, but they are very sensitive to damage. Structural health monitoring is therefore required in safety-critical applications. Many non-destructive evaluation techniques are not suitable for in-service monitoring, which calls for new approaches. We investigate the use of full-field digital image correlation (DIC) for detecting damage in UD CFRP components. Stereo-DIC data is used to analyze changes in vibration modes due to artificial defects. Finally, the effect of the defects is assessed and the suitability of the DIC method is evaluated.

Kim-Niklas Antin, Lauri Harhanen, Sven Bossuyt

Chapter 8. Experimental Quantification of Bend-Twist Coupling in Composite Shafts

Bend-twist coupling refers to the ability of a structure (beam, plate, shaft, etc.) to twist when subjected to pure bending. This characteristic is used in wind turbine blades, marine propellers, and beams subjected to off-axis loadings. A method is presented for designing carbon fiber shafts to maximize the degree of bend twist coupling while maintaining structural integrity. The unique composite ply lay-up utilized in the design of hollow, circular shafts with bend-twist coupling creates certain difficulties in the manufacturing process which are addressed. A method is presented for quantifying the amount of bend-twist coupling by experimentally locating the shear center. 3D DIC is used to measure tip deflection and rotation of the cantilevered shaft when subjected to cantilever bending. Several techniques are presented for minimizing the uncertainty in the final measurement of shear center. An analytical model capable of predicting shaft performance is presented and compared to the experimental results. The model predicts that shear center is a function of shaft length. Good agreement is found among the analytical, FEA, and experimental methods.

S. Rohde, P. Ifju, B. Sankar

Chapter 9. Evolution of Speckle Photography: From Macro to Nano and from 2D to 3D

In this review we traces the genesis of the speckle photography technique, its evolution from using laser speckle to white speckle to electronic speckle patterns. The change from a purely optical technique to digital technique is also described. More importantly we introduce a newly developed 3D technique which can be applied to mapping interior strain field of opaque materials.

Fu-pen Chiang, Lingtao Mao

Chapter 10. A Nonlocal Strain Measure for DIC

It is well known that the derivative-based classical approach to strain is problematic when the displacement field is irregular, noisy, or discontinuous. Difficulties arise wherever the displacements are not differentiable. We present an alternative, nonlocal approach to calculating strain from digital image correlation (DIC) data that is well-defined and robust, even for the pathological cases that undermine the classical strain measure. This integral formulation for strain has no spatial derivatives and when the displacement field is smooth, the nonlocal strain and the classical strain are identical. We submit that this approach to computing strains from displacements will greatly improve the fidelity and efficacy of DIC for new application spaces previously untenable in the classical framework.

Daniel Z. Turner, Richard B. Lehoucq, Phillip L. Reu

Chapter 11. Highly Accurate 3D Shape and Deformation Measurements Using Fluorescent Stereo Microscopy

Biomechanics has been developing at rapid pace in recent decades. For investigation of the biotissues, biomaterials or biofilms under the microscale or the nanoscale, it urgently demands an accurate measurement technique for three-dimensional (3D) surface profilometry and deformation in real-time. Traditional stereo microscope with stereo-based digital image correlation (DIC) works well on common materials, but it is hard to apply to moisture sample due to the specular reflections which could cause large decorrelation among those stereo images. In this paper, we described a fluorescent stereo microscopy (FSM) measurement method for surface profilometry and deformation based on stereo-based DIC. Due to the complex lens combination of a microscope, the distortion of the optical system is hard to formulate accurately using ordinary distortion models. Thus, it could cause large reconstruction errors, particularly in Z-direction in height. In order to improve the accuracy, a new distortion correction scheme is introduced along with a new calibration board. This distortion correction method is intended for use prior to stereo-vision calibration by mapping sensor coordinates of generic image coordinates to a virtual ideal plane. In order to demonstrate this technique, a sequence pair of images of a biofilm is captured during growth, and the 3D surface profilometry and deformation was measured with high accuracy accordingly. A detailed description of this technique is presented in this paper.

