Advancement of Optical Methods in Experimental Mechanics, Volume 3

This paper discusses the use of an iterative back projection (IBP) super-resolution (SR) image reconstruction technique on the carbon epoxy laminates with simulated porosity defects. In order to first validate and evaluate the application of the proposed method, three artificially simulated delamination defects in carbon epoxy laminates were considered. Based on the preliminary results, it was verified that the contrast signal-to-noise ratio (CNR) of the SR image was higher than the bi-cubic interpolation image. Further, the peak signal-to-noise ratio (PSNR) value of SR result had an average increase of 5.7088 dB compared to the bi-cubic interpolation method. This validates the proposed approach used to generate the reconstructed SR images with image quality similar to the original simulated UT images. After the validation, the UT image reconstruction algorithm was applied to the ultrasonic C-scan amplitude images of a porosity sample. Based on the results, it was demonstrated that the SR image achieved better visual quality with an improved image resolution. It was also demonstrated that this method was capable of detecting the defects with more confidence by recovering the defect outline compared to the LR C-scan image. The defect outline in SR images is more distinct to recognize, allowing post-processing work such as measurement of defect size, shape, and location to be much easier.

The images projected by scanned laser pico-projectors (SLPPs) contain speckle noise caused by the reflection of the high coherent laser light source from the projection screen. A speckle pattern can be used to detect the concentration of liquid solution with nanoparticle. Three samples: solid solution, liquid solution, and collagen solution are tested. The experimental results show that the green laser within the SLPP provides a better sensitivity and resolution than the red or blue lasers. In solid solution measurement, different concentrations of nanoparticles embedded in a poly (methyl methacrylate) (PMMA) matrix are tested. In liquid solution measurement, different concentrations of nanoparticles dissolved in deionized water are also tested. Finally, the system shows ability to measure the collagen concentrations from 0.125 % to 0.025 %. Accordingly, the proposed system provides a viable, low-cost solution for high-sensitivity in biomedical, chemical and environmental applications.

High-speed shape measurement is required to analysis the behavior of a breaking object, a vibrating object or a rotating object. A shape measurement by a phase shifting method can measure the shape with high spatial resolution because the coordinates can be obtained pixel by pixel. A light source stepping method (LSSM) using linear LED array was proposed by authors. Accurate shape measurement can be performed by a whole space tabulation method (WSTM). The response speed of the LED array is more than 12 kHz. In this paper, high-speed shape measurement is performed with a high-speed camera by WSTM and LSSM using a linear LED array. The phase shifting is performed in 12,000 Hz and the shape measurement of a rotating fan is performed in 4,000 Hz.

The use of various deconvolution techniques to enhance strain maps obtained with the grid method is addressed in this study. Since phase derivative maps obtained with this measurement technique can be approximated by their actual counterparts convolved by the envelope of the kernel used to extract phases and phase derivatives, non-blind restoration techniques can be used to perform deconvolution. Six deconvolution techniques are compared here in order to restore a synthetic phase derivative map. Obtained results are analyzed and discussed.

Full field measurements and inverse methods can be conveniently used to identify the constitutive properties of materials. Several methods are available in the literature which can be applied to many different types of materials and constitutive models (linear elasticity, elasto-plasticity, hyper-elasticity, etc.). The effectiveness of the identification procedure is related to the specimen geometry and the quality of the optical measurement technique. A method to improve and optimize the identification procedure is to numerically simulate the whole process. In such a way it is possible to compare different configurations and chose the one that shows the lowest identification error.

A test simulator was already developed by the authors and, in this paper, an improved version is presented. The simulator is now able to deal with small deformations thanks to a super-sampling algorithm which allows to reduce the numerical artefacts introduced during the image deformation. The simulated experiments are compared with actual ones. A series of experiments has been performed using aluminium specimens. The reliability of the simulated experiments is evaluated looking at the simulated and experimental displacement and strain maps and comparing the obtained identification errors

The Virtual Fields Method (VFM, Pierron and Grediac, 2012) is an inverse technique for computing mechanical properties of materials from full-field deformations obtained from techniques such as Digital Image Correlation (DIC). VFM is based on the principle of virtual work, which is a weak statement of the equations of motion. Central to VFM is the appropriate choice of virtual fields, which in prior work, have been assumed to be polynomials that are continuously differentiable, either piece-wise or over the entire domain of interest. In this work, we propose a new method for systematically identifying virtual fields by performing a principal component analysis (PCA) of the strain field measured from experiments. The virtual strain components to be used in VFM are then chosen to be the eigenfunctions so determined. In addition to being a physically meaningful set of virtual fields, such a choice exploits the orthogonality of the computed eigenfunctions while simultaneously eliminating computation of a large number of coefficients that define the virtual fields in prior approaches. In the case of linear elastic behaviour, we show that this new approach, named the Eigenfunction Virtual Fields Method (EVFM), leads to a compact system of equations that can be solved for the unknown material parameters.

