Showing papers on "Digital image correlation published in 2002"

Systematic errors in digital image correlation due to undermatched subset shape functions

Abstract: Digital image correlation techniques are commonly used to measure specimen displacements by finding correspondences between an image of the specimen in an undeformed or reference configuration and a second image under load. To establish correspondences between the two images, numerical techniques are used to locate an initially square image subset in a reference image within an image taken under load. During this process, shape functions of varying order can be applied to the initially square subset. Zero order shape functions permit the subset to translate rigidly, while first-order shape functions represent an affine transform of the subset that permits a combination of translation, rotation, shear and normal strains. In this article, the systematic errors that arise from the use of undermatched shape function, i.e., shape functions of lower order than the actual displacement field, are analyzed. It is shown that, under certain conditions, the shape functions used can be approximated by a Savitzky-Golay low-pass filter applied to the displacement functions, permitting a convenient error analysis. Furthermore, this analysis is not limited to the displacements, but naturally extends to the higher-order terms included in the shape functions. This permits a direct analysis of the systematic strain errors associated with an undermatched shape function. Detailed numerical studies are presented for the case of a second-order displacement field and first- and second-order shape functions. Finally, the relation of this work to previously published studies is discussed.

An automated approach for direct measurement of two-dimensional strain distributions within articular cartilage under unconfined compression.

Abstract: An automated approachfor measuring in situ two-dimensional strain fields was developed and validated for its application to cartilage mechanics. This approach combines video microscopy, optimized digital image correlation (DIC), thin-plate spline smoothing (TPSS) and generalized cross-validation (GCV) techniques to achieve the desired efficiency and accuracy. Results demonstrate that sub-pixel accuracies can be achieved for measuring tissue displacements with this methodology with a measurement uncertainty ranging from 0.25 to 0.30 pixels. The deformational gradients (from which the strains are determined) can be evaluated directly using the optimized DIC, with a measurement uncertainty of 0.017 to approximately 0.032. In actual measurements of strain in cartilage, TPSS and differentiation can be used to achieve a more accurate measurement of the gradients from the displacement data. Using this automated approach, the two-dimensional strain fields inside immature bovine carpometacarpal joint cartilage specimens under unconfined compression were characterized (n=21). The depth-dependent apparent elastic modulus and Poisson's ratio were also determined and found to be smallest at the articular surface and increasing with depth. The apparent Poisson's ratio is found to decrease with increasing compressive strain, with values as low as 0.01 observed near the articular surface at 25% compression. The variation of the apparent Poisson's ratio with depth is found to be consistent with a theoretical model of cartilage which accounts for the disparity in its tensile and compressive moduli.

Full-field speckle pattern image correlation with B-Spline deformation function

Abstract: A full-field speckle pattern image correlation method is presented that will determine directly the complete, two-dimensional deformation field during the image correlation process on digital images obtained using computer vision systems. In this work, a B-Spline function is used to represent the object deformation field throughout the entire image area. This is an improvement over subset-based image correlation methods by implicitly maintaining position and derivative continuity constraints among subsets up to a specified order. The control point variables within the B-Spline deformation function are optimized iteratively with the Levenberg-Marquardt method to achieve minimum disparity between the predicted and actual deformed images. Results have shown that the proposed method is computationally efficient, accurate and robust. The general framework of this method can be applied ton-dimensional image correlation systems that solve for multi-dimension vector fields.

Multiscale displacement field measurements of compressed mineral-wool samples by digital image correlation

Abstract: We propose a multiscale approach to determine the displacement field by digital image correlation. The displacement field is first estimated on a coarse resolution image and progressively finer details are introduced in the analysis as the displacement is more and more securely and accurately determined. Such a scheme has been developed to increase the robustness, accuracy, and reliability of the image-matching algorithm. The procedure is used on two different types of examples. The first one deals with a representative image that is deformed precisely and purposefully to assess the intrinsic performances. In particular, the maximum measurable strain is determined. The second case deals with a series of pictures taken during compression experiments on mineral-wool samples. The different steps of the procedure are analyzed and their respective role is assessed. Both reflection and transmission images are tested.

