Showing papers on "Digital image correlation published in 2003"

In-plane strain measurement by digital image correlation

Abstract: This paper presents a "fast and simple" (FAS) detection algorithm based on the digital image correlation for measurement of the surface deformation of planar objects. The proposed algorithm uses only fine search at the pixel level resolution and surface fitting for sub-pixel level. Two different specimens are investigated to explore the feasibility of this proposed algorithm. The displacements calculated by the FAS algorithm are compared with the ones obtained from Newton-Raphson method (N-R) and Enhanced Sequential Similarity Detection Algorithm (ESSDA). The results show that the experimental data are in good agreement with the theoretical solution. The proposed algorithm is found to be much faster than Newton-Raphson method with inferior, yet reasonable, accuracy for displacement and strain evaluation in the cases of uniaxial tension and disk under diametrical compression tests.

Three-dimensional optical tomography: resolution in small-object imaging.

Abstract: Near-infrared (NIR) optical tomography can provide estimates of the internal distribution of optical absorption and transport scattering from boundary measurements of light propagation within biological tissue. Although this is a truly three-dimensional (3D) imaging problem, most research to date has concentrated on two-dimensional modeling and image reconstruction. More recently, 3D imaging algorithms are demonstrating better estimation of the light propagation within the imaging region and are providing the basis of more accurate image reconstruction algorithms. As 3D methods emerge, it will become increasingly important to evaluate their resolution, contrast, and localization of optical property heterogeneity. We present a concise study of 3D reconstructed resolution of a small, low-contrast, absorbing and scattering anomaly as it is placed in different locations within a cylindrical phantom. The object is an 8-mm-diameter cylinder, which represents a typical small target that needs to be resolved in NIR mammographic imaging. The best resolution and contrast is observed when the object is located near the periphery of the imaging region (12–22 mm from the edge) and is also positioned within the multiple measurement planes, with the most accurate results seen for the scatter image when the anomaly is at 17 mm from the edge. Furthermore, the accuracy of quantitative imaging is increased to almost 100% of the target values when a priori information regarding the internal structure of imaging domain is utilized.

Two-photon image correlation spectroscopy and image cross-correlation spectroscopy

Abstract: We introduce two-photon image correlation spectroscopy (ICS) using a video rate capable multiphoton microscope. We demonstrate how video rate two-photon microscopic imaging and image correlation analysis may be combined to measure molecular transport properties over ranges typical of biomolecules in membrane environments. Using two-photon ICS, we measured diffusion coefficients as large as 10(-8) cm2 s(-1) that matched theoretical predictions for samples of fluorescent microspheres suspended in aqueous sucrose solutions. We also show the sensitivity of the method for measuring microscopic flow using analogous test samples. We demonstrate explicitly the advantages of the image correlation approach for measurement of correlation functions with high signal-to-noise in relatively short time periods and discuss situations when these methods represent improvements over non-imaging fluorescence correlation spectroscopy. We present the first demonstration of two-photon image cross-correlation spectroscopy where we simultaneously excite (via two-photon absorption) non-identical fluorophores with a single pulsed laser. We also demonstrate cellular application of two-photon ICS for measurements of slow diffusion of green fluorescent protein/adhesion receptor constructs within the basal membrane of live CHO fibroblast cells.

Simulation of the global response of a friction stir weld using local constitutive behavior

Abstract: The local and global mechanical responses of friction stir welded AA-2024 were studied experimentally and modeled using the finite element technique. A technique employing tensile testing and digital image correlation was used to obtain the full field strain distribution in transversely loaded welds, as well as the local constitutive behavior of the various weld regions. Two-dimensional, plane stress and plane strain, finite element models of the welded specimen were constructed using the local constitutive properties as input data and the local and global responses in tension were simulated. Nearly plane stress conditions were found to exist in the specimen as demonstrated by the correspondence between experimental results and the predictions of the 2-D models. These findings were further corroborated by use of a 3-D finite element model. The 3-D model was also used to examine the constraint developed in specimens of various widths.

