Showing papers on "Digital image correlation published in 1998"

Submicron deformation field measurements: Part 2. Improved digital image correlation

Abstract: This is the second paper in a series of three devoted to the application of scanning tunneling microscopy (STM) to mechanics problems. In this paper, improvements to the digital image correlation method are outlined, a technique that compares digital images of a specimen surface before and after deformation to deduce its two-dimensional surface displacement field and strains. The necessity of using the framework of large deformation theory for accurately addressing rigid body rotations to reduce associated errors in the strain components is pointed out. In addition, the algorithm is extended to compute the three-dimensional surface displacement field from STM data; also, significant improvements are achieved in the rate as well as the robustness of the convergence. For (STM) topographs, the resolution yields 4.8 nm for the in-plane and 1.5 nm for the out-of-plane displacement components spanning an area of 10 μm×10μm.

Error assessment for strain mapping by digital image correlation

Abstract: The accuracy and sensitivity of strain mapping by DIC have been experimentally evaluated for a sheet metal tensile specimen and the error level of local strains in terms of their standard deviations from the means is found to be around 100 μstrain for the imaging system and data reduction processes used. In general, as the error in DIC measurements are affected both by digitization and optics for a given DIC processing routine, it is suggested that a system error calibration procedure similar to the one described here should be carried out for each batch of specimens prior to testing. A series of tests on a stationary specimen with a typical contrast pattern should be first conducted to identify the minimum number of frames needed for averaging during the image acquisition to achieve the desired accuracy in displacements and strains for the given image formation system. Then translation tests along the three axes and in-plane rotation tests that simulate the motion of a specimen during testing should be carried out and errors in displacements and strains should be computed immediately. Depending on the nature of the error and its magnitude, optics realignment, optimization of surface contrast features, and increase of digital image pixel resolution may further be explored to reduce the system errors to an acceptable level for each specific application. The reliability and accuracy of the strain maps obtained in the subsequent testing can thus be ensured.

Submicron deformation field measurements: Part 1. Developing a digital scanning tunneling microscope

Abstract: A new experimental method has been developed for studying deformations of micromechanical material systems at the submicron scale. To that end, a special digital scanning tunneling microscope (STM) was designed to be coupled to a mechanically deforming specimen. Operating in constant current mode, this digitally controlled STM records detailed topographies of specimen surfaces with a resolution of 10 nm in-plane and 7 nm out-of-plane over a 10 μ × 10 μ area. Three-dimensional displacement field information is extracted by comparing topographies of the same specimen area before and after deformation by way of a modified digital image correlation algorithm. The resolution of this (combined) displacement measuring method was assessed on translation and uniaxial tensile tests to be 5 nm for in-plane displacement components and 1.5 nm for out-of-plane motion over the same area. This is the first paper in a series of three in which the authors delineate the main features of this specially designed microscope and describe how it is constituted, calibrated and used with the improved version of the digital image correlation method to determine deformations in a test specimen at the nanoscale.

High-Speed PIV Analysis Using Compressed Image Correlation

Abstract: With the development of Holographic PIV (HPIV) and PIV Cinematography (PIVC), the need for a computationally efficient algorithm capable of processing images at video rates has emerged. This paper presents one such algorithm, sparse array image correlation. This algorithm is based on the sparse format of image data - a format well suited to the storage of highly segmented images. It utilizes an image compression scheme that retains pixel values in high intensity gradient areas eliminating low information background regions. The remaining pixels are stored in sparse format along with their relative locations encoded into 32 bit words. The result is a highly reduced image data set that retains the original correlation information of the image. Compression ratios of 30:1 using this method are typical. As a result, far fewer memory calls and data entry comparisons are required to accurately determine tracer particle movement. In addition, by utilizing an error correlation function, pixel comparisons are made through single integer calculations eliminating time consuming multiplication and floating point arithmetic. Thus, this algorithm typically results in much higher correlation speeds and lower memory requirements than spectral and image shifting correlation algorithms. This paper describes the methodology of sparse array correlation as well as the speed, accuracy, and limitations of this unique algorithm. While the study presented here focuses on the process of correlating images stored in sparse format, the details of an image compression algorithm based on intensity gradient thresholding is presented and its effect on image correlation is discussed to elucidate the limitations and applicability of compression based PIV processing.

