Study Of Digital Image Correlation-A Computer Vision Technique for Stress Analysis: Its Accuracy and Reliability

The computer vision methods for stress analysis were emerged in early 1980s. Among them the method of digital speckle/image correlation (DSC) was first proposed in the year of 1983 (by Sutton et al). The latest technology advance has made possible to build affordable systems to handle applications that conventional techniques are unable to deal with. Researchers worldwide have been studying to improve the accuracy and reliability of the DSC in order to extend its applications to emerging areas of science and engineering. The study of DSC at Ryerson was initiated in early 1990s and the existing system at Ryerson, named “AutoStrain,” was made available early in 2000. The recent applications have been concentrated in areas where stress analysis in the microscopic scale is needed. The applicability of the method has been studied and verified by the predecessors in many ways, mostly via comparison with the results obtained from analytical solutions, numerical modelling and experimental measurement using proven techniques. Yet new applications usually demand revelation of stress/strain for problems with no prior knowledge and under unusually subtle conditions (e.g., new material/structure of micro or sub-micro scale of geometry, multi-material interface, ultra-hassle environment and etc.). For an application facing such challenges, the accuracy and reliability of the measurements are constantly a concern. The goals of the current project are as follows: to review the recent research on the sources resulting in errors from both fundamental and technical fronts; to give a close analysis of the key factors that cause errors in the existing system; and to finally propose measures to cope with the factors and to lead to improvements. The study has covered these aspects o f the method including the image formation and the effect of light sources, the speckle patterns, the algorithms of the search schemes used and the proper interface among these schemes, the convergence criterion and the adaptive method to determine/adjust the values of the criterion to fit specific applications, etc. The project has reached the goals by having made substantial improvements as follows. First, the importance has been clarified of consistency among the different coordinate systems defined in the different parts of the software and improved the interface among these