Showing papers on "Digital image correlation published in 2006"

Digital Image Correlation: from Displacement Measurement to Identification of Elastic Properties – a Review

Abstract: The current development of digital image correlation, whose displacement uncertainty is well below the pixel value, enables one to better characterise the behaviour of materials and the response of structures to external loads. A general presentation of the extraction of displacement fields from the knowledge of pictures taken at different instants of an experiment is given. Different strategies can be followed to achieve a sub-pixel uncertainty. From these measurements, new identification procedures are devised making use of full-field measures. A priori or a posteriori routes can be followed. They are illustrated on the analysis of a Brazilian test.

"Finite-Element" Displacement Fields Analysis from Digital Images: Application to Portevin-Le Châtelier Bands

Abstract: A new methodology is proposed to estimate displacement fields from pairs of images (reference and strained) that evaluates continuous displacement fields. This approach is specialized to a finite-element decomposition, therefore providing a natural interface with a numerical modeling of the mechanical behavior used for identification purposes. The method is illustrated with the analysis of Portevin–Le Châtelier bands in an aluminum alloy sample subjected to a tensile test. A significant progress with respect to classical digital image correlation techniques is observed in terms of spatial resolution and uncertainty.

Performance of sub-pixel registration algorithms in digital image correlation

Abstract: Developments in digital image correlation in the last two decades have made it a popular and effective tool for full-field displacement and strain measurements in experimental mechanics In digital image correlation, the use of the sub-pixel registration algorithm is regarded as the key technique to improve accuracy Different types of sub-pixel registration algorithms have been developed However, little quantitative research has been carried out to compare their performances This paper investigates three types of the most commonly used sub-pixel displacement registration algorithms in terms of the registration accuracy and the computational efficiency using computer-simulated speckle images A detailed examination of the performances of each algorithm reveals that the iterative spatial domain cross-correlation algorithm (Newton–Raphson method) is more accurate, but much slower than other algorithms, and is recommended for use in these applications

Stress intensity factor measurements from digital image correlation: post-processing and integrated approaches

Abstract: Digital image correlation is an appealing technique for studying crack propagation in brittle materials such as ceramics. A case study is discussed where the crack geometry, and the crack opening displacement are evaluated from image correlation by following two different measurement and identification routes. The displacement uncertainty can reach the nanometer range even though optical pictures are dealt with. The stress intensity factor is estimated with a 7% uncertainty in a complex loading set-up without having to resort to a numerical modelling of the experiment.

Digital image correlation: performance and potential application in photogrammetry

Abstract: A procedure for digital image correlation is described which is based on least squares window matching. The immediate aim is high precision parallax assessment, point transfer, and point measurement. Experiments and theory have confirmed the high accuracy potential of the method. By implementation of charge coupled device (CCD) video cameras in an analytical plotter, an experimental hardware and software configuration has been established with which the operational on line application of digital image correlation for conventional photogrammetric measuring tasks can be tested. First results of calibration and performance of the system are presented. They allow optimistic conclusions as to the further development and practical application of digital image processing in photogrammetry.

Automatic Evaluation of Mixed‐mode Stress Intensity Factors Utilizing Digital Image Correlation

Abstract: A method for determining mixed-mode stress intensity factors from displacement fields obtained by digital image correlation is studied. To perform automatic evaluation, not only stress intensity factors but crack-tip location, higher-order terms in the series expansion of displacement fields and rigid-body displacement components are also determined simultaneously using nonlinear least squares based on the Newton–Raphson method. Experimental results show that the mixed-mode stress intensity factors are evaluated accurately from either radial or circumferential displacement components. As mixed-mode stress intensity factors can be evaluated easily, simply and automatically by the technique with digital image correlation and nonlinear least squares, it is expected that the proposed method can be applied to solve various fracture problems.