Mechanical response of friction stir welded AA2024: experiment and modeling

Abstract: The mechanical response of heterogeneous structures, such as weldments, is largely governed by the response of the local constituents. In the present paper, the global and local mechanical response of friction stir welded AA2024 is examined experimentally and numerically. Full field strain measurements are obtained on transversely loaded tensile specimens via the digital image correlation technique. Assuming an iso-stress configuration, local constitutive data were determined for the various weld regions and used as input for a 2-D finite element model. The simulation results were compared with the experimental results to assess the viability of the modeling approach and the validity of the iso-stress assumption

Digital volume correlation including rotational degrees of freedom during minimization

Abstract: Digital volume correlation is a new experimental technique that allows the measurement of the full-field strain tensor in three dimensions We describe the addition of rotational degrees of freedom into the minimization problem for digital volume correlation in order to improve the overall performance of the strain measurement A parameterization of rotations that is particularly suited to the minimization problem is presented, based on the angle-axis representation of finite rotations The partial derivative of both a normalized cross-correlation coefficient and the sum-of-squares correlation coefficient are derived for use with gradient-based minimization algorithms The addition of rotation is shown to greatly reduce the measurement error when even small amounts of rigid body rotation are present in an artificially rotated test volume In an aluminum foam sample loaded in compression, including rotational degrees of freedom produced smoother contours of minimum principal strain Renderings of the aluminum foam architecture in areas of low, medium and high rotation showed material deformation pattern in detail

Analysis of a multiaxial test on a C/C composite by using digital image correlation and a damage model

Abstract: The “planar” digital image correlation technique needs a single CCD camera to acquire the surface patterns of a zone of a specimen in the underformed and deformed states. With these two images, one can determine in-plane displacement and strain fields. The digital image correlation technique used herein is based on Fast Fourier Transforms, which are very effective in reducing the computation cost. Its performance is assessed and discussed on artificial signals and in a real experimental situation. The technique is utilized to analyze experimental results of a plane shear experiment and validate a damage meso-model describing different degradations in a C/C composite material.

Evaluating the Mechanical Behavior of Arterial Tissue using Digital Image Correlation

Abstract: In this study, digital image correlation (DIC) was adopted to examine the mechanical behavior of arterial tissue from bovine aorta. Rectangular sections comprised of the intimal and medial layers were excised from the descending aorta and loaded in displacement control uniaxial tension up to 40 percent elongation. Specimens of silicon rubber sheet were also prepared and served as a benchmark material in the application of DIC for the evaluation of large strains; the elastomer was loaded to 50 percent elongation. The arterial specimens exhibited a non-linear hyperelastic stress-strain response and the stiffness increased with percent elongation. Using a bilinear model to describe the uniaxial behavior, the average minor and major elastic modulii were 192±8 KPa and 912±40 KPa, respectively. Poisson's ratio of the arterial sections increased with the magnitude of axial strain; the average Poisson's ratio was 0.17±0.02. Although the correlation coefficient obtained from image correlation decreased with the percent elongation, a correlation coefficient greater than 0.8 was achieved for the tissue experiments and exceeded that obtained in the evaluation of the elastomer. Based on results from this study, DIC may serve as a valuable method for the determination of mechanical properties of arteries and other soft tissues.