In the last few decades Experimental Mechanics, helped by advanced technologies to gather 3-D spatial information in non-transparent media, has evolved into a very general tool. It has become possible to observe the internal volume of engineering materials and in the area of biomechanics living internal tissues. This paper contains a brief review of Continuum Mechanics mathematical models that are available to formulate problems in 3-D including large deformations. The extension of the experimental methods that measure displacements in 2-D to 3-D is presented. Two important cases are considered: (a) use of deterministic signals, (b) use of random signals. In order to separate the complexity of the subject of 3-D analysis from the difficulties that arise from the use of random signals, the connection between mathematical models and their experimental determination is presented utilizing deterministic signals. The extension of the use of random signals to the determination of displacements in 2-D to 3-D is outlined. A new method to extract displacement information from random signals is developed and an example of application is provided. Two methods to extract displacement information in 3-D, the classical method based on displacement projections and discrete image correlation (DIC) based on following gradients of intensities are compared. There are many complex steps involved in data processing aside the basic approach, this circumstance makes difficult a comparison between the two methods, however it is possible to conclude that the results are in fair agreement.

Digital Image Correlation is widely used for shape, motion and deformation measurements. Basically, the main steps of 3D-DIC for shape measurement applications are: off-line camera calibration, image matching and triangulation. The matching of each pixel of an image to a pixel in another image uses a so-called subset (correlation window). Subset size selection is a tricky issue and is a trade-off between a good spatial resolution, achieved with small subsets that preserve image details, and a low displacement uncertainty achieved with large subsets that can smooth image details.

In this paper, we present a new multi-step DIC algorithm specially designed for measuring the 3D shape of objects with sharp edges. With this new algorithm an accurate 3D reconstruction of the whole object, including the sharp edges that are preserved, can be achieved.

Structured light projection is a fast and flexible optical method for measuring the 3D shape of objects. The measurement is performed within a few seconds. The result is a dense cloud of points that accurately describes the shape of the surface. The information could be used e.g. to generate CAD models from an item designed by hand, or it could be used for product quality inspection. The desired information, such as distances, angles, profiles, etc. can conveniently be extracted from the measurement results. It is also possible to calculate and visualize the deviation between a measurement and a CAD model. Most of the research work made with this technique has been carried out in a terrestrial normal environment. However sometimes it is convenient to make measurements inside hazardous or wet media, such as fogy, cloudy, raining, scattering, or underwater, or even biologic and toxic media. In this paper are presented the first steps toward obtain 3D inform shape information of a structure which is immersed inside water. Two of the most important steps are the optical setup characterization and the study optical properties of the environment. Both give us mainly conditions of illumination and restrictions on the object surface texture. In order to characterize the CCD optics, experiments related to fringe visibility varying distance to target and density and absorption of the media are carrying out. The first experimental results are presented.

We are developing an advanced computer-controlled digital holographic system (DHS) with the ability to measure both shape and acoustically induced deformations of the tympanic membrane of several species, including humans. The DHS has been deployed in the clinic and is currently being optimized for in-vivo measurements.

The clinical environment presents numerous challenges such as disturbances due to patient’s heartbeat, breathing, patient’s tremor as well as environmentally induced mechanical vibrations of several orders of magnitude larger (1–10 Hz, 0.1–100 μm) than the nanometer measuring resolution of the system. Biological samples and the tympanic membrane in particular represent numerous challenges for optical systems including non-uniform light scattering, random internal and external reflections as well as low reflectivity (<10 %) and high transparency (>50 %). Design and optimization of the system for clinical use includes the development and implementation of various acquisition strategies and algorithms for minimization of measurement disturbances in clinical conditions as well as in-vivo measurements.

In this work we show implementation of several single frame acquisition algorithms based on both lens and lensless optical holographic configurations. We also evaluate their performance in terms of acquisition speed, external mechanical disturbance tolerance, appropriate depth of field, as well as tolerance to non-uniform light scattering typical in biological samples.

Minimally invasive percutaneous fixation techniques play a role of crucial relevance in the clinical practice. In spite of their consolidated use, little is reported in the literature to provide a mechanobiological explanation on how design of fixation devices can affect the healing process within fractured vertebrae.

The aim of the study is to develop a multi-scale mechano-regulation model capable of predicting how the patterns of tissue differentiation within a vertebral fracture change in the presence or in the absence of fixation devices and how the dimensions of the device, and the materials it is made from, can affect the outcome of the healing process.

To this purpose, a multi-scale mechano-regulation model is developed that combines a macro-scale model representing the spinal segment L3-L4-L5 including the fractured body of the L4 vertebra, and a micro-scale model of a fractured portion of cancellous bone. The macro-scale model includes also a minimally invasive percutaneous fixation device. The above mentioned model allows us to investigate how spatial and temporal patterns of tissue differentiation in the fracture gap change for different dimensions of the fixation device components and for different materials (Ti-6Al-4V alloy and Co-Cr alloy). Furthermore, the model provides information on the stress state in the fixation device and hence allows the risk of failure of the device itself to be estimated.

The mechanical properties of the forming tissue change as the healing process progresses. In order to validate the mechano-regulation model, displacement fields will be measured with moiré and holography and compared with numerical computations.