Zhenxing Hu, Huiyang Luo, Hongbing Lu

Chapter 12. Displacement Measurement by DIC Method with Cameras of Different Formats

In this study, towards implementation of surveillance cameras as an optical infrastructure health monitoring system, camera pair consists of different formats are considered. In contrast to the typical three dimensional digital image correlation system; whenever surveillance system is implemented as optical capture devices, optics and sensors of the camera pairs are not always identical. Gray levels, spatial resolutions, fields of view, synchronization and sizes of captured images might be different; the obtained images must be normalized into same format by image transformations with pre-obtained CCD camera calibration data and geometrical relations before the images analyzed by DIC method. To perform the experiments, a home-made imaging system consists of three camera are first implemented. Two of the cameras have the same optical head but the other is different. A cylindrical tube is used as the specimen, capturing images before and after moving the cylindrical tube. All captured images of different cameras are combined into three different image-pairs and then analyzed by commercial 3D-DIC software to evaluate mean displacement U, V, W, and the associated strain εxx, εyy, εxy to evaluate the potential concerns while different cameras are used for displacement determination.

Chi-Hung Hwang, Wei-Chung Wang, Yung-Hsiang Chen, Chih-Yen Chen, Hsi-Chiao Ya

Chapter 13. Evaluating Thermal Stresses and Strains from Measured Displacements Using an Experimental-Numerical Hybrid Method

An experimental-numerical hybrid method is proposed for obtaining reliable and accurate strains induced by temperature change. Strains obtained from measured displacement distributions are suffered from the measurement errors. Therefore, the measured displacements are used as the input data for determining boundary condition of a finite element model. Nodal forces at all nodes in the finite element model are identified from the measured displacements by the proposed method. Simultaneously, the reliable displacements and the strains are obtained. Effectiveness is validated by applying the proposed method to the displacement fields in dissimilar materials under thermal load obtained by digital image correlation. Results show that the nodal forces for a local finite element model obtained by the proposed method. It is expected that the proposed method can be powerful tool for stress analysis of electronic devices under thermal load.

S. Yoneyama, S. Arikawa, Y. Kurosu

Chapter 14. Stress Analysis of a Perforated Asymmetrical Vehicle Cooling Module Structure from Unidirectional DIC Displacement Information

The stresses in a perforated asymmetrical cooling module of a heavy commercial vehicle are determined by processing Digital Image Correlation (DIC)-recorded unidirectional displacement data with a series representation of an Airy stress function. Typical of many real engineering problems, the external loads and boundary conditions applied to the cooling module are not well known, thereby challenging the determination of an accurate analytical or Finite Element Method (FEM) Solution. On the other hand, full-field stress information could potentially be obtained from DIC measured displacements. This would traditionally involve differentiating measured displacement data; a process which can be ill-conditioned and adversely influenced by data noise and quality. A hybrid approach which processes the recorded displacement data using a stress function and determines stresses in finite members has been published previously, but it was restricted to symmetrically loaded structures. The earlier concepts are extended to analyze perforated and asymmetrical isotropic structural member from DIC recorded values of a single component of the displacement field and without having to physically differentiate the measured displacement data.

S. Paneerselvam, K. W. Song, W. A. Samad, R. Venkatesh, R. F. El-Hajjar, R. E. Rowlands

Chapter 15. Thermo-mechanical Properties of Metals at Elevated Temperatures

Results from novel DIC-based experiments focused on quantifying the high temperature thermo-mechanical properties of steel alloys at elevated temperatures are presented. In these studies, the vision-based high temperature measurement system is comprised of (a) an induction coil heating system, (b) blue and white light illumination of the specimen, (c) optical band-pass notch filters for each camera centered on the blue light illumination wavelength and (d)~a stereo-camera configuration for acquisition of synchronized image pairs at the desired temperature. The entire system is portable and is sufficiently flexible such that it can be employed with a wide range of specimen geometries. Each pair of stereo images acquired at a specified temperature is analyzed using three-dimensional digital image correlation (stereo-DIC). Results from laboratory studies confirm that the blue light-illumination with optical notch filtering allows high contrast images to be obtained even when the specimen is glowing white hot. The effectiveness of the system is demonstrated by successfully performing experiments to obtain the coefficient of thermal expansion of a steel alloy as a function of temperature. Additionally, technical challenges (e.g., sensitivity, spatial resolution, and repeatability of the measurements) when performing high temperature experiments with the proposed methodology will be discussed.