Grain-scale processes governing shear band initiation and evolution in sands

Abstract: A variety of experimental techniques have been used to advance understanding of strain localization phenomena in sands. However, all of these methods have fallen short in characterizing the evolution of the grain-scale processes that necessarily control shear band formation and growth in sands. This paper presents results of application of the non-destructive displacement measurement technique of digital image correlation (DIC) to measure two- and three-dimensional surface displacements on plane strain and axisymmetric sand specimens over short time steps. The abundance of local displacement data, high level of accuracy, and nearly continuous (spatially and temporally) record of displacement evolution afforded by the DIC technique has finally enabled a means to quantify local displacements to particulate-scale intensity. The data have been used to evaluate the local displacement mechanisms leading to the triggering of the formation of persistent shear bands, the timing of shear band formation with regard to the achievement of peak stress, and the character of displacements within fully formed shear bands. Insights are offered regarding the relation between strain localization and global stress–strain behavior, and the ensuing interpretations of shear banding as a hardening or softening phenomenon. Comparison of behavior between plane strain and triaxial tests offer additional perspective on the influences of three-dimensional stresses and boundary conditions on shear banding. The results further shed light on the micro-deformation mechanisms (i.e. buckling columns) responsible for the observed local strain non-uniformities that characterize “steady-state” shear band evolution.

Metrology in a scanning electron microscope: theoretical developments and experimental validation

Abstract: A novel approach for correcting both spatial and drift distortions that are present in scanning electron microscope (SEM) images is described. Spatial distortion removal is performed using a methodology that employs a series of in-plane rigid body motions and a generated warping function. Drift distortion removal is performed using multiple, time-spaced images to extract the time-varying relative displacement field throughout the experiment. Results from numerical simulations clearly demonstrate that the correction procedures successfully remove both spatial and drift distortions. Specifically, in the absence of intensity noise the distortion removal methods consistently give excellent results with errors on the order of +/- 0.01 pixels. Results from the rigid body motion and tensile loading experiments at 200 x indicate that, after correction for distortions, (a) the displacements have nearly random variability with a standard deviation of 0.02 pixels; (b) the measured strain fields are unbiased and in excellent agreement with previous full-field experimental data obtained with optical illumination; (c) the strain field variability is on the order of 60 microstrain in all components with a spatial resolution on the order of 25 pixels. Taken together, the analytical, computational and experimental studies clearly show that the correction procedures successfully remove both spatial and drift distortions while retaining excellent spatial resolution, confirming that the SEM-based method can be used for both micromaterial and nanomaterial characterization in either the elastic or elastic-plastic deformation regimes.

Lens distortion correction for digital image correlation by measuring rigid body displacement

Abstract: A method of lens distortion correction is proposed in order to improve the measurement accuracy of digital image correlation for two-dimensional displacement measurement. The amounts of lens distortion are evaluated from displacement distributions obtained in a rigid body in-plane translation or rotation test. After detecting the lens distortion, its coefficient is determined using the method of least squares. Then, the corrected displacement distributions are obtained. The effectiveness of the proposed method is demonstrated by applying the correction method to an in-plane translation test and tension tests. The experimental results show that the proposed distortion correction method eliminates the effect of lens distortion from measured displacements.

Comparative study on local and global mechanical properties of 2024 T351, 2024 T6 and 5251 O friction stir welds

Abstract: The effect of the microstructure heterogeneity on the global and local tensile properties of friction stir welded joints in 5251 (O temper) and AA2024 (T351 and T6 tempers) aluminium alloys has been investigated. Micro-tensile tests parallel to the welding direction have been carried out in the regions representative of the main microstructural zones. The digital image correlation technique (DIC) has been used during transverse tensile tests for mapping the strain distribution and to determine the local stress–strain curves. A 3-D finite elements model has been developed to predict the weld behaviour from the tensile curves of the individual regions of the weld. The tensile properties of the 5251 O weld are relatively homogeneous leading to high ductility and fracture in the base material. In contrast, the tensile properties of the various regions of the 2024 T351 and 2024 T6 welds are very heterogeneous and essentially controlled by the state of precipitation. The thermo-mechanically affected zone is the weakest region where the strain localises during a transverse tensile test. The 2024 T6 base material is stronger than the 2024 T351 alloy, leading to a more pronounced strain localisation during transverse tensile tests and a lower overall ductility. Local tensile data obtained by strain mapping are in good agreement with the curves obtained by micro-tensile tests, and these results can be safely used in a finite elements model to predict the behaviour of the overall weld assembly.

Method and apparatus for measuring motion of a subject using a series of partial images from an imaging system

Abstract: A line scan imager is used to determine the motion of a subject. Each line of image data from the line scan imager is compared with a reference image. The location of a matching line in the reference image reveals the displacement of the subject. The current subject displacement can be determined based on each line of image data. The resulting displacement information can be used to correctly place other optical beams on the subject. The method can be applied to tracking the human eye to facilitate measurement, imaging, or treatment with a beam of optical radiation.