Fracture energy from indirect tension testing

Abstract: This paper proposes that fracture energy determined from the indirect tension test (IDT) can serve as a simple performance indicator for fatigue cracking of asphalt mixtures. The effectiveness of the loading apparatus and deformation measurement device is evaluated through an experimental study. The Strategic Highway Research Program (SHRP) Load Guide Device (LGD) and surface-mounted Linear Variable Differential Transducers (LVDTs) with a 50-mm gauge length are proposed as the indirect tension testing system components. The Digital Image Correlation (DIC) technique is also proposed as a possible noncontact, full-field displacement/strain measurement method. In this research, the DIC technique was applied to determine the proper gauge length for a 100-mm diameter IDT specimen. The theory of viscoelasticity was used to develop analytical solutions for creep compliance and center strain from displacements measured on the specimen surface. Indirect tensile creep and strength tests were performed on field cores obtained from WesTrack sections constructed from fine and coarse mixtures with varying asphalt contents and air void contents. Various parameters determined from the analysis of these data were compared against the known fatigue performance of these mixtures from the WesTrack testing facility. Fracture energy obtained from the viscoelastic analysis of the indirect tensile strength test at 20 deg C correlated highly with the field performance of these mixtures at WesTrack.

Simple Performance Test for Fatigue Cracking and Validation with WesTrack Mixtures

Abstract: Viscoelastic characterization of asphalt concrete in indirect tensile (IDT) testing and development of a simple performance test for fatigue cracking are described A 50-mm gauge length was adopted to measure the horizontal and vertical deformations with surface-mounted linear variable differential transducers on an IDT specimen with a 100- or 150-mm diameter and 38-mm thickness The effect of a concentrated load under loading strips on vertical displacement within the 50-mm gauge length was evaluated by the digital image correlation method, a noncontact, full-field displacement and strain measurement technique The theory of viscoelasticity was used to develop analytical solutions for creep compliance and center strain from displacements measured on the specimen surface These solutions were verified by three-dimensional finite element viscoelastic analysis IDT creep and strength tests were performed on fine and coarse mixtures from WesTrack with various asphalt contents and air void contents, and vario

Limits and possibilities of laser speckle and white-light image-correlation methods: theory and experiments.

Abstract: A laser-speckle method and a white-light image-correlation method are used for strain mapping. A schematic model of the correlation function of two speckle patterns is proposed for investigation of strain influence on displacement-measurement accuracy. A specific software has been developed to calculate by direct correlation the displacement values between two pictures with a grainy pattern at any point on an object’s surface. Its efficiency is demonstrated in several tests. Moreover, theoretical results are checked through experimental measurements. The limitations and performances of both optical techniques are discussed.

Tools for multiaxial validation of behavior laws chosen for modeling hyper-elasticity of rubber-like materials

Abstract: We present an experimental approach to discriminate hyper-elastic models describing the mechanical behavior of rubber-like materials. An evaluation of the displacement field obtained by digital image correlation allows us to evaluate the heterogeneous strain field observed during these tests. We focus on the particular case of hyper-elastic models to simulate the behavior of some rubber-like materials. Assuming incompressibility of the material, the hyper-elastic potential is determined from tension and compression tests. A biaxial loading condition is obtained in a multiaxial testing machine and model predictions are compared with experimental results.

Application of Digital Image Correlation Method to Mechanical Testing of Asphalt-Aggregate Mixtures:

Abstract: The digital image correlation (DIC) method, a noncontact, full-field displacement measurement technique, has been applied to mechanical testing of asphalt concrete. A single couple charged device camera acquires images of an area of interest from a specimen in the undeformed and deformed states. These images are correlated to determine deformations, and advanced mathematical procedures are applied to these deformations to calculate strains. To verify the DIC measurements, vertical displacements for the middle and bottom sections of a specimen subjected to monotonic tension are compared with conventional linear variable differential transformer measurements. A series of DIC images captured during the monotonic and cyclic tests visualizes the evolution of the failure zone (i.e., the fracture process zone) at the crack tip. Also, it is demonstrated that the full-field measurement and post-processing nature of DIC allows a more accurate determination of the stress-strain behavior of the fracture process zone....