Behrad Koohbor, Guillermo Valeri, Addis Kidane, Michael A. Sutton

Chapter 16. Correlation of Microscale Deformations to Macroscopic Mechanical Behavior Using Incremental Digital Volume Correlation of In-Situ Tomography

An incremental digital volume correlation (DVC) technique was developed to measure large nonlinear deformations on volumetric images acquired using X-ray micro-computed tomography (μ-CT). A series of bridging volumetric images are acquired during the loading of a specimen. The deformation in the neighboring images is sufficiently small to allow DVC calculation. The displacements are accumulated, and analyzed to determine their gradients for deformation measurements. The technique was applied for observation of internal deformations of Polymethacrylimide (PMI) foam, polymer bonded sugar (PBS), and granular materials in compression experiencing large nonlinear deformations. When PMI foam underwent compression, 17 states of the PMI were captured and large nonlinear deformations were observed. The PBS cylindrical specimen, in which sugar grains are embedded in hydroxylterminated polybutadiene (HTPB) binder matrix, was compressed up to 32 % of compressive strain without confinement. The debonding evolution was observed and discussed. On granular materials, a methodology was developed to determine for the force chains. These applications demonstrate that incremental DVC is a powerful technique for linking the microstructure with macroscopic mechanical behavior.

Zhenxing Hu, Huiyang Luo, Yingjie Du, Hongbing Lu

Chapter 17. Sparse Spherical Marker Tracking in Volumetric Images: Assessment of Local Measurement Errors

Simulations are performed to assess the potential of sparsely distributed volumetric marker tracking for estimation of local internal deformations. Assuming rigid spherical markers embedded in a compliant matrix, algorithms are developed to accurately locate the centroid of individual markers in both undeformed and deformed positions. To assess variability in the measured marker positions, Monte-Carlo simulations using Gaussian noise in the image data are analyzed. Results for a 2 % Gaussian noise indicate that variability in the marker tracking measurements is a strong function of marker radius and a weak function of sub-voxel marker position.

Ning Li, Michael Sutton, Hubert Schreier

Chapter 18. Flapping Wing Deformation Measurement in Hover Flight Conditions

Deformation during the flapping cycle can be used as a determinant of hover-mode performance. The wing used here is a small hummingbird sized wing constructed from carbon fiber and machined acetal plastic. Digital image correlation measurements while the wing was flapped at 25 Hz is compared with aerodynamic thrust performance to reveal characteristics that might translate to favorable or poor wing performance. This is aimed to provide flapping micro air vehicle designers with knowledge of how to tailor their wings for desired aerodynamic performance. Three different wing designs were tested; all of which were the same planform area and tested using a single active degree of freedom flapping mechanism. Tip deflection and twist angle at 50 % and 75 % are extracted from the full-field deformation and used as metrics. The twist measurements showed advanced rotation in all the wings tested, though to different extents. The wing which had the most advanced rotation was also found to have produced the most thrust at 25 Hz, while the wing with the least tip deflection and was most efficient. This suggests that the larger tip deflection seen with the other wings may be detrimental to the efficiency of the wing.