Accuracy of Digital Image Correlation for Measuring Deformations in Transparent Media

Abstract: Experimental models to measure spatial deformation patterns within a soil mass are typically limited by the fact that soil sensors do not provide a continuous image of the measured continuum. Additionally, soil sensors exhibit static and dynamic characteristics that are different from those of the surrounding soils and therefore can change the response of the measured continuum. The fundamental premise of this research is that transparent synthetic soil surrogates can be used to overcome these difficulties using digital image correlation (DIC). A system consisting of a laser source and a line-generating lens was used to optically slice the transparent synthetic soil models. A digital camera was used to capture images of the slices before and after deformation. This paper presents a new technique for quantifying spatial deformation throughout transparent synthetic soil models using DIC. The accuracy of the DIC technique was evaluated based on a scheme of predefined digital movement of synthetic soil images. Finally, a model consisting of a strip footing on a synthetic transparent soil is presented. The spatial deformations in the model are evaluated using the proposed DIC methodology and compared with the result of finite-element analysis.

Deformations in wide, center-notched, thin panels, part I: three-dimensional shape and deformation measurements by computer vision

Abstract: The response of wide, thin, center-notched, 2024-T3 aluminum panels undergoing far-field tensile load is investigated. Three panels with a notch length to panel width of 0.33 and widths of 305, 610, and 1016 mm were subjected to far-field tensile loading. As part of the experimental program, two pairs of cameras were configured into separate stereovision systems and used to simultaneously capture both the global response of the sheet and the local response near a notch tip. Global areas, ranging in size from 250x 250 mm to 550x 550 mm, were imaged for each panel. A second stereovison system recorded images of a small area, 10×20 mm, ahead of one notch tip. Postprocessing of the stereovision measurement data from global and local systems using three-dimensional digital image correlation was used to obtain the complete displacement field at each point in the region of interest. In general, results demonstrate that the combination of stereovision and three-dimensional digital image correlation is capable of accurately measuring true, three-dimensional structural deformations in regions undergoing both large out-of-plane displacements and large displacement gradients. Furthermore, 3-D measurements on the panel specimen near the grip location are shown to provide an independent assessment of the true boundary conditions, with specimen slippage clearly noted in the 1016-mm specimen. Results from the extensive notched, wide panel experimental program demonstrate that (a) each panel has an initial shape that deviates up to 3 mm from planarity, with the greatest deviations occurring at the center of the notch, (b) the global load-displacement response is essentially linear for load levels that are well beyond the onset of large, out-of-plane displacements in the notch region, and (c) increasing the size of the notched, thin panel specimen results in distinctly different surface deformations and deformed shapes, with three separate maxima/minima in the out-of-plane component of the largest panel. The region where tensile opening strains are above 2% extends several millimeters ahead of the hole, while compressive strains parallel to the notch direction are contained within a few millimeters of the hole. The in-plane shear strains are concentrated along circular lobes at +/-45 deg from the horizontal direction, a trend which is generally consistent with plane stress conditions.

Imaging apparatus and image recovery method

Abstract: PROBLEM TO BE SOLVED: To recover an image to the frequency characteristics of the original image through a filter process using an inverse function, by previously defining the inverse function of the amplitude transfer function according to the change, even when the amount of zooming, the amount of defocusing and the amount of diaphragming are changed. SOLUTION: In the imaging apparatus, an image is imaged after passing an optical system by an imaging means. In an imaging process of an image A photographed by an image processing means, an image B obtained by the imaging means is so treated that the image after passing through the optical system is restored to an image 8, having frequency characteristics of before passing through the optical system through an MTF filter 5 as the inverse function of the transfer function, according to an optical adjustment amount to the optical system. COPYRIGHT: (C)2005,JPO&NCIPI

Accurate Displacement Measurement by Correlation of Colored Random Patterns

Abstract: The use of colored random patterns for the calculation of the correlation coefficient is proposed to improve the accuracy of the digital image correlation technique for displacement measurements. The colored random pattern is created by spraying color paints on the surface of a sample. Then, not only the displacement distributions but the displacement gradients are determined using a set of color images before and after deformation. The effectiveness is demonstrated by applying the proposed method to the displacement measurement of rigid body rotation, and the results are compared with those obtained from the monochromatic images. The results show that the use of the color image in digital image correlation improves the measurement accuracy both of the displacements and the displacement gradients. In addition, the smaller size subset can be used for the calculation of the correlation. The proposed technique is effective in the fields of experimental mechanics and experimental stress analysis.

