Stephen R. McNeill

Digital image correlation using Newton-Raphson method of partial differential correction

Digital image correlation is finding wider use in the field of mechanics. One area of weakness in the current technique is the lack of available displacement gradient terms. This technique, based on a coarse-fine search method, is capable of calculating the gradients. However the speed at which it does so has prevented widespread use. Presented in this paper is the development and limited experimental verification of a method which can determine displacements and gradients using the Newton-Raphson method of partial corrections. It will be shown that this method is accurate in determining displacements and certain gradients, while using significantly less CPU time than the current coarse-fine search method.

Advances in Two-Dimensional and Three-Dimensional Computer Vision

The foundations of two- and three-dimensional image correlation, as well as recent developments, are described in detail. The versatility and robustness of these methods are illustrated through application examples from diverse areas including fracture mechanics, biomechanics, constitutive property measurement in complex materials, model verification for large, flawed structures and nondestructive evaluation. A detailed description of experimental and data-reduction procedures is presented for the application of the two-dimensional image-correlation method to thin-sheet mixed-mode I/II fracture studies, local crack-closure measurements using optical microscopy and the measurement of constitutive properties. Application examples using three-dimensional image correlation include profiling of components for reverse engineering and manufacturing and the measurement of full-field surface deformation during wide cracked panel tensile tests for verification of buckling and crack-growth models. Results from nearly sixteen years of use have demonstrated that both two-dimensional and three-dimensional image-correlation methods are robust and accurate tools for deformation measurements in a variety of applications. The range of uses for the two-and three-dimensional image-correlation methods is growing rapidly as scientists and engineers begin to understand their true capabilities.

Improved three‐dimensional image correlation for surface displacement measurement

A PC-based, 3-D surface profile and displacement measure- ment system capable of micron-level accuracy using moderately priced off-the-shelf equipment has been developed. For use in field applica- tions, a simplified calibration process using precision grids and camera translations is developed. An improved image correlation process is de- veloped which corrects for perspective distortions due to viewpoint dif- ferences between the two cameras. The accuracy of the system was assessed experimentally and results expressed using several different error measures, including a new error measure proposed by the authors. The accuracy for both the profile and displacement measurement sys- tems was established through a series of profile and translation tests. The baseline tests confirmed that the measurement system is capable of highly accurate full-field measurements. The system was also used suc- cessfully to measure both the bending of a clamped circular plate under pressure loading and the local buckling which occurs during tension loading of a cracked plate.

Effects of subpixel image restoration on digital correlation error estimates

Recently, a method has been developed that uses computer vision to determine the deformations of subsets of an object. Although the method has been used successfully in a variety of applications, to date there has been no critical assessment of the key parameters in the system and their effect on the accuracy of the measured deformations. The present work presents the results of initial studies of this system. The system components are modeled, and a representative intensity pattern is chosen and deformed by known amounts. Then, the effects of varying the various parameters in the model are analyzed numerically. The most significant parameters are found to be (1) the number of quantization levels in the digitization process (i.e., the number of bits in the A/D converter), (2) the ratio of the frequency of the signal to the frequency of the sampling, and (3) the form of the intensity interpolation function.

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

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.

Measurement of surface profile using digital image correlation

Presented in this paper is the development of a system for measuring surface profile using digital image correlation. All needed equations for camera calibration and system profile measurements are shown. Equations included needed transformation to account for subset perspective distortions. The system is capable of an accuracy of 1/20,000 of the camera-to-object distance. Experimental results are shown for several cases, with results giving a maximum error of 0.05 mm with camera-to-object distance of 1000 mm.

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

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.

Development and assessment of a single-image fringe projection method for dynamic applications

A single-image fringe projection profiling method suitable for dynamic applications was developed by combining an accurate camera calibration procedure and improved phase extraction procedures. The improved phase extraction process used a modified Hilbert transform with Laplacian pyramid algorithms to improve measurement accuracy. The camera calibration method used an accurate pinhole camera model and pixel-by-pixel calibration of the phase-height relationship. Numerical simulations and controlled baseline experiments were performed to quantify key error sources in the measurement process and verify the accuracy of the approach. Results from numerical simulations indicate that the resulting phase error can be reduced to less than 0.02 radians provided that parameters such as fringe spacing, random measured intensity noise, fringe contrast and frequency of spatial intensity noise are carefully controlled. Experimental results show that the effects of random temporal and spatial noise in typical CCD cameras for single fringe images limits the accuracy of the method to 0.04 radians in most applications. Quantitative results from application of the fringe projection method are in very good agreement with numerical predictions, demonstrating that it is possible to design both a fringe projection system and a measurement process to achieve a prespecified accuracy and resolution in the point-to-point measurement of the spatial (X, Y, Z) positions.

Three-dimensional point cloud registration by matching surface features with relaxation labeling method

Automated approaches for the conversion of multiple overlapped three-dimensional (3D) point clouds into an integrated surface shape measurement in the form of a complete polygon surface are important in the general field of reverse engineering. Traditionally, the conversion process is achieved in a semi-automated manner that requires extensive user interaction. In this work, automated methods for point set registration are developed and experimentally validated using polygon surface reconstruction to represent raw, 3D point clouds obtained from non-contacting measurement systems. Using local differential properties extracted from the polygon surface representation for a measurement data set, a robust sculpture surface feature-matching method is described for automatically obtaining the initial orientation and mismatch estimates for each overlapped data set. Using both simulated and measured experimental data to quantify the performance of the method, it is shown that differential local surface features are appropriate metrics for identifying common features and initializing the relative positions of individual point clouds, thereby providing the basis for automating the registration and integration processes while improving the speed of the surface distance minimization method developed for the initial registration process.

Three-dimensional image correlation for surface-displacement measurement

This paper describes a 3D surface profile and displacement measurement system capable of micron level accuracy using moderately priced off-the-shelf equipment. A non-linear optimization based calibration system is presented. The calibration system determines the position and operating characteristics of the cameras as well as correcting for lens distortion. Also presented is a surface profile and displacement measurement system base on projections into space of subsets of the recorded images. This method provides information about both the location and orientation in space of the subset. The accuracy of the system is established through a series of experiments. The calibration is assessed and the results are expressed using several different error measurements including a new error measurement proposed by the authors. The baseline accuracy of the measurement system was determined through a series of profile and translation tests. The system is capable of measurements to an accuracy of 0.003 mm over a 14 mm X 18 mm field from a distance of 416 mm using a 512 X 480 CCD camera and a magnification factor of 27 pixels/mm. The system was also used to measure the bending of a circular plate under pressure loading. The experimental results are analyzed and compared with theoretical prediction.