Kelvin Chang, Archit Nazare, Peter Ifju

Chapter 19. Characterization of Cover-Plate Bolted Steel Joints with Full-Field Measurements

The combination of cover-plate bolted joints commonly used in steel structures allows the transfer of various internal loads such as bending, shear or axial forces. The mechanical behavior of these joints is complex because it depends on both the contact evolution due to load transfer between bolts and holes, and the elastoplastic response of the constitutive material of the plates. The interaction between the holes in the plates is also of great importance regarding the various configurations of joint and hole positions. The different failure modes such as net section fracture, gross section plastic yielding, shear out and bearing, are difficult to predict because they combine local compression under bolt and shear at loaded end distance of the plate. Many studies analyze the behavior of cover-plate joints by combining finite element approaches and/or experiments, but for obvious practical or economic reasons, only limited numbers of strain gages are employed to estimate the strain distribution in the components of the joints. Here the grid method is used to analyze in details the strain distribution on the plates with various configurations of bolts and holes positions. This approach enables us to observe the strain fields in the plate around the bolts during both the elastic and plastic phases of the material response. The obtained results help understanding the evolution of the failure zones regarding mainly the bearing mode. They can also be used as a reference to calibrate sophisticated finite element models.

Evelyne Toussaint, Abdelhamid Bouchaïr, Michel Grédiac, Sébastien Durif

Chapter 20. Connecting Rod FEA Validation Using Digital Image Correlation

Standard structural analyses were performed on a connecting rod in order to assess its durability. The finite element (FE) model predicted a significant amount of ovalization at the crankpin (i.e. big) end of the connecting rod when subjected to high tensile loads. This ovalization imposed a large bending moment on the crankpin end cap screws. Accounting for the cap screw’s axial preload and the bending moment, the combined stress state was very near the yield strength of the cap screw. It was desired to experimentally validate the FE model. Therefore, an axial tension test was conducted on a sample connecting rod and the deformation near the cap screws was measured using digital image correlation (DIC). Both the axial and lateral deformations between the analytical and experimental data were very well matched. The results of this investigation showed that with good modeling and experimental methods, DIC can be an effective method for model validation.

Jordan E. Kelleher, Michael D. Hayward, Paul J. Gloeckner

Chapter 21. Comparison of Subset-Based Local and Finite Element-Based Global Digital Image Correlation

Subset-based local DIC and finite element-based (FE-based) global DIC are the two primary image matching methods that have been extensively investigated and regularly used in experimental mechanics community. Due to its straightforward implementation and high efficiency, subset-based local DIC has been used in almost all commercial DIC packages. However, it is assumed by some researchers that element-based global DIC offers better accuracy because of the enforced continuity between element nodes. Thus there is a pressing need to comprehensively examine the performance of these two DIC approaches. In this work, theoretical analyses of the standard deviation errors of classic subset-based DIC and two FE-based DIC techniques are first performed. Then, by measuring displacements of the same calculation points using the same calculation algorithms and identical calculation parameters, the performances of subset-based local DIC and two FE-based global DIC approaches are compared experimentally in terms of measurement error and computation efficiency using numerical tests and real experiments. A detailed examination of both the theoretical and experimental results reveals that, when subset (element) size is not very small, standard subset-based local DIC approach not only provides better results in measured displacements, but also demonstrates much higher computation efficiency. However, several special merits of FE-based global DIC approaches are indicated.

Bing Pan, Bo Wang, Gilles Lubineau, Ali Moussawi

Chapter 22. A Meshless Global DIC Approach

The formulation of global Digital Image Correlation (DIC) is almost identical to standard (i.e. subset-based) DIC: indeed, in both cases the solution algorithm tries to minimize the error between the reference and test image by adjusting the shape-function-controlling parameters, but in the former, a modification of the parameters affects the displacement field globally, whereas in standard DIC the modification is local.There are several ways to implement a global DIC program, but the approach most used relies on a Finite-Element-like mesh. This solution gives maximum flexibility with regard to description of the computational domain, but it makes it difficult to implement adaptive algorithms because a complete re-meshing infrastructure has to be employed. Considering that normally DIC codes are used to analyze relatively simple domains, this work propose to replace the standard FEA formulation with a mesh-less description of the displacement field; using Shepard’s approach, a partition of unity can easily be constructed, thus allowing for a simple and flexible description of the global displacement field.