Full-field structural response of composite structures: analysis and experiment

Abstract: Several applications of a three-dimensional digital image correlation system to the investigation of the static response of composite structures tested and analyzed at NASA Langley Research Center are presented. The digital image correlation system that was used to measure full-field three-dimensional displacement components over global and/or local areas of the structure is briefly described. Representative results obtained from several examples are then presented. These examples include the compression response of a notched composite panel, the compression response of an anisotropic composite panel with and without impact damage, the geometric shape measurement and compression response of a composite cylinder with a reinforced cutout, and the structural response of a shear-loaded stiffened composite panel. Results that illustrate recent enhancements to the digital image correlation system are also presented. These enhancements include an improved capability to compute in-plane strains from the full-field displacements and the capability to measure the surface profile of closed structures, such as cylinders. Experimental and analytical results are presented and compared. The results illustrate the benefits of obtaining full-field displacement and strain measurements in validating analytical models and in understanding the structural response of the test specimens.

A new digital image correlation algorithm for whole-field displacement measurement

Abstract: We have developed a new digital image correlation (DIC) algorithm for non-contact, two-dimensional, whole-field displacement and strain measurement. Relative to existing algorithms, our algorithm substantially reduces the calculation expense by using neighborhood information while processing the data to determine the displacement field in a sub-region of interest. The new algorithm also uses higher-order interpolations of the displacement field, allowing for better accuracy in estimating strain distributions when the deformation field is non-homogeneous. Numerically-generated digital images are used to show that the new algorithm accurately reproduces the imposed displacement fields. The algorithm is also tested on actual images from deformed specimens from a variety of experiments, and shown to perform satisfactorily. Keywords— A. Experimental Mechanics B. Non-contact strain measurement. C. Mechanical testing

High fidelity imaging in electrical impedance tomography

Abstract: This thesis addresses the computational reconstruction of images using Electrical Impedance Tomography (EIT). EIT is an imaging method, in which electrical currents are injected through electrodes into a conducting volume and the resulting potential distribution is measured at surface electrodes. From these potentials, an image of the electrical conductivity can be obtained using numerical reconstruction techniques. This non-linear reconstruction is mathematically difficult and computationally intensive. Most applications in medicine and industry rely upon a fast and accurate image acquisition. The aim of this investigation is to find methods which improve the speed and accuracy of EIT by a range of improvements to the numerical methods used in the forward solution and inverse reconstruction. We investigate the impact of the finite element discretization on the performance of computing the electric field forward solution. We derive an a posteriori error estimate on the finite element mesh and implement 2D adaptive mesh refinement techniques in an optimised forward solver. Our results of this novel approach show that a speed-up of approximately an order of magnitude can be obtained. We extend the developed iterative Newton-Raphson algorithm to include image smoothness constraints and adaptive mesh refinement based on conductivity gradients in the image. The results show that the image resolution can be made independent of the underlying numerical discretization and therefore is limited only by the level of noise present in the measurements. An additional benefit of this new technique is the automatic focus of available computational resources on key regions for forward solution and inverse reconstruction. As 3D impedance imaging becomes computationally too expensive for the Newton-Raphson method, we develop a novel non-linear conjugate gradient algorithm incorporating 3D adaptive mesh refinement routines, and present results showing the decrease of memory requirements and the increase in image reconstruction performance. In addition, a Matlab software package containing optimised routines for the finite element-based computations in EIT has been developed as part of this work. Finally, we outline a method for obtaining a map for the determination of the reconstruction reliability and image correlation of an EIT algorithm. With the improvements to reconstruction accuracy and speed investigated in this thesis, we conclude that efficient non-linear 3D impedance imaging is feasible

Optical flow estimation in omnidirectional images using wavelet approach

Abstract: The motion estimation computation in the image sequences is a significant problem in image processing. Many researches were carried out on this subject in the image sequences with a traditional camera. These techniques were applied in omnidirectional image sequences. But the majority of these methods are not adapted to this kind of sequences. Indeed they suppose the flow is locally constant but the omnidirectional sensor generates distortions which contradict this assumption. In this paper, we propose a fast method to compute the optical flow in omnidirectional image sequences. This method is based on a Brightness Change Constraint Equation decomposition on a wavelet basis. To take account of the distortions created by the sensor, we replace the assumption of flow locally constant used in traditional images by a hypothesis more appropriate.