Micro- and nanomaterials characterization by image correlation methods

Abstract: The authors present a new approach to deformation analysis based on localized correlation analysis on load state images. Displacement and strain fields are extracted from images, originating from different kinds of highest resolution equipment, e.g., from scanning electron and scanning force microscopes. As a result, object load response can be recorded within microscopic or nanoscopic material areas. This unique technique is utilized for deformation mapping as well as for direct determination of thermo-mechanical material properties. The method has been established as microDAC/nanoDAC for strain field mapping and as mTest for the measurement of material properties, currently for coefficients of thermal expansion (CTE) and Poisson ratios. This paper includes an introduction in the measurement technique, a view on the method’s capability and the developed hardware for mechanical and thermal testing. The application of the tools is illustrated by examples from electronics and microsystem packaging. Deformation measurements on chip scale packages and measurement of CTE are discussed more in detail. Front-end application of correlation techniques using atomic force microscope imaging is demonstrated.

Image processing issues in digital strain mapping

Abstract: We have developed high density image processing techniques for finding the surface strain of an untreated sample of material from a sequence of images taken during the application of force from a test rig. Not all motion detection algorithms have suitable functional characteristics for this task, as image sequences are characterised by both short- and long-range displacements, non-rigid deformations, as well as a low signal-to-noise ratio and methodological artifacts. We show how a probability-based motion detection algorithm can be used as a high confidence estimator of the strain tensor characterising the deformation of the material. An important issue discussed is how to minimise the number of image brightness differences that need to be calculated. We give results from two studies of materials under axial tension: a sample of aluminium alloy exhibiting a propagating plastic deformation, and a preparation of deer antler bone, a natural composite material.

High-contrast images of semiconductor sites via one-photon optical beam-induced current imaging and confocal reflectance microscopy.

Abstract: We demonstrate a computationally efficient procedure for determining only the semiconductor sites in a confocal reflectance image of an integrated circuit. It utilizes a one-photon optical beam-induced current (1P-OBIC) and confocal reflectance images that are generated from the same focused excitation beam. A 1P-OBIC image is a two-dimensional map of the currents induced by the beam as it is scanned across the circuit surface. A 1P-OBIC is produced by an illuminated semiconductor material if the excitation photon energy exceeds the bandgap. The 1P-OBIC image has no vertical resolution because the 1P-OBIC is linear with the excitation beam intensity. The exclusive high-contrast image of semiconductor sites is generated by the product of the 1P-OBIC image and the confocal image. High-contrast images of the metal sites are also obtained by the product of the complementary OBIC image and the same confocal image.

Spectral interferometric optical coherence tomography with nonlinear β-barium borate time gating

Abstract: A high-speed, all optical coherence tomography system was designed and constructed. This tomography system employs spectral interferometry and optical Fourier transformation to reduce the number of mechanical scanning dimensions required for multidimensional profilometry. The system also employs a time gate comprising a beta -barium borate crystal driven by a femtosecond laser pulse to improve measurement time. This system has 43-mum depth resolution and 150-fs temporal resolution and is capable of taking 1000 cross-sectional image frames per second.

Focus errors and their correction in microscopic deformation analysis using correlation

Abstract: Subpixel digital image correlation has been applied to microscope images to analyze surface deformation. Nonintegral pixel shifting and successive approximation are used to calculate the subpixel element of the sample displacement without introducing systematic interpolation errors. Although in-plane displacement precision of better than 2% of a pixel, or < 15 nm at x10 magnification, is shown to be achievable, the use of even moderate numerical aperture microscope objectives render the technique sensitive to errors or variations in sample focusing. The magnitude of this effect is determined experimentally and a focus compensation method is described and demonstrated.