Displacement/strain measurements using an optical microscope and digital image correlation

Abstract: We conduct displacement/strain measurements on the micro- scale using light microscopy and digital image correlation DIC. Errors in the measurements attributed to the optical arrangement and aberration induced at high magnification are identified using a warping function. Coefficients of the warping function are determined using a simple tech- nique that employs a precisely made orthogonal cross-grating plate. By acquiring images of the grating and identifying the nodes using subpixel techniques, a relationship between the object and the image planes is established. Thus, the displacement/strain derived by means of DIC is corrected by converting the displacement components in the image plane to the coordinate system existing on the object's surface. The approach is validated through a determination of the elastic properties of common metals; errors in estimation of the elastic modulus were within 4%. Although surface preparation generally plays a critical role in suc- cessful application of DIC, it is found to be of minimal importance under high magnification. Instead, the natural surface texture can be used with adjustment of the light incident angle. Results of the study show that DIC is a powerful tool in performing displacement/strain measurements on the microscale using a light microscope provided that an adequate cor- rection is employed for image distortion. © 2006 Society of Photo-Optical Instru-

Digital image correlation analysis of crack behavior in a reinforced concrete beam during a load test

Abstract: A displacement-measuring technique using digital image cross-correlation was applied to study the in situ behavior of a shear crack in a reinforced concrete beam during a bridge static load test. A...

Fluorescent Image Correlation for Nanoscale Deformation Measurements

Abstract: Recent advances in nanotechnology have enabled the fabrication of a new generation of materials with highly complex structures. The characteristic length scale of these materials has now outpaced the ability of current techniques to make full-field, nanoscale mechanical property measurements in real time. In addition, biological materials also possess complex structures at the nanoscale, which can affect their resulting larger-scale response. Measurements of bulk properties, while important, offer little information about how the nanostructure influences performance. Localized measurements, such as nanoindentation, provide data that are often difficult to extend to larger length scales. High-resolution imaging techniques such as atomic force or scanning tunneling microscopy (AFM/STM) and near-field scanning optical microscopy (NFSOM), despite being full-field techniques, involve bringing a tip on or very near the surface and thus are not entirely suitable for use with soft (e.g., biological) materials. In addition, these methods require rastering, which limits real-time visualization capability (e.g., of fracture, of the motion and growth of cells, and so on). Fluorescent dyes and particles have enabled a different set of experimental tools for imaging displacements. Fluorescent particles have been widely used in flow visualization and measurement techniques. In biological studies, fluorescent dyes indicate the presence of particular microorganisms or track the growth and development of cellular structures. Recent advances in optical techniques have improved discrete fluorescent particle imaging, making laboratory nanoscale measurements feasible. Single-particle tracking (SPT) via confocal laser scanning microscopy is a tool used in biophysical research to observe trajectories of small fluorescent particles with nanometer-scale precision. Here, we present a powerful full-field technique, fluorescencebased digital image correlation (FDIC), to measure nanoscale deformation in materials using fluorescent nanoparticles. To demonstrate the capabilities of the method, we characterize the complex deformation fields generated around silica microspheres embedded in an elastomer under tensile loading. Displacement resolutions of 20 nm are obtained over a 100:100 mm field of view. Digital image correlation (DIC) is a data analysis method, which applies a mathematical correlation algorithm to obtain kinematic information from digital images acquired during deformation. For conventional two-dimensional (2D) DIC, samples are prepared for testing by the application of a random speckle pattern to their surface. Comparison of successive images reveals a deformed speckle pattern relative to the initial, nondeformed one. The correlation works by matching small square subsets of the nondeformed image (Figure 1a) to locations in the deformed image (Figure 1b). The core of the DIC method lies in the optimization of a correlation coefficient between the two subsets over six parameters characterizing the in-plane deformation; namely the displacements components u and v, and the displacement gradients @u/@x, @u/@y, @v/@x, and @v/@y. The accuracy of the method is approximately 0.1 pixels in displacement. A novel aspect of the current work is that fluorescent nanoparticles are used to create the speckle pattern, enabling the characterization of any material to which the particle pattern can be applied. Fluorescent silica nanoparticles were synthesized and labeled with a rhodamine fluorescent dye (555-nm peak excitation wavelength) according to the procedure outlined by van Blaaderen and Vrij, resulting in a 0.1% solution (by volume) of nanoparticles in ethanol. The average particle diameter was 180 nm (Figure 1c). The fluorescent particle solution was spin-cast onto samples of polydimethylsiloxane (PDMS), which had been allowed to fully cure at room temperature. Prior to deposition, all PDMS samples were subjected to a 10-min ultraviolet (UV) surface treatment to enhance particle adhesion. Following deposition and evaporation of the ethanol solution, a single layer of particles remained adhered to the sample surface. The spin-casting rate was varied from 500 to 2500 rpm to produce different pattern densities. The lower spinning rates resulted in patterns with too many agglomerated particles and little dispersion, while the higher spinning rates produced completely uniform patterns with no contrast. A spin[*] Prof. N. R. Sottos Beckman Institute for Advanced Science and Technology Department of Theoretical and Applied Mechanics 104 S. Wright Street, University of Illinois Urbana-Champaign Urbana, IL 61801 (USA) Fax: (+1)217-244-5707 E-mail: n-sottos@uiuc.edu