Antonio Baldi, Filippo Bertolino

Chapter 23. Out-of-Plane Motion Evaluation and Correction in 2D DIC

2D and stereo Digital Image Correlation (DIC) allows to retrieve complex displacement and strain fields on a specimen’s surface. Although 2D DIC is strongly affected by out-of-plane motions, in many situations, it is preferred over stereo DIC because of its ease to use and because only one camera is required. The out-of-plane movements can be ascribed mainly to three causes: the camera positioning, the imperfections of the used test device, and the camera self-heating. These effects gain importance when the distance between the camera and the specimen is reduced. The positioning of the camera aims to have its optical axis perfectly perpendicular to the specimen to observe. Nevertheless small but effective misalignments can easily happen even if suitable devices are used for the alignment. This contribution concerns the experimental evaluation of these movements considering a cyclic uni-axial tensile test performed on an aluminium specimen. The study is particularly focused to the out-of-plane motions that occur at every cycle because of the tensile bench, which are the more critical ones. Finally a compensation method, based on fixed compensation plates, is presented. The method allows to properly correct the data coming from a 2D DIC set-up.

Michele Badaloni, Pascal Lava, Marco Rossi, Gianluca Chiappini, Dimitri Debruyne

Chapter 24. A Realistic Error Budget for Two Dimension Digital Image Correlation

There has been a lot of interest in the matching error for two-dimensional digital image correlation (2D-DIC), including the matching bias and variance; however, there are a number of other sources of error that must also be considered. These include temperature drift of the camera, out-of-plane sample motion, lack of perpendicularity, under-matched subset shape functions, and filtering of the results during the strain calculation. This talk will use experimental evidence to demonstrate some of the ignored error sources and compile a complete “notional” error budget for a typical 2D measurement.

Phillip L. Reu

Chapter 25. Accuracy Comparison of Fringe Projection Technique and 3D Digital Image Correlation Technique

Fringe projection technique and 3D digital image correction technique are widely used in experimental mechanics field for 3D shape and deformation measurements. In spite that the two techniques have fundamental similarities, they also have considerable differences. This paper presents an experimental comparison of the two techniques in terms of measurement accuracy. Considering that 3D imaging and 3D printing are very hot topics in today’s world, we believe that the accuracy comparison can be of great interest to the technical community.

Hieu Nguyen, Zhaoyang Wang, Jason Quisberth

Chapter 26. Continuous Development of 3D DIC by Using Multi Camera Approach

The 3D DIC method is a well-established full-field measurement technique in experimental mechanics which is used for the characterization of e.g. displacements, vibrations and strains. The results are often employed in order to determine material properties, to validate numerical models and many more.Nowadays, the application of the DIC technique moves from academic to industrial fields. In this interest, one needs to prove the DIC technique on real components. The conventional two-cameras 3D DIC inspection becomes then non-optimal since complex geometries imply hidden areas. An efficient way to overcome these limitations is to take advantage of the use of multi-cameras DIC system.This publication describes the idea and principle of using the Cluster Approach for multi-cameras DIC systems. In order to illustrate the implications of this new technique, we present tensile/compression test measurements on an automotive carbon rim, on a transverse link and on a plastic sample.

Thorsten Siebert, Karsten Splitthof, Marek Lomnitz

Chapter 27. On Noise Prediction in Maps Obtained With Global DIC

A predictive formula giving the measurement resolution in displacement maps obtained using Digital Image Correlation was proposed some years ago in the literature. The objective of this paper is to revisit this formula and to propose a more general one which takes into account the influence of subpixel interpolation for the displacement. Moreover, a noiseless DIC tangent operator is defined to also minimizes noise propagation from images to displacement maps. Simulated data enable us to assess the improvement brought about by this approach. The experimental validation is then carried out by assessing the noise in displacement maps deduced from a stack of images corrupted by noise. It is shown that specific image pre-processing tools are required to correctly predict the displacement resolution. This image pre-processing step is necessary to correctly account for the fact that noise in images is signal-dependent, and to get rid of parasitic micro-movements between camera and specimen that were experimentally observed and which corrupt noise estimation. Obtained results are analyzed and discussed.