Submicron deformation field measurements: Part 3. Demonstration of deformation determinations

Abstract: This is the third and last paper is a sequence devoted to an experimental investigation of deformation mechanisms at the submicron scale through the use of a specially designed scanning tunneling microscope. Its application, when used jointly with digital image correlation, as a tool for strain and deformation determinations is explored by way of two demonstrations. First, deformations in a uniaxially stressed, unplasticized (poly)vinylchloride sample are analyzed to yield the three-dimensional surface displacement field over a 10 μm×10μm area. Homogeneous deformations occur at the micrometer and large size scales. However, at the 100-nm scale, inhomogeneous deformations embedded in a homogeneous deformation field appear. The second example addresses the deformation field in the vicinity of an interface between a carbon fiber and the surrounding matrix under shear stresses along the fiber. This loading leads to shearing a sheath from the carbon fiber that is about half a micron thick.

Strain inhomogeneity and discontinuous crack growth in a particulate composite

Abstract: A full field method for visualizing strain fields around a crack tip under large strains is developed. Digital image correlation is used to compute strains and displacements incrementally between consecutive images in a process of large deformations. Values of strain and displacements for these consecutive deformations are added such that convergence of the DIC algorithm is assured. The method is used to investigate the strain distribution in a globally homogeneously strained particulate composite (solid propellant) as well as in a zone close to (∼2 mm) the crack tip in such a material by using a microscope. It is found that maximal strain variations deviate by as much as a factor of three from the average strain ; additionally, observations on the interaction of strain inhomogeneities with the tip of a macroscopic crack are discussed.

Mechanical properties of thin polysilicon films by means of probe microscopy

Abstract: A new method for tensile testing of thin films is being developed. An electrostatic grip apparatus was designed and implemented to measure the elastic and ultimate tensile properties (Young's modulus, Poisson's ratio and tensile strength) of surface micromachined polysilicon specimens. The tensile specimens are 'dog-bone' shaped ending in a large 'paddle' for electrostatic gripping. The test section of the specimens is 400 micrometers long and with 2 micrometer X 50 micrometer cross section. The method employs Atomic Force Microscope (AFM) or Scanning Tunneling Microscope (STM) acquired surface topologies of deforming specimens to determine (fields of) strains. By way of the method of Digital Image Correlation (DIC), the natural surface roughness features are used as distributed markers. The effect of markers artificially deposited on the surface is examined computationally. Also the significance of other parameters on property measurements, such as surface roughness, has been examined computationally. Initial results obtained using the tensile test apparatus are presented.

Strain characterization of propagative deformation bands

Abstract: An experimental study on the formation and evolution of strain inhomogeneity in the form of deformation bands on the surface of polycrystalline sheet metals is presented. A whole field in-plane deformation measurement technique based on computer vision has been applied for in situ monitoring the development of plastic deformation patterns in annealed Al–5%Mg and recovered AA5182 aluminum sheet metals. The strain inhomogeneity over a rectangular area about 5 mm×3 mm on the sheet metal surfaces is found to increase steadily with increasing elongation during a uniaxial tensile test. Propagative deformation bands have been detected in both aluminum alloys by incremental strain mapping although no band-like deformation patterns can be identified by direct visual observation of Al–5%Mg specimen surfaces. The effect of plastic deformation patterns on surface finish, tensile ductility, and formability of sheet metals are elucidated. The approach and results of this experimental investigation can be valuable to physicallybased multiscale plasticity modeling of sheet metal forming.

Person tracking by integrating optical flow and uniform brightness regions

Abstract: This paper describes a method to track a person by integrating two cues, optical flow and uniform brightness regions, where optical flow cannot be obtained. This method works even if tracking with either optical flow or uniform brightness regions may fail. The proposed method has been implemented on a real time image processor with multiple DSPs and successfully tracked a target person using real image sequences.