Full-field characterization of mechanical behavior of polyurethane foams.

Abstract: The foam material of interest in this investigation is a rigid closed-cell polyurethane foam PMDI with a nominal density of 20 pcf (320 kg/m{sup 3}). Three separate types of compression experiments were conducted on foam specimens. The heterogeneous deformation of foam specimens and strain concentration at the foam-steel interface were obtained using the 3-dimensional digital image correlation (3D-DIC) technique. These experiments demonstrated that the 3D-DIC technique is able to obtain accurate and full-field large deformation of foam specimens, including strain concentrations. The experiments also showed the effects of loading configurations on deformation and strain concentration in foam specimens. These DIC results provided experimental data to validate the previously developed viscoplastic foam model (VFM). In the first experiment, cubic foam specimens were compressed uniaxially up to 60%. The full-field surface displacement and strain distributions obtained using the 3D-DIC technique provided detailed information about the inhomogeneous deformation over the area of interest during compression. In the second experiment, compression tests were conducted for cubic foam specimens with a steel cylinder inclusion, which imitate the deformation of foam components in a package under crush conditions. The strain concentration at the interface between the steel cylinder and the foam specimen was studied in detail.more » In the third experiment, the foam specimens were loaded by a steel cylinder passing through the center of the specimens rather than from its end surface, which created a loading condition of the foam components similar to a package that has been dropped. To study the effects of confinement, the strain concentration and displacement distribution over the defined sections were compared for cases with and without a confinement fixture.« less

The Compressive Behavior of Isocyanate-crosslinked Silica Aerogel at High Strain Rates

Abstract: Aerogels are low-density, highly nano-porous materials. Their engineering applications are limited due to their brittleness and hydrophilicity. Recently, a strong lightweight crosslinked silica aerogel has been developed by encapsulating the skeletal framework of amine-modified silica aerogels with polyureas derived by isocyanate. The mesoporous structure of the underlying silica framework is preserved through conformal polymer coating, and the thermal conductivity remains low. Characterization has been conducted on the thermal, physical properties and the mechanical properties under quasi-static loading conditions. In this paper, we present results on the dynamic compressive behavior of the crosslinked silica aerogel (CSA) using a split Hopkinson pressure bar (SHPB). A new tubing pulse shaper was employed to help reach the dynamic stress equilibrium and constant strain rate. The stress-strain relationship was determined at high strain rates within 114–4386 s−1. The effects of strain rate, density, specimen thickness and water absorption on the dynamic behavior of the CSA were investigated through a series of dynamic experiments. The Young’s moduli (or 0.2% offset compressive yield strengths) at a strain rate ∼350 s−1 were determined as 10.96/2.08, 159.5/6.75, 192.2/7.68, 304.6/11.46, 407.0/20.91 and 640.5/30.47 MPa for CSA with densities 0.205, 0.454, 0.492, 0.551, 0.628 and 0.731 g cm−3, respectively. The deformation and failure behaviors of a native silica aerogel with density (0.472 g cm−3), approximately the same as a typical CSA sample were observed with a high speed digital camera. Digital image correlation technique was used to determine the surface strains through a series of images acquired using high speed photography. The relative uniform axial deformation indicated that localized compaction did not occur at a compressive strain level of ∼17%, suggesting most likely failure mechanism at high strain rate to be different from that under quasi-static loading condition. The Poisson’s ratio was determined to be 0.162 in nonlinear regime under high strain rates. CSA samples failed generally by splitting, but were much more ductile than native silica aerogels.