Image registration based on Hough transform and phase correlation

Abstract: In this paper, Hough transform and phase correlation are combined to solve the problem of image registration. Based on 1D phase correlation, a method is proposed to obtain the angle of rotation by searching a angle ridge. After de-rotation, the traditional 2D phase correlation is imported to get the translation. The method can take full advantage of the high efficiency of FFT. Experiments show that the proposed method can detect the rotation and translation between images efficiently and exactly.

Evaluating microdefect structures by AFM-based deformation measurement

Abstract: The rapid development of a wide variety of new devises in microelectronics, MEMS, NEMS and nano technology will lead to new challenges for their mechanical characterization and reliability assessment. Measurement of deformations and stresses in microscopic and even nanoscopic regions becomes a key issue. The authors make use of load state images captured by Atomic Force Microscopes (AFM) in order to measure object deformations. Out-of-plane deformation is determined from usual topography scans by computing surface profile differences. NanoDAC, a recently established approach, allows to meet these goals with regard to in-plane deformation. The method bases on cross correlations analysis performed on AFM scans, which are captured from thermally and/or mechanically loaded samples. Finally, local 3D displacement fields and in-plane strain fields are measured. A description of the basic principles and the capability of the technique are given. Furthermore, the authors demonstrate the potential of the mentioned method by its application to microcrack evaluation and the study of sensor and MEMS structure degradation. The first application corresponds to the measurement of crack opening displacement in the very vicinity of crack tips. As a consequence, fracture mechanics parameters are derived and allow to assess the defect with regard to possible crack propagation and component failure. This approach is used to study the influence of nanoscale material structures on crack behavior. The second example illustrates how the impact of thermal loading to the constitution of sensor or MEMS submicron layers is investigated by deformation analysis. The devices had been heated actively under the AFM. Degradation processes due to a severe thermal material mismatch were observed and monitored.

Nanoscale Displacement and Strain Measurement

Abstract: The mechanical testing of micro-electro-mechanical systems (MEMS) and nano-electro-mechanical systems (NEMS) requires precise measurements of displacement and strain in the nanometer scale. Integrated testing devices, including specimens, actuators and sensors, were designed and fabricated to overcome the aligning and gripping difficulties faced by previous separated micromechanical testing apparatuses. The specimens were “dog-bone” shaped and were deformed in-plane by electrothermal actuators based on bent-beam suspensions. The displacement field was obtained by comparing surface topologies, acquired with an Atomic Force Microscope (AFM), of undeformed and deformed specimens using the Digital Image Correlation (DIC) method. The change in capacitance of the integrated capacitive sensors was measured to determine the total displacement. The behavior of the whole device was simulated by ANSYS multiphysics, which agreed well with the measured results.

Deformation characteristics of tin-based solder joints

Abstract: A novel experimental configuration was devised to measure the evolution of the deformation field and the corresponding toughness in solder joints for microelectronic packaging. The utilized material system comprised a ductile layer of tin-based solder encapsulated within relatively hard copper shoulders. The experimental configuration provided pure shear state within the constrained solder layer. Different Pb/Sn compositions were tested with grain size approaching the film thickness. The in-plane strain distribution within the joint thickness was measured by a microscopic digital image correlation system. The toughness evolution within such highly gradient deformation field was monitored qualitatively through a two-dimensional surface scan with a nanoindentor. The measurements showed a highly inhomogeneous deformation field within the film with discreet shear bands of concentrated strain. The localized shear bands showed long-range correlations of the order 2–3 grain diameters. A size-dependent macroscopic response on the layer thickness was observed. However, the corresponding film thickness was approximately 100–1000 times larger than those predicted by nonlocal continuum theories and discreet dislocation.

Application of digital speckle correlation to micro-deformation measurement of a flip chip assembly

Abstract: A digital speckle image correlation system was developed to measure the in-situ micro-deformation of the solder joints in a flip chip on board (FCOB) assembly kubject to thermal cycling. The digital images acquired from deformed solder joint images captured at higher temperatures were correlated to the undeformed base image using a correlation algorithm. The deformation distributions in the solder joint were determined. A fmite element model was established to simulate the deformation of the assembly. The solder material constitutive model is viscoplastic and underfill material model is viscoelastic. The measurement results by correlation were then compared with the fmite element simulation results. It was found that U and V displacement fields of the solder joint obtained from the experiment and simulation agree well with each other. the strain fields are comparable.