Experimental characterization of crack tip deformation fields in Alloy 718 at high temperatures

Abstract: A series of fracture mechanics tests were conducted at temperatures of 650 C and 704 C in air, using Inconel 719. A noncontacting measurement technique, based on computer vision and digital image correlation, was applied to directly measure surface displacements and strains prior to and during creep crack growth. For the first time, quantitative comparisons at elevated temperatures are presented between experimentally measured near-crack-tip deformation fields and theoretical linear elastic and viscoelastic fracture mechanics solutions. The results establish that linear elastic conditions dominate the near-crack-tip displacements and strains at 650 C during crack growth, and confirm that K{sub 1} is a viable continuum-based fracture parameter for creep crack growth characterization. Postmortem fractographic analyses indicate that grain boundary embrittlement leads to crack extension before a significant amount of creep occurs at this temperature. At higher temperatures, however, no crack growth was observed due to crack tip blunting and concurrent stress reduction after load application.

Real-time 3D topography by speckle image correlation

Abstract: A speckle image correlation method is proposed for the application to 3D topography at ultra-high speeds. With randomdensely-seeded laser speckles projected onto an object to be measured, taking snapshots from two different viewing anglesusing a pair of CCD cameras, we are able to correlate the two speckle images in real time, 30 Hz, by a sparse-arraycompressed-image correlation algorithm. As soon as the correlation is established, a fast triangulation reconstruction givesthe 3D coordinates of the speckles on the object in the global coordinate system, and thus the topography of the surface ofthe object is obtained. This method has an accuracy of pixels and a resolution equal to that of its cameras. Mostimage correlation algorithms suffer from their lack of speed. The method discussed here has overcome this barrier andmade wide applications possible. Real-time correlation enables applications in areas such as on-line inspection, non-stillobject measurement, and instant 3D model acquisition.Keywords: surface inspection, surface geometry measurement, image compression.

An automated image alignment system for the scanning electron microscope

Abstract: We have developed an automated image alignment system for the scanning electron microscope (SEM). This system enables specific locations on a sample to be located and automatically aligned with submicron accuracy. The system comprises a sample stage motorization and control unit together with dedicated imaging electronics and image processing software. The standard SEM sample stage is motorized in the X and Y axes with stepping motors which are fitted with rotary optical encoders. The imaging electronics are interfaced to beam deflection electronics of the SEM and provide the image data for the image processing software. The system initially moves the motorized sample stage to the area of interest and acquires an image. This image is compared with a reference image to determine the required adjustments to the stage position or beam deflection. This procedure is repeated until the area imaged by the SEM matches the reference image. A hierarchical image correlation technique is used to achieve submicron alignment accuracy in a few seconds. The ability to control the SEM beam deflection enables the images to be aligned with an accuracy far exceeding the positioning ability of the SEM stage. The alignment system may be used on a variety of samples without the need for registration or alignment marks since the features in the SEM image are used for alignment. This system has been used for the automatic inspection of devices on semiconductor wafers, and has also enabled the SEM to be used for direct write self-aligned electron beam lithography.

Computational model of image formation process in DIC microscopy

Abstract: A computational model of the image formation process has been developed for the Nomarski differential interference contrast (DIC) microscope. The DIC microscope images variations of the phase of the light wave transmitted through the specimen. In the study of biological phenomena, the DIC microscope is used to visualize live cells which are highly transparent in the visible spectra but distort the phase of the impinging light wave. Within the microscope, a birefringent prism splits the transmitted light wave into two laterally sheared wavefronts. An interference pattern is imaged when the wavefronts recombine. The computational model we developed uses polarization ray-tracing techniques. Rays propagating through different microscope components and the specimen are traced. A specimen is represented by a 3D grid of voxels, each containing a complex refractive index. At the image plane, a coherent superposition of the diffracted field due to each ray contributes to the image intensity. Partial coherence at the image plane is also simulated by wavefronts with different propagation directions. By computing the image intensity at different positions along the axial direction, we can obtain optically sectioned images. In order to evaluate our model, we compared simulated images to the images taken under a real DIC microscope. We constructed test specimens of known shape and properties, using polystyrene beads in optical cement and an etched glass wafer. As the next step, we plan to use this computational model for the reverse problem, i.e. to reconstruct the 3D refractive index distribution of an imaged specimen.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Digital Image Correlation Procedure to Characterize Soil Surface Layer Cracking