The model predicts that fixation devices significantly shorten healing times. Increasing values of the rod diameter

D

and decreasing values of its radius of curvature

R

lead to shorter durations of the healing period. Manufacturing the rods in Cobalt-Chrome alloy is predicted to reduce slightly the healing period by providing greater mechanical stability within the fracture callus.

The optical methods of photoelasticity and caustics have extensively been used for the determination of stress intensity factors (SIFs) in static and dynamic crack problems. Both methods present their potentialities and limitations. In crack problems the state of stress in the neighborhood of the crack tip changes from plane strain near the tip to plane stress away from the tip through an intermediate three-dimensional region. This affects the determination of SIFs using experimental methods. In the present work the methods of photoelasticity and caustics applied to crack problems are briefly presented and compared regarding the following criteria: the optical set-up, the efficiency in the determination of stress intensity factors, the effect of crack tip radius, the effect of plate boundaries, the location of the crack tip and the changing state of stress near the crack tip.

Displacement field in AA5052 alloy during tensile test was visualized using Electronic speckle pattern interferometry. The process of nucleation and propagation of Portevin-Le Chatelier deformation band during the plastic deformation were investigated by a dynamic observation of the images of displacement contours (fringe pattern). Various types of bands were observed depending on the applied strain rate and the total strain at which the deformation band (DB) appeared. The characteristics of nucleation and propagation of DBs were discussed being compared with the serrated curve in the load. Type A and B serration observed at the high strain rate was characterized by local strain in the DB, and it was found that the difference between type A and B arises from the increase in the local strain rate. Deformation band at low strain rate showed complicated change in response to the nucleation and propagation processes. The correlation between the band propagation and the serrated curve was discussed based on the variation in density and velocity of mobile dislocation.

Because both the accuracy and spatial resolution of digital image correlation (DIC) are directly related to the field-of-view and the number of pixels, it is sometimes advantageous to have a tight view for high resolution measurements and a wide view for overall object deformation. This approach will be demonstrated using a high-speed measurement of the deformation and strain of a riveted thin plate with an explosive loading. Overall plate deformation was provided by a wide-view stereo system, while a tight view of a section of the rivets was imaged with a second stereo pair to measure the strain around the rivet holes. The challenge is creating a speckle pattern which will work with both systems without creating holes in the overall measurement data. This was accomplished by creating a black/white course pattern for the wide view and a black/grey/white fine pattern for the tight view. The grey speckles were sized such that they are not resolved by the wide view and therefore do not compromise the full-field measurement. Details of the process and example results will be presented.

The uncertainty of a Digital Image Correlation (DIC) displacement field is estimated using a generic post-processing method based on statistical analysis of the intensity patterns. First the second image is dewarped back onto the first one using the computed displacement field which provides two almost perfectly matching images. Differences are analyzed regarding the effect on shifting the minimum of the correlation function. A relationship is derived between the standard deviation of intensity differences over a local region (subset or facet size) and the expected asymmetry of the correlation peak, which is then converted to the uncertainty of the displacement vector. This procedure is tested with synthetic data for various types of noise (random Gaussian noise, photon shot noise, image degradation) and provides accurate estimate of the true error. Finally the technique is applied to experimental data where the true error is estimated independently by other means. The proposed technique provides in many cases a reliable uncertainty estimate for different error sources related to variation in surface pattern as well as illumination and viewing angle changes.

Attempt to indicate the potential correlation errors of DIC method, the modulation transfer function (MTF) test method is proposed in this paper. An Alumnus plate with random pattern on the surface was moved by a linear stage and commercial DIC software was used to calculate the displacement filed while the reference image was taken at focus and the second image set was taken at different field of view. The calculated displacement fields are corrected with a linear function to eliminate unexpected displacement gradient. Meanwhile, the MTF values are also calculated with the same random pattern images. Finally, the MTF values and the slop coefficient of the linear fitting function are related, by this way, once the camera MTFs are known then the possible displacement error can be estimated.

Full-field thermal deformation experiments on electronic packaging materials for areas from 50 × 50 to 10 × 10 μm

2

and temperatures from RT to ≈200

°

C have been successfully performed in a Zeiss Ultraplus Thermal Field Emission SEM using 2D-DIC. First, polishing methods for heterogeneous electronic packages containing silicon, Cu bump, dielectric layer, substrate and FLI (First level interconnect) have been studied to achieve sub-micron surface flatness. Using novel self-assembly techniques, a dense, randomly isotropic high contrast pattern has been successfully applied over the surface of test samples. A high precision Physik Instrumente (PI) Piezo nanopositioning stage has been used to help implement essential drift and spatial distortion correction procedures, which were recently shown to be effective in removing distortions from SEM images. Using thin ceramic films to reduce thermal effects on the FEG SEM source, results indicate that the method is capable of measuring local thermal expansion in selected regions, improving our understanding of these heterogeneous material systems under controlled thermal environmental conditions.

In this study, the thermal expansion of bi-metal specimen is measured by digital image correlation (DIC). A measurement system is developed for the evaluation of complex thermal strain distribution on electronic packages. A heating chamber is designed for applying the thermal load and DIC provides the full-field thermal deformation distribution of the bi-metal specimen due to temperature changes. The in-plane strain distribution measured by DIC is influenced by the out-of-plane displacement. By measuring the thermal expansion of the materials having known thermal expansion coefficient at same time, the effect of the out-of-plane displacement on the in-plane strain measurement is corrected. Experimental Results show that the thermal strain of the bi-metal specimen can be obtained by the measurement system including the out-of-plane displacement correction.