B. Blaysat, M. Grédiac, F. Sur

Chapter 28. Full-Field 3D Deformation Measurement of Thin Metal Plates Subjected to Underwater Shock Loading

In this paper, the dynamic deformation of thin metal plates subject to underwater shock wave loading are studied by using high-speed 3D digital image correlation (3D-DIC). An equivalent device consist of a gas gun and a water anvil tube was used to supplying an exponentially decaying pressure in lieu of explosive detonation which acted on the panel specimen. The thin circular specimen panels with different support boundary conditions ware fixed on the end of the anvil tube by the flange ring cover. The thin circular plates with different thickness were made of pure copper. The intensity of impulsive loading is controlled by changing the impact velocity of the flyer. The pressure history of the underwater impulsive wave was measured by a pair of dynamic pressure transducers. The transient responses of the panels were measured by a High-speed three-dimensional Digital Image Correlation (3D DIC) photography system. The 3D DIC system records full field shape, displacement profiles velocity profiles and strain profiles of the specimen surface in real time. The influences of intensity of loading and support condition on the deformation of specimen panels are discussed based on the DIC results. The strain gauges also were used to monitor the structural response on the selected position for comparison. The DIC data and the strain gauges results show a high level of correlation, and 3D-DIC is proven to be an effective method to measure 3D full-field dynamic response of structures under underwater impact loading.

Peng-wan Chen, Han Liu, Shaolong Zhang, Ang Chen, Baoqiao Guo

Chapter 29. A Multi-camera Stereo DIC System for Extracting Operating Mode Shapes of Large Scale Structures

Stereophotogrammetry and three-dimensional (3D) digital image correlation (DIC) have recently received attention for the collection of operating data on large wind turbine blades due to their non-contacting, rapid, and distributed measurement capability. Unlike conventional methods that only provide information at a few discrete points on a wind turbine blade, photogrammetry can provide a wealth of distributed data over the entire structure. One of the challenges with using a camera pair to observe a structure is the limited field of view. Because utility-scale wind turbines are so large and the physical limitations within a blade test facility, a single pair of DIC cameras may not be able to accurately measure the desired area of the structure. Thus, in order to perform a DIC measurement on a utility-scale wind turbine blade, it is desirable to couple several pairs of cameras to simultaneously measure the deformations of the entire blade. The measured deformations of each measured section of the blade needs to be stitched together to extract the deformation for the entire blade. In this paper, a multi-camera 3D DIC measurement is used to identify resonant frequencies and corresponding operating shapes of an individual 2.3-m wind turbine blade placed in a cantilevered boundary condition. The setup is composed of two pairs of synchronized stereo cameras in which each pair of cameras measures a part of the blade’s deformation. The individual measurements include the geometries and the displacements from each pair of cameras that are mapped into a universal coordinate system. Afterwards, operational modal analysis is carried out to extract mode shapes of the cantilevered blade and the extracted results are compared to a validated finite element model for the blade. The results obtained using two camera pairs and DIC demonstrate the great potential of the proposed approach to identify the entire dynamic behavior of utility-scale wind turbine blades.

Peyman Poozesh, Javad Baqersad, Christopher Niezrecki, Peter Avitabile

Chapter 30. Metrology of Contours by the Virtual Image Correlation Technique

The Virtual Image Correlation (VIC) technique consists in finding the optimal fit between a contour or silhouette embedded in an image and a contour or silhouette described from an analytical description in a virtual image. The optical parameters of the virtual image are obtained thanks to a minimization process. The sub-pixel precision and the robustness of the method are shown onto practical experiments. Examples of use are given in the fields of strain measurement, metrology of engineered parts, mechanical testing and metrology of highly deformable inflatable structures. In that last case, the analytical description of contour in the virtual image is obtained from the Timoshenko beam theory relevant of inflatable structures.

M. L. M. François, A. Bloch, J. C. Thomas

Chapter 31. Uncertainties of Digital Image Correlation Due to Pattern Degradation at Large Strain

A method to measure the displacement uncertainty increase, due in part to pattern degradation at large strains, is described. The method is similar to rigid body displacement methods used in prior work. Multiple pattern assessment metrics from the literature are calculated for the same experiments to determine if the trends in the uncertainty are consistent with the trends seen in the metrics. The pattern assessments are carried out over the entire region of interest for correlation, but some of the metrics are based on the size of the correlation subset. The results show better agreement between the increase in displacement uncertainties and the subset size based metrics, than the metrics that measure the entire region of interest regardless of the subset size.