Abstract: This study was conducted to characterize surface layer cracking in Vertisols by determining surface displacement trends. A disturbed sample from the Ap horizon of a Randall clay (fine, smectitic, thermic Ustic Epiaquerts) was evaluated. Digital image correlation (DIC) was used to track the movement of points on the surface as the soil dried and to assess the development of soil surface cracking. Displacement fields among a group of images were obtained by using a cross-correlation algorithm. The displacement vectors resulting from the analysis of the displaced fields can be used to show cracking patterns on a ground-up soil sample from a Vertisol before they become visually apparent. Digital image correlation may be used to calculate the deformation of the surface layer with relatively high precision.

Physics-model-based 3D facial image reconstruction from frontal images using optical flow

Abstract: image synthesis is one of the most realistic approaches to realize lifelike agents in computers. A facial-muscle model 1 is composed of facial tissue elements and muscles. In this model, forces affecting facial tissue elements are calculated by contraction of each muscle, so the combination of each muscle parameter determines a specific facial expression. Then each muscle parameter is specified in a trial-and-error procedure comparing the sample photograph and generated image using our Muscle-Editor to generate a specific face image. In this sketch, we propose a strategy for automatic estimation of facial muscle parameters from optical flow using a neural network. This corresponds to 3D facial-motion tracking from a 2D image under the physics-model-based constraint. This technique is also 3D-motion estimation from 2D tracking in a captured image under the constraint of the physics-based face model. We use optical flow of the facial image to measure transformation of the face when an expression appears. However, all of the flows are not used independently. They are compressed in the window depending on the position of each muscle. Optical flow is calculated by a block-matching method, and the block size is 8 by 8 pixels in an image size of 720 by 486 pixels. Then we can determine the difference between any specific expression and a neutral with 96 dimensional vectors including 48 windows with x-y directions. The learning pattern is composed of a data pair: a muscle-parameter vector for the output layer and an optical-flow vector for the input layer. Neural networks were trained using back propagation. They include 55 images (i.e., five keyframes for transition from neutral to each of six basic expressions and mouth shapes of five vowels. Their learning pattern is not given at once but gradually increased to escape the local minimum. To confirm whether the learning process of the neural network is successfully completed, we input the learning data into the input layer of the neural network and resynthesize the facial image using muscle parameters from the output layer of the neural network. Example results are shown in the figures. There are slight differences in fine detail between the original and regenerated images, but subjectively, the overall facial features and expressions are almost the same as the original image. So the mapping rules work well for the training data. Muscle parameters are decided only from the 2D image, but the 3D facial image is well regenerated. …

Image reconstruction schemes for optical tomography

Abstract: The authors have developed a novel approach to the image reconstruction problem in optical tomography. Unlike currently available schemes, this method does not assume that the solution is a small perturbation to a known reference medium. In the authors' approach the reconstruction problem is viewed as an optimization problem, which can be solved by gradient-based optimization techniques.

Full Field Strain Measurements in Welds.

Abstract: : The intent of this study is to examine the stress-strain response of small material subsets in a weld subjected to some nominal loading using a full-field surface strain mapping technique. This process makes use of the digital image correlation program (VIC_2D) developed by the University of South Carolina. Full-Field surface strain mapping will be used to help determine the basic material properties of the various regions within a weld, which are the heat-affected zone and the mechanically affected zone (weld nugget). During development of this process, various specimens were tested to verify the accuracy of the strain mapping procedure. A tapered stainless steel tensile specimen was studied and the results verified by a finite element model of the test specimen. The results from the full-field strain mapping matched very closely with the strain distribution predicted by the finite element model. To further prove the validity of the strain mapping process, a .0135 inch diameter hole was drilled in a 5454-0 aluminum tensile specimen. The strain measurement process detected the perturbations in the strain field caused by the defect at displacements as low as three-hundredths of an inch within the three-inch gage length. Furthermore, the strain measured near the edge of the specimen using the mapping technique matched to within .01 % the strain measured by an extensometer attached to the test specimen.