Polymeric structural foams are widely used in many engineering applications due to their exceptional properties including high specific strength and energy absorption. The mechanical properties depend strongly on their microstructures, which also dictate their load-bearing capability under deformation. However, the mechanical behavior of polymer foams in compression is not well understood, due to the complex local deformation and strain characteristics associated with the cellular microstructure. In this paper, unconfined uniaxial compression of a polymeric structural foam was conducted while its microstructure was determined using micro-computed tomography (micro-CT) subjected to large deformations. The detailed local deformations and strains are obtained by using three dimensional digital volume correlations (DVC) method. This incremental DVC allows the use of intermediate bridging images to determine large nonlinear deformations in the foam under compression. The evolution and deformation mechanism of the microstructure are observed during different compression stages using the incremental DVC techniques.

An experimental technique is introduced to measure full field strains using three dimensional digital image correlation at temperatures up to 800°C. Challenges include: thermal air gradients, speckle pattern adhesion, image distortion due to viewing window deformation, camera calibration, and infrared light pollution of the camera sensor. Elements of the test setup are designed to address all of these challenges. The technique is used to measure full-field strains on Ti-6Al-4V specimens as they are loaded to failure in tension. The technique provides substantially more data than traditional elevated temperature strain measurement methods.

Ceramic composites are being developed as the next generation accident tolerant fuel cladding for light water reactors. In this study, we report a novel method to evaluate thermo-mechanical robustness of ceramic nuclear fuel cladding tube under simulated accident conditions. A ceramic surrogate core is used as the “pressurizing media” inside the cladding tube. To mimic accident condition, the surrogate core is electrically heated up to 1,200°C to create a temperature gradient; stress in the cladding tube is generated by the mechanical interference of the surrogate core and the cladding tube. In order to apply digital image correlation (DIC) technique for surface strain measurement, a method to produce temperature-resistance speckle pattern was developed. By using a narrow bandpass filter and a LED source, stable reflective images of speckle pattern was achieved from room temperature up to over 1,000°C. This enabled the first successful full field strain mapping of nuclear fuel cladding at high temperature using 3D DIC technique. The surface strain distribution as well as the temperature data obtained can be used to validate numerical simulation models of nuclear fuel cladding.

Metal-plate-connected wood trusses for roofs and floors have been used in light-frame residential, industrial, and commercial construction for several decades. The behavior of a metal-plate connector (MPC) joint is very complex and is influenced by many variables, including metal-plate properties, joint geometry, and the natural variability of wood. Moreover, boundary conditions of such connectors are not well defined in practice, thus ruling out meaningful analytical or numerical stress analyses. With very little prior research on the state of stress of a MPC, this paper presents a hybrid method for stress analyzing a MPC in a beam subjected to 3-point bending using 3D Digital Image Correlation (DIC). The recorded vertical displacements in the vicinity of one of the holes of the MPC are coupled with an Airy stress function, along with imposed appropriate boundary conditions on the edge of the MPC hole. All stress components are determined from only the vertically recorded displacements. Obtaining reliable experimental data near edges can be challenging, but this is overcame here by filling the MPC holes with extremely compliant clay material and applying the speckle pattern over both materials such that measured displacements are obtained continuously throughout the perforated metal plate.

Understanding the load resisting mechanisms and failure modes of masonry walls loaded in their plane is key to validate mechanics-based analysis and simulation algorithms. Proof load tests are performed on specimens that are extensively instrumented with transducers (e.g., LVDTs, potentiometers, strain gauges), which are typically mounted on a specimen at specific locations. The resulting measurements are local and cannot describe in detail the complex response of infill and confined masonry systems, which may combine bricks, mortar joints, reinforced concrete (RC), and the associated interfaces. In addition, crack maps are typically marked by hand based on visual inspection, making it likely to overlook cracks especially when they have a relatively small width or close after unloading.

This paper discusses the feasibility of using digital image correlation (DIC) as a non-contact method to measure displacements on large masonry wall specimens and provide faithful crack maps. Feasibility is assessed based on evidence from in-plane reverse-cycle load tests of two full-scale confined masonry walls. The specimens were designed for two different performance levels in terms of in-plane strength and deformability. Deformability was maximized for the case of a wall that was retrofitted using in-plane (horizontal) reinforcement embedded along the bed joints. The DIC measurements were validated vis-à-vis relevant counterparts from linear displacement transducers, including in-plane drift, diagonal deformations, and interface slip between the RC tie columns and the masonry panel. In addition, faithful DIC-based crack maps were obtained where it is easier to recognize cracks compared with hand marked crack maps on the specimens.