Mark A. Iadicola

Chapter 32. Optimization Analysis of Large-Area Full-Field Thickness Measurement Interferometry in Thin Glass Plates

With the market requirements, glass plates used in flat panel display must be large and thin. In thin glass plates, thickness uniformity has been a key standard in quality identification. To response the requests of industries and manufacturers to large-size glass plate in on-line real-time inspection, the authors recently developed a large-area full-field thickness measurement method named angular incidence interferometry (AII). AII is based on thin film interferometry and uses the point-expanded laser light to illuminate the specimen with an incident angle to generate the particular interference fringes. Due to the regular correlation between the interference fringes, the full-field thickness distribution can be obtained from only one interference fringe pattern (IFP).However, to the thinner glass plates, the thickness non-uniformity may be more serious. In the measurement of AII, modulating the incident angle can effectively eliminate the influence of the serious thickness non-uniformity to the regular correlation between the interference fringes. Therefore, in this paper, the optimization of the incident angle and distance parameters in AII setup was analyzed. The commercially available glass plates with 0.70, 0.55, and 0.33 mm nominal thicknesses were employed to verify the optimization analysis and confirm the measurement feasibility of AII in thinner glass plates.

Po-Chi Sung, Wei-Chung Wang, Meng-Hsiu Li

Chapter 33. A New Approach to Calibration of Polycarbonate Material for Photoplastic Studies

Polycarbonate material is calibrated for photoplastic studies using reflection photoelasticity and digital image correlation (DIC) techniques. A modified dog bone specimen is used for easy ordering of photoelastic fringes and to obtain optimum information from a single photoelastic image. The specimen surface is coated with a reflective aluminium spray paint. On top of this a random speckle pattern required for DIC experiment is achieved by spraying black speckles on white background. Both reflection photoelasticity and DIC experiments are performed simultaneously on the dog bone specimen. A maximum principal strain difference of 7 % is measured in the experiment. The photoplastic constant is evaluated by relating the fringe order, N from reflection photoelasticity to the principal strain difference, (ε1 − ε2) data from DIC.

M. Subramanyam Reddy, K. Ramesh

Chapter 34. Revealing Dynamic Banding During High Temperature Deformation of Lightweight Materials Using Digital Image Correlation

In this work, digital image correlation (DIC) is used to investigate the unique banding behaviors of selected steel and aluminium alloy sheets at various temperatures. The materials are deformed in simple tension at different temperatures ranging between −50 and 300 °C. DIC is integrated in the tension testing setup, and the impact of the testing setup (glass and heat) on the accuracy of the measurements is evaluated. DIC reveals the accumulation of localized plastic strains and strain rates, as well as their evolution during testing. Depending on the mechanism of mobility of the material surface during deformation, different dynamic localization bands are noted. Selected examples highlighting a dramatic shift in the dynamic banding behavior in these materials are discussed. The correlations between the bands revealed in DIC measured strain and strain rate maps, and the corresponding macro-mechanical stress-strain behaviors are explored for different types of bands.