Strain measurement in tests on fibre fabric by image correlation method

Abstract: Strain measurements are investigated by an image correlation method, for tensile tests, in order to analyse the non-linearities due to crimp. The interest of the method is the possibility to study the phenomena at different scales and so to contribute to a better understanding of the yarn interactions and of their influence on the fabric mechanical behaviour

Measurements of pressure distribution by integrating the material acceleration

Abstract: This paper describes the development of a measurement system that is capable of measuring the instantaneous pressure distribution over a sample area in a turbulent flow field. The technique utilizes four-exposure PIV to measure the material acceleration of the flow and integrating it to obtain the pressure distribution. If necessary, e.g., for cavitation test, a reference pressure at a single reference point is also required. Two 2K×2K CCD cameras and perpendicularly polarized Nd:Yag lasers are used for recording four exposures, with images one and three recorded by camera one to provide the first velocity vector map, and images two and four recorded by camera two to provide the second velocity vector map. The material acceleration is calculated from the velocity difference in sample areas that are shifted relative to each other according to the local velocity. Averaged omni-directional integration of the material acceleration, avoiding regions dominated by viscous diffusion, provides the pressure distribution. To improve the accuracy of the acceleration measurement, cross-correlation of the corresponding image correlation maps is implemented at areas with high velocity gradient. Applications of these procedures to synthetic images of rotational and stagnation point flows show that the relative error of the measured pressure is 4.0% at the 95% confidence level. The system is presently being used to measure the instantaneous pressure and velocity distributions of a 2D cavity flow field in a water tunnel.

Semiautomated analytical image correlation.

Abstract: Machine vision refers to computer programs consisting of a collection of pattern recognition and digital image processing algorithms (Fabel, G. Motion Control 2000, 53−54). A version of machine vision has been applied to correlating digital images generated by optical microscopy and secondary ion mass spectrometry (SIMS). By suitable application of image processing algorithms, semiautomated correlation between optical and secondary ion images is possible. For correlation of minor constituents evident in secondary ion images but invisible in optical images, correlation is performed by reference to the relative position of minor to major constituents. Precise coordinates of features apparent in one analytical image can be translated into the corresponding coordinates of an analytical image obtained by a different method. In principle, this capability yields a semiautomated system to combine complementary features of disparate imaging methods, such as secondary ion and optical microscopy.

Optical Design of a Miniature Fourier Transform Lens System for a Hybrid Digital-Optical Correlator

Abstract: The availability of high resolution spatial light modulators (SLMs) and modern digital signal processors (DSPs) allows a convenient hybridization of digital and optical image processing to produce a small and compact correlator system. The hybridization allows the optical Fourier transform lens system for a correlator to be compact, such that it will fit inside a conventional PC. We describe the design of the optical system. © 2002 Society of Photo-Optical Instrumentation Engineers.

Non-contacting measurement technology for component safety assessment

Abstract: This paper describes the use of the digital image correlation (DIC) method of deformation measurement suited to finite element method modelling. The technique has applications in component manufacture in the automotive industry. Displacement of the tested material is inferred by tracking the deformation of a random speckle pattern, applied to a component's surface, in digital images acquired during load testing. More recently, the DIC method has been extended to use multiple cameras, permitting measurement of three-dimensional shape and deformation. One particular DIC algorithm is preferred for deformation analysis because it can measure arbitrarily large rotations and strains and performs with superior accuracy and efficiency. It is based on an iterative solution process for finding the maximum of the cross-correlation coefficient in an n-dimensional parameter space. DIC has an advantage when locally varying material properties are being measured, as the modulus and yield strength can be measured in a single tension test. Examples of use are given. For the covering abstract see ITRD E126782.

Optical methods for the measurement of MEMS materials and structures

Abstract: In this paper, optical methods for the surface profile and deformation measurement of MEMS materials and structures are introduced. The methods are based on the digital image correlation, interferometry and fringe projection. The deformations of a micro-mirror and a micro-beam are measured by phase shifting fringe projection and interferometry. The surface profile of an electrode pad was used for the demonstration of the microscopic surface contouring. For the MEMS material test, using a long working distance microscope and digital speckle correlation method the tensile strain of a thin copper wire is measured.