Ball impact experiments were conducted on unstrengthened and strengthened glass bars at 261 and 345m/s, respectively. Damage propagation was recorded using a high-speed camera at frame rates of 281,000 frames per second. Immediately after the ball impact on the unstrengthened glass, the damage front reached a maximum velocity of 1,967m/s before falling to zero within a short distance. However, the longitudinal wave created due to the impact continued down the bar towards the rear-end. Upon reflection from the rear-end of the bar, a secondary damage front was initiated at 3,192m/s, which eventually arrested. On the other hand, the damage front in the strengthened glass reached a maximum of 2,275m/s immediately after impact, and then stabilized at 1,921m/s until the bar was consumed. It was determined that the stored elastic energy in the strengthened glass fueled the self-sustained damage and allowed it to propagate at a near constant rate. For both glasses, high-speed imaging allowed for observation of energy dissipation modes such as fracture propagation (fracture surface area), radial bar dilation, and high velocity jetting of fine glass particles at the impact site. In addition to the triangular dilation observed in the unstrengthened glass at the impact site, the strengthening process also led to uniform dilation of the entire rod.

Advances in multiple view computer vision techniques have made it possible to make highly accurate three-dimensional (3D) measurements using calibrated stereo image systems. Recent experiments conducted at Sandia National Laboratories have demonstrated the feasibility of applying these techniques on an X-Ray system. Acquiring measurements from stereo image systems, be it visible or x-ray, require the estimation of the system’s intrinsic and extrinsic parameters via a calibration process. There are several calibration methods depending on the system’s configuration and its intended use. In most cases, one or more image pairs of a calibration artifact such as a 3D object of known dimension or a 2D target board are processed to estimate the system’s calibration parameters. For this paper, methods based on both types of calibration artifacts will be discussed along with experimental results.

To analyze the displacement field in volumes imaged by X-ray tomography at several deformation states, a new approach is proposed whereby the displacement is measured down to the voxel scale and determined from a mechanically regularized system using the equilibrium gap method and additional boundary regularizations. It is then possible to compute a displacement vector for each voxel, inducing lower residuals (in terms of experimental data). As representative reconstructed volumes lead to large numbers of degrees of freedom, a dedicated GPU computational strategy has been developed and implemented. A set of volumes of size 100×170×256 voxels (i.e., more than 13 million kinematic unknowns), which corresponds to a part of a sample made of nodular graphite cast iron and tested in tension, is analyzed.

Volumetric Digital Image Correlation (VDIC) or Digital Volumetric Correlation (DVC) is becoming more widely used in biological research, micro-structural studies in materials, geological sciences and other fields. With DVC, volumetric image sets obtained by X-Ray tomography, magnetic resonance imaging or confocal microscopy can be analyzed to obtain volumetric deformation measurements throughout the interior of a specimen, provided that there is sufficient contrast (local changes in contrast) within the interior region of interest. Using software developed by the authors [1], results from a series of experimental studies on a small circular disk specimen are presented that demonstrate the accuracy of the method when adequate contrast exists.

Light guide plate (LGP) is one of the essential components of a back light module (BLM). Warpage may be produced during the manufacturing process due to the lower structural strength of the LGP in a thinner thin film transistor liquid crystal display (TFT-LCD) as required by the market. Basically, the fringe pattern produced by the digital phase-shifting shadow moiré (DPSSM) method is generated from interference between the reference grating and its projected grating on the test surface. For a transparent object such as a LGP, reflective painting is generally sprayed on the object’s surface to make the projected grating sufficiently visible. However, the LGP may no longer be re-used or may be damaged during the process of painting or de-painting. To investigate the feasibility of obtaining the surface topography of a transparent object by employing the DPSSM method without painting, fringe patterns generated from different capture angles with the same incident angle were examined. With the help of the image correction program, it was found that skewed images obtained from different capture angles can be successfully recovered. The surface topography of a LGP can then be correctly determined.

We propose a method for determining phase distribution of interference fringes utilizing a CCD camera equipped with a micro-polarizer array. An optical setup of polarization interferometry using a Mach-Zehnder Interferometer with two polarizers is constructed to analyze the distribution of the thickness change of the transparent sample. Light emerging from the interferometer is recorded using a CCD camera that has a micro-polarizer array on a CCD plane. This micro-retarder array has four different principal directions. The four images separated from the image recorded by the CCD camera are reconstructed using gray level interpolation. Subsequently, the distributions of the Stokes parameters that represent the state of polarization are calculated from the four images. The phase distribution of the interference fringe pattern produced by the Mach-Zehnder interferometer is then obtained from these Stokes parameters. This method is applicable to dynamic phenomena because multiple exposures are unnecessary for sufficient data acquisition for phase analysis of fringes.

Phass-shifted shadow Moire has gained more applications in electronic industry. However, printed circuit boards (PCB) may contain many cavities or specular materials on the surface that make the phase-unwrapping of Moire fringe patterns more difficult or fail. In this paper, a method is proposed to overcome this difficulty to process the phase-shifted fringe patterns effectively and automatically for surface profile measurement. Firstly the intensity values of the original four phase-shifted fringe patterns are averaged and differentiated to enhance the erroneous spots. Then the median grey-level value of the enhanced image is used as the threshold to binarize the enhanced image to find the erroneous bright and black spots. According to the largest size of erroneous spot, the size of a structuring element is determined for morphology filtering. Thereafter the phase can be calculated and unwrapped correctly. Test of the method on a PCB is demonstrated and discussed.