Jun Hu, Nan Zhang, Fadi Abu-Farha

Chapter 35. Strains in Shallow and Deep Notches Using Two DIC Algorithms

In the current work, a high magnification 3D stereoscopic digital image correlation (DIC) system and a 3D micro-stereoscopic DIC system were used to determine elastic and elastic-plastic strain distributions in the proximity of shallow and deep notches. Specimens used were a plate with a shallow circumferential notch under bending load, and two deep U-Notch specimens under tensile load. The data analysis employed two methodologies. One was the direct application of a conventional and commercial DIC analysis tool. Another, applied to the same gathered data, using a novel combination of image analysis technique with a meshless numerical method. The Scale Invariant Feature Transform (SIFT) computer vision algorithm was used to extract distinctive points or features of the captured images. Then, the displacements were obtained by tracking the positions of successfully matched SIFT points of the undeformed-deformed pair of images. In sequence, the points provided by SIFT were selected as nodes of a meshless formulation to generate a numerical approximation of the displacement fields and their derivatives. For that, the moving least square method was employed. The advantages of the novel proposed method were the automaticity in the process correlation and the good performance when low and high strain gradient strain fields co-existed in the same analysis. The SIFT-Meshless strain distributions consistently matched with those obtained from the traditional DIC subset-step type of analysis applied to the same sets of images and with results obtained from finite element models. An important conclusion was that the SIFT-Meshless method provided reliable measurements, especially at the vicinity of the notches where the maximum strain values were expected.

G. L. G. Gonzáles, L. D. Rodrigues, M. A. Meggiolaro, J. L. F. Freire

Chapter 36. Towards the Development of a Global Cn-Continuous DIC Procedure?

In this work, a novel yet easy to use Global Digital Image Correlation algorithm is presented. The proposed method is developed to counter one of the important drawbacks of the most frequently adopted DIC techniques, namely the non-continuity of the strain field. In both the local subset method (no continuity) as in the widely known global Q4-DIC method (C0-continuity) this problem arises. The proposed method is developed to obtain a C1-continuous displacement field, and thus obtain a continuous strain field. In this way, strains can be derived directly from the algorithm without any post processing or filtering of the displacement, this in contrast to the previously mentioned DIC techniques. In these, the strain calculation is mostly done by using polynomial smoothing (e.g. strain window) of the displacement field. The extent of smoothing is controlled by user settings, making the measurement results significantly influenced by the user. The proposed algorithm uses well known principles both from the standard global DIC method as well as principles from finite element analysis to obtain a continuous strain field. Here an algorithm is presented using rather simple, specifically developed element shape functions, making the algorithm extremely easy to be implemented in a standard global approach.

L. Wittevrongel, D. Debruyne, S. V. Lomov, P. Lava

Chapter 37. Extraction of Linear Anisotropic Parameters with Scattering Property by Mueller Optical Coherence Tomography for Stress Analysis

Using a Mueller optical coherence tomography (OCT) to extract linear birefringence (LB) and linear dichroism (LD) parameters of the material with scatters is proposed in this study. The analytical model is derived by the differential Mueller matrix formalism that considering the reflected mode for measuring an optically anisotropic sample containing LB, LD and depolarization effect in full range measurements. In contrast to the conventional Mueller OCT system, the proposed model provides the exact solutions for the optically anisotropic parameters decoupled from the depolarization effect for actual materials. Also in comparisons with an analytical model based up a decomposition method, it is found that one based upon a differential method is much easier and more realistic for simulating the materials simultaneously containing multiple optical properties. The validity of the system is proved by the simulation results. As authors’ knowledge, this is the first concept to merge a differential method in the reflection mode in a Mueller OCT system.

Chia-Chi Liao, Yu-Lung Lo

Chapter 38. Field Strain Measurement on the Fiber-Epoxy Scale in CFRPs

Laminated composites are materials with complex architecture made of continuous fibers (usually glass or carbon) embedded within a polymeric resin. The properties of the raw materials can vary from one point to another due to different local processing conditions or complex geometrical features for example. A first step towards the identification of these spatially varying material parameters is to image with precision the displacement fields in this complex microstructure when subjected to mechanical loading. Secondary electron images obtained by scanning electron microscopy (SEM) and then numerically deformed are post-processed by either local subset-based digital image correlation (DIC) or global finite-element based DIC to measure the displacement and strain fields at the fiber-matrix scale in a cross-ply composite. It is shown that when global DIC is applied with a conformal mesh, it can capture more accurate local variations in the strain fields as it takes into account the underlying microstructure. In comparison to subset DIC, global DIC is better suited for capturing gradients across the fiber-matrix interfaces.

Ran Tao, Ali Moussawi, Jian Zhou, Gilles Lubineau, Bing Pan
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