Digital shearing method for three-dimensional data extraction in holographic particle image velocimetry

Abstract: We report a new digital shearing method for extracting the three-dimensional displacement vector data from double-exposure holograms. With this method we can manipulate both the phase and the amplitude of the recorded signal, which, like optical correlation analysis, is inherently immune to imaging aberration. However, digital shearing is not a direct digital implementation of optical correlation, and a considerable saving in computation time results. We demonstrate the power of the method by MATLAB simulation and discuss its performance with reference to optical analysis.

Speckle correlation methods applied to ballistics and explosives

Abstract: Image correlation techniques developed for speckle metrology have recently been applied to a wide range of ballistics and explosives studies at the Cavendish Laboratory. White-light image correlation applied to quasi-static testing of explosives, at a range of magnifications, is discussed. High-speed ballistic measurements were performed using Digital Speckle Radiography (DSR): a combination of iamge correlation with flash X-rays to study the movement of a plane of X-ray opaque filings within a sample. DSR operates by capturing one image before and a second during the event, allowing the displacements on the plane to be measured to sub-mm accuracy. Varying the delay time and the depth of the seeded plane gives a full three-dimensional flow field in the sample. The use of two X-ray heads stereoscopically increases accuracy by allowing all three components of displacement to be determined. Measurements at velocities ranging from 100 m s-1 to 6 km s-1 have been achieved using this technique.© (2003) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Measurement of Deformation of Epoxy Resin Plates with an Embedded SMA Wire Using Digital Image Correlation

Abstract: The deformation of the surface of epoxy resin plates with an embedded linear shape-memory alloy (SMA) wire as an actuator in smart structure members is measured as a function of the duration of the supply of electric current to the SMA wire, using a digital image correlation technique. Results show that the two-dimensional deformation of the plate surface can be measured, and that the maximum deformation of the plate surface can be obtained on both sides of a crack and increases with the duration of the current supply. Therefore, the deformation of the plate surface measured using the digital image correlation technique suggests the possibility of its application in the prediction of crack closure in epoxy resin plates with embedded SMA wires heated by supplying electric current.

Analysis of Strain Distribution of Softwood in Transverse Compression Measured by Digital Image Correlation Method

Abstract: Strain distributions on a cross section of softwood under transverse compression were analyzed in detail using digital image correlation method. We discussed the relationship between compression strain distributions in an annual ring and anatomical structure (earlywood and latewood). In radial compression, local large shear strains were observed in the edge of earlywood near annual ring boundary where mechanical properties change abruptly along radial direction. In a plastic region of stress-strain curve we found a row of large compression strain that is thought to be cell-wall collapse in earlywood. A start point of the row of large compression strain was the edge of earlywood in which the large shear strain were observed, because the radial wall of sheared cell are easily buckled. In the case of 45 degree compression, large compression strain along loading direction was observed in earlywood. Large expansion strain across the loading direction was also observed in the same area because of Poisson's ratio effect. As the strains were transformed to radial-tangential coordinate system, large shear strain and little normal strain were observed in early wood. The shear strain under 45 degree compression was larger than that under radial compression. The former is the shear strain that is usually called rolling shear.

DIC for Deformation Assessment: a Case Study

Abstract: The aim of this paper is to present in detail case study, where the deformation of a complex connection system of a modular bridge are measured by means of the digital image correlation (DIC) technique, in order to study the displacements in the joint. The connection system is composed of metallic and composite parts. The paper shows that DIC is a complementary tool of the conventional measurement systems such as LVDT and strain gages. The use of strain gages and LVDT only allows to suggest where the major part of the deformation is located, but does not allow to understand the origin of the deformations. The DIC analysis permits to locate and to quantify the different sources of deformation, which may lead to the improvement of the design and the assembly procedure of the connection. The paper also points out the set-up procedure for the use of the CCD camera, and the precautions which must be taken concerning the contrast pattern to place on the specimen, the light conditions, the calibration procedure and the magnification factor, which must be used especially in this case where different planes are measured.