With the advent of CCD camera and the rapid development of computer technology, digital holographic technique has recently attracted much attention from various research fields. This paper presents spatial phase retrieval techniques in digital holography. In the spatial domain, a novel method using the concept of complex phasor is proposed to determine the phase difference map in digital holography. A simple method based on finite difference is also proposed to compute high quality phase derivatives. Based on a deformation phase map obtained by the complex phasor method, slope, curvature and twist maps can be determined. Unwrapped derivative maps can be determined directly from a wrapped phase map without any phase unwrapping process. Simulation and experimental results demonstrate that the proposed method has high measurement accuracy and can effectively determine high resolution phase derivative with less computational effort. It is shown that the proposed techniques can effectively and accurately overcome theoretical and application problems in digital holography.

The deformation behavior of crystal grains of polycrystal aluminum is observed using speckle interferometry. Coarse grained pure aluminum specimens with grains about 5 mm in diameter are fabricated to observe the deformation behavior easily. The specimens are plastically deformed by a tensile and a compression test. Then, the bidirectional in-plaine deformation distributions of the specimen surfaces containing multiple crystal grains are measured. The measurement results show that the various deformation behaviors in each grain are observed. Additionally, the longitudinal, transverse and shearing strains in each crystal grain are analyzed from the measurement results. In the strain maps, the complex deformations and slip bands are observed.

Optical interferometry and contact angle goniometry are applied to evaluate the adhesion strength of nano-scale thin-film systems. The optical interferoemtry is used to characterize the harmonic response of the film-substrate interface, and the contact angle goniometry is used to measure the surface energy of the substrate. The thin-film specimen is configured as one of the end-mirrors of a Michelson interferometer and driven from the rear with an acoustic transducer at audible frequencies. The resultant film’s displacement is detected as interferometric fringe patterns behind the beam splitter. A mathematical procedure is developed to estimate the film’s displacement from the Fourier spectrum of the fringe pattern. Specimens of a titanium-film coated on a silicon substrate, a gold-film coated on a titanium pre-coated silicon substrate, and a platinum-film coated on a titanium pre-coated silicon substrate are tested. In all these specimens, the titanium film is coated either on the silicon substrate whose surface is treated with plasma bombardment (the surface-treated specimens), or directly on the silicon substrate without pre-coating treatment (the untreated specimens). The surface treatment is known to enhance the adhesion. The surface energy measurement by contact angle goniometry indicates that the surface treatment substantially alters the surface energy of the silicon substrate. Interestingly, however, the harmonic response of the film surface measured by the interferometer indicates that in a certain frequency range the surface-treated specimen shows greater oscillation amplitude than the non-surface treated cases.

Digital Shearography is an optical non-contacting non- destructive testing technique. The inspection method has found many applications and is particularly suited for amongst others, in-situ inspection of composite components for defects such as low velocity impact damage and delaminations. Due to advancing technologies smaller and more cost effective components are readily available for integration into such an inspection system.

This paper presents the latest portable prototype developed at the University of Cape Town. The technique and it’s application is described in detail. The layout and design of the prototype including camera, laser and optical configuration is outlined and presented. Using the system, initial results from inspections of a known defect sample is presented in an attempt to evaluate the performance of the prototype. The paper is concluded with a discussion of the results obtained and possible recommendations for refinements and improvements.

Shearography is an interferometric method that produces full-field displacement gradients of the inspected surface. In high-technology industry it is often used qualitatively to detect material defects, but quantitative applications are still rare. The reasons for that are the complicated calibration procedure as well as the denoising, unwrapping, the local sensitivity vector estimation and the local shearing angle estimation needed to get quantitative gradient-maps. To validate the technique and its calibration, results obtained from shearography are compared to results obtained from scanning laser vibrometry. Beams are acoustically excited to vibrate at their first resonant frequency and the mode shape is recorded using both shearography and scanning laser vibrometry. Outputs are compared and their properties discussed. Separate inverse method algorithms are developed to process the data for each method. They use the recorded mode shape information to identify the beam’s local stiffness distribution. The beam’s stiffness is also estimated analytically from the local geometry. The local stiffness distributions computed using these methods are compared and the results discussed.

To better evaluate current condition standards commonly used for the exhibition of canvas paintings, it is necessary to have a quantitative technique capable of measuring degradation components induced by changes in temperature and relative humidity, as well as the effects of ambient vibration and the thermomechanical effects of museum lighting. This paper presents advances in our development of a customized laser shearography system for temporal characterization of in-plane displacements of canvas paintings when subjected to changes in exhibition conditions. The shearography system performs concomitant measurements of gradients of displacement along two orthogonal shearing directions and is synchronized with a thermal IR camera to provide thermal maps of the area being analyzed. Recent innovations incorporated into the system include a real-time temporal phase unwrapping algorithm, and high-resolution Fast Fourier Transform (FFT) methods to calibrate applied shearing levels that allow a wide range of measuring resolutions. Examples will be presented that illustrate the system’s capabilities to detect cracks in the paint surface and measure and map associated strain vectors as a function of changes in condition parameters. Included are representative results of continuous 30 h recordings on American nineteenth century oil on canvas painting. Multi-domain data has been combined and correlated using the shearography and IR data from our system, temperature and humidity data from the museum’s climate control system, as well as activity log from museum’s security system.