Residual Stress Determination Using Hole Drilling and 3D Image Correlation

Abstract: In recent years, the hole drilling method for determining residual stresses has been implemented with optical methods such as holographic interferometry and ESPI to overcome certain limitations of the strain rosette version of hole drilling. Although offering advantages, the interferometric methods require vibration isolation, a significant drawback to their use outside of the laboratory. In this study, a 3D image correlation approach was used to measure micron-sized surface displacements caused by the localized stress relief associated with hole drilling. Residual stresses were then found from the displacements using non-dimensional relations previously derived by finite element analysis. A major advantage of image correlation is that it does not require interferometric vibration isolation. Experiments were performed to check the ability of this new approach for uniaxial and equi-biaxial states of stress. Stresses determined by the approach were in good agreement with computed values and those determined by hole drilling using holographic interferometry.

AFM image reconstruction for deformation measurements by digital image correlation.

Abstract: The scanner drift of the atomic force microscope (AFM) is a great disadvantage to the application of digital image correlation to micro/nano-scale deformation measurements. This paper has addressed the image distortion induced by the scanner drifts and developed a method to reconstruct AFM images for the successful use of AFM image correlation. It presents such a method, that is to generate a corrected image from two correlated AFM images scanned at the angle of 0° and 90° respectively. The proposed method has been validated by the zero-deformation test. A buckling test of a thin plate under AFM has also been demonstrated. The in-plane displacement field at the centre point of the buckling plate has been successfully characterized by the application of the image correlation technique on reconstructed AFM images.

In-Plane Displacement Measurement Using Digital Image Correlation with Lens Distortion Correction

Abstract: Two-dimensional displacement measurement using digital image correlation with lens distortion correction is described in this paper. A single cross-grating is used as a calibration reference. Using two-dimensional Fourier transform, the phases of the grating pattern are analyzed and lens distortion distribution is obtained from the unwrapped phase maps. After detecting lens distortion, the coefficients of lens distortion are determined using the least-squares method. Then, the displacement distributions without the lens distortion are obtained. The effectiveness of the method is demonstrated by applying the proposed method to the rigid body translation test and the uniaxial tension test. The results show that the proposed distortion correction method removes the effect of lens distortion from the measured displacements. By the proposed method, accurate measurements can be performed even if images are deformed by lens distortion.

Digital image correlation of nanoscale deformation fields for local stress measurement in thin films

Abstract: In this paper, the application of an in situ stress measurement technique to a silicon nitride thin film deposited onto a thick silicon substrate is presented. The method is based on the measurement of the displacement field originated when a slot is milled into the material under study. Displacements are obtained by digital correlation analysis of scanning electron microscope (SEM) images, whereas milling is performed by ion milling in focused ion-beam equipment. Due to the mechanical constraint introduced by the substrate and the small thickness of the tested layer, the displacements generated by the milling process are in the range 0?5?nm, which is one of the smallest displacement ranges measured up to now in a relaxation-based measurement technique. The local stress value determined with this new method is in good agreement with values obtained by a classical method like the wafer bending test.

The constitutive behavior of laser welds in 304L stainless steel determined by digital image correlation

Abstract: A digital image correlation (DIC) method has been used to characterize the constitutive tensile stress-strain response in 304L austenitic stainless steel weldments produced by both continuous-wave (CW) and pulsed-wave (PW) laser welding. The method provides quantitative two-dimensional (2-D) strain maps of the deformation field across the transverse weld samples throughout the tensile test. Local stress-strain response was extracted from regions within the fusion zone and compared to base metal response. The weldments were found to have a higher yield strength than the base metal. The metallurgical origin for the fusion zone strengthening was largely attributed to Hall-Petch and ferrite content effects. While failures localized in the fusion zone with little appreciable necking, the material within the fusion zone retained considerable local ductility: more than 45 pct strain at failure. Significant weld root porosity found in the PW condition and absent in the CW condition appeared to have no deleterious effect on the mechanical performance under the present test conditions in this very ductile, flaw-tolerant alloy.

Error estimations of 3D digital image correlation measurements

Abstract: Systematical errors of digital image correlation (DIC) measurements build a limiting factor for the accuracy of the resulting quantities. A major source for introducing systematical errors is the system calibration. We present a 3D digital image correlation system, which provides error information not only of diverse error sources but even more the propagation of errors throughout the calculations to the resulting contours, displacements and strains. On the basis of this system we discuss error sources, error propagation and the impact on correlation results. Performance tests for studying the impact of calibration errors on the resulting data are shown.