Quantitative diffuse optical tomography requires nonlinear reconstruction algorithms

Abstract: Summary form only given. Many diffuse optical tomography (DOT) algorithms rely on assumptions that linearize the relationship between the optical contrast of an optical inhomogeneity and measurements of diffuse light. In this paper, we show that this linearisation results in incorrect moments and thus accurate quantitative imaging is not possible. Common examples of linearized image reconstruction algorithms for DOT are based on the Born approximation. Quantitative imaging is possible only if the linearized solution produces scattered waves that agree well with exact solutions. To check this, we calculated the Born approximation for a spherical object embedded in an otherwise uniform, infinite medium, and compared the numerical results with the analytic solution for diffuse photon density waves (DPDW) scattering from spherical objects.

Characterizing and Modeling Plastic Strain Inhomogeneity in Thin Metallic Sheets

Abstract: Using a newly developed plastic strain measurement technique based on digital image correlation, surface plastic deformation of polycrystalline aluminum alloys in a thin sheet form has been experimentally characterized at a length scale comparable to that of the thickness of the aluminum sheets but much larger than the average size of individual grains. Both static and dynamic local straining patterns in these aluminum alloys have been observed and these strain patterns can not be simulated using the conventional plasticity models. The texture clustering of grains may contribute to the static local plastic strain patterns detected in an Al-Mg alloy. Two distinctive dynamic straining behaviors resulted from the dynamic strain aging of dislocations due to the alloying elements have been experimentally established for 5XXX and 6XXX alloys, respectively. First observation of dynamic strain inhomogeneity is also made in a sheet metal specimen deforming predominately in a plane strain state.

Measuring Errors Analysis and Improvement of Digital Image Correlation Method Measurement System

Abstract: In this paper, aII measuring errors of Digital—Image Correlation Method and their origins are analyzed theoretically in detail. Experimental technology of the method are studied. Some methods to eliminate these errors by experimertal technology are proposed.

Inverse characterization of nonlocal behaviour using digital image correlation

Abstract: In the present study, the relation between the initial microstructure, the proper damage mechanisms and the length parameter is investigated for three materials with a gradient-enhanced damage model. For this purpose, a series of experiments has been carried out on polypropylene-based composite materials with different glass-fibre lengths or microstructures. The process-zone close to the crack-tip has been investigated with a digital image correlation technique. Herein, the images of the deformed process-zone are digitized and stored. The correlation analysis between the successive images permits quantification of the displacements in the zone of interest. An inverse mixed numerical-experimental approach permits the determination of the length parameter and the governing parameters for the damage evolution. The influence of the microstructural characteristics of each material and the failure mechanisms on the parameters is scrutinized. Conclusions are drawn with respect to the physical nature of some nonlocal aspects, which relate the transforming microstructure to the macroscopically observed behaviour.

Associative retrieval of holographic digital data

Abstract: Summary form only given. One of the strengths of optical signal processing is the ability to rapidly perform two dimensional correlation of images. A template image is first presented on a spatial light modulator (SLM), Fourier transformed by a lens, and recorded as a hologram. Then, the input image to be compared is used to reconstruct the reference beam of the hologram, which is Fourier transformed by a second lens onto a detector camera. Matches between the input image and the template result in a bright output peak.

Hierarchical estimation of optical flow using spectral energy method

Abstract: The estimation of optical flow is an important problem in the dynamic image analysis. There are various methods of estimating the optical flow. The gradient method and matching method are based on the operation in the space-time domain. While spectral energy method is based on the frequency analysis and has robustness against noise. In this paper, we propose a hierarchical estimation of the optical flow using spectral energy method. In the proposed method, we make a set of hierarchical images for each frame. A global motion which is estimated at the coarsest resolution image sequences is corrected by using finer resolution image sequences. Consequently we can estimate both global and local motion accurately with robustness to noise.