Interferometric strain/slope rosette technique is an optical technique based on laser interferometric. It has advantages such as short gage length, non-contacting, thermal resistant, measuring both in-plane strains and out-of-plane deformations’ slopes simultaneously. Six-faced delta rosette has been always utilized. In this paper, a new type of rosette, the eight-faced rectangular rosette was first applied. Besides the advantages described above, the new rosette has the advantage for measuring strains/slopes directly in two orthogonal directions. This avoids the strain transformation process which was generally required by the delta rosette. Also, since the rectangular rosette measures strains directly along the two objective directions instead of measuring in the direction with an angle, higher sensitivity will be achieved. Two preliminary tests using the new rosette were presented in this paper. First, it was adapt to measure strains in a compression test of an aluminum block at each loading step. Resistance strain gage rosette was also applied for evaluating the result. Second, the new rosette was introduced for measuring the real time strain during an arc welding process. Strain history during the welding process was successfully obtained and compared well with results using delta rosettes.

Accurate and high-speed three-dimensional (3D) shape measurements are becoming increasingly important in many industries. Recent advances in the development of high-resolution detectors, light sources, MEMS, and computational power are enabling the availability of noncontact and noninvasive optical methods, which can be applied to new and challenging applications requiring 3D and high-speed measurements.

In this paper, we describe our recent advances in the development of a fringe projection-based system for high-speed 3D shape measurements, as part of the Surface Optical Profilometry Roadway Analysis (SOPRA) system for evaluation of the public road quality within the urban infrastructure. The system is mounted on a vehicle moving at speeds of up to 100 km per hour and it consists of a custom built structured light projector and a recording and analysis sub-system. Fast-Fourier Transform (FFT) and discrete wavelet transform (DWT) single frame methods as well as optimized spatial phase unwrapping methods have been developed as part of the analysis sub-system. We have also developed innovative Graphics Processing Unit (GPU) accelerated algorithms to improve the analysis and post-processing speeds to run live phase retrieval at up 40 fps. Automatic road curvature and crack detection algorithms quantify the road quality index. The system has been field tested for real-time road measurements in daylight conditions.

In this paper, we consider the advantages and limitations of two existing techniques: structured light fringe projection (FP) and reflectance transformation imaging (RTI), and we propose a new hybrid shape measurement approach that combines their advantages. FP allows direct full field-of-view measurements of shape; however, due to the nature of the technique, the high frequency shape details of the object may not be captured. RTI is a recently developed technique that allows high resolution measurements of surface normal vectors, which are related to high frequency details of shape of the object; however, extracting the object shape from RTI data requires numerical integration, which leads to cumulative low frequency bias errors. We present representative results that demonstrate the ability of our approach to perform high resolution shape measurements and non-destructive testing of structures.

This investigation applied, simultaneously, two or three experimental strain measurement techniques to different structural models in order to highlight their specific advantages and fields of application. The applied techniques were the digital image correlation method (DIC), the electrical resistance strain gage method (SG), and the reflection photoelasticity method (RP). The DIC, SG and RP results for strains measured at the same points or at similar points of the tested models proved to be satisfactorily close.

A spatial-temporal hybrid approach for retrieving the isoclinic and isochromatic phase messages of practical photoelastic problems is proposed. With the aid of the proposed method, the undefined problem caused by the null term of denominator in the phase map calculation as well as the ambiguity zone problem during temporal phase unwrapping can be respectively solved. Four plane polariscope phase stepping frames are utilized first to correctly restore the isoclinic data of the test sample, of which a white lighting source with full color CCD is used for the colored photoelastic interferograms recording to circumvent the undefined problem. Then after, the retrieved isoclinic is further substituted into the isochromatic formulation to decouple its isoclinic dependence and get a decoupled isochromatic phase map for the following spatial phase unwrapping without much difficulty. In addition, a temporal retrieving of isochromatic phase map is also implemented too; a different wavelength approach is applied to check the performance of the proposed algorithm. It is proved that the robustness and effectiveness of the proposed method are both acceptable.

We adopt an improved co-precipitation method to prepare the Fe

3

O

4

magnetic nanoparticles (MNPs). Influence factors such as surfactant amount, stirring speed, dispersion mode, and Fe

3+

/Fe

2+

molar ratio are considered. Using the TEM and XRD, we characterize the dispersibility and size of the products. The appropriate values of experimental parameters are determined, such as stirring speed is 1,000 rpm in titration, simultaneous ultrasonic vibration and mechanical stirring in titration and surface coating, and surfactant amount of oleic acid is 1.2 ml for Fe

3+

/Fe

2+

molar ratios including 1.7, 1.8, and 1.9. The average diameters of these Fe

3

O

4

MNPs are determined around 11 nm by XRD analysis. In addition, the saturation magnetization for the MNP produced by Fe

3+

/Fe

2+

molar ratio as 1.7 is 50.17 emu/g with near paramagnetism. Above all, the linear birefringence and dichroism of the kerosene-based ferrofluids are investigated by a developed Stokes polarimerter. Finally, compared to the results with those for a commercial product and the influences of particle size distribution and magnetization are discussed.

The aim of this study is to measure the (complex) shape of an object by using a priori information given by its CAD representation in a 3D-DIC framework. In order to follow a 3D object during its deformation and to determine 3D surface displacement fields, a first measurement of the object shape is necessary. The main goal of the present method is to obtain a CAD representation of this measurement by updating the CAD reference via a global approach to 3D-DIC. Although in the majority of the shape measurement methods a cloud of points is obtained and subsequently interpolated to create a continuous description of the surface, CAD-based stereoDIC is devised to directly measure a 3D shape described by NURBS. This approach allows a direct link to be established between the theoretical CAD model and its true geometry thereby yielding the metrology of the measured surface in the CAD language.

A new method for local strain measurement of soft materials like wood is proposed. Norway spruce samples were subjected to radial compression in an encapsulated split-Hopkinson device (ESHD). High speed photography was used at two magnifications for image based analysis. The strain estimation was made from high magnification images showing compression on local, fiber level for 1–2 growth rings and from low magnification images showing compression on sample level, for 5–8 growth rings. Strain gauges on the ESHD bars give stress and average strain for comparison. Image analysis based on PIV technique gives local and average strain propagation as a function of time. Wood is an inhomogeneous material and thus, local strain is a more proper measure of the response of the material. The high magnification captures differences between earlywood and latewood while the low magnification gives the strain distribution over the whole sample. Both magnifications are important in order to understand the response of the wood material to the sudden compression. A way to estimate the stress field was developed. The results showed similarity to the strain gauge measurement results.

The potential use of pulse phase thermography (PPT) and thermoelastic stress analysis (TSA) to identify artificial defects in CFRP single lap joints has been studied. PPT was able to identify defects in adhesive bonds where there was a contrast between defect and non-defect thermal properties. Where a lack of thermal contrast occurred it was found that the application of a small load was sufficient to reveal a previously unidentified defect. TSA was able to reveal defects where there was a change in stress distribution caused by the defect; however it did not identify inclusions that had very little effect on stress distributions. It may be argued that such an inclusion is not a defect. It is necessary to clarify this as false identification of defects may lead to unnecessary repair work.

Several evidences of anomalous nuclear reactions occurring in condensed matter have been observed during electrolysis, solid fracture and liquid cavitation. Despite the great amount of experimental results coming from the so-called

Cold Nuclear Fusion

and Low Energy Nuclear Reaction research fields, the comprehension of these phenomena still remains unanswered. On the other hand, as reported by most articles devoted to

Cold Nuclear Fusion

, one of the principal features is the appearance of micro-cracks on the electrode surfaces after the experiments. In the present paper, a mechanical explanation is proposed considering a new kind of anomalous nuclear reactions, the piezonuclear fissions, which are a consequence of

hydrogen embrittlement

of the electrodes during electrolysis. Energy emissions in the form of neutrons and alpha particles were measured during the experiments, where the electrolysis is obtained using Ni-Fe and Cr-Co electrodes in an aqueous solution. The electrode compositions were analyzed both before and after the experiments recognizing the effects of piezonuclear fissions occurring in the host lattices.

Stress during welding is of high interest while complex due to the high gradient transient thermal effect. All published literatures studied it by finite element simulations. For the first time, the authors tried to derive the stress history based on measured temperature and in-plane strains. However, the measured strains were the total strains consisted of coupled thermal, elastic and plastic strains. To determine the stress based on the constitutive equations, measured total strains should be decoupled.

Three in-plane strain (strain X, strain Y and shear strain XY) histories which were general strain history during welding process were utilized for illustrating the strain decoupling process. Thermal strain was first determined by the measured temperature history. To separate the elastic strain and plastic strain, the sum of mechanical strain determined by subtracting thermal strain from total strain was divided into three stages: compression, recovery and tension. Yielding was detected at both the compression and tension regions using von Mises’ yielding criteria. Similar to cyclic tension/compression testing, same amount of elastic strain will be recovered before the tension region starts. All material properties used were temperature dependent, and a plane stress condition was assumed since welding was applied on thin steel plates.

Fe-Ni-Cr alloys have been used for more than 40 years to manufacture metal foil strain gages for specific applications. The main characteristics of this metal are high gage factor, high fatigue strength, and high thermal output. To circumvent the effect of high thermal output, typical applications in stress analysis have involved dynamic loading where the high gage factor and fatigue strength could be used to advantage and the effect of thermal output could be minimized or ignored. Typical applications in precision transducers have also involved dynamic loading and/or careful exercise of Wheatstone bridge circuit cancelation of like-thermal output in adjacent arms. In this paper, we report uniform thermal output variation in Fe-Ni-Cr metal foil strain gages. Quarter bridge and half bridge thermal output data are presented, which illustrated the suitability of this type of strain gage for stress analysis and precision transducers even when loading conditions are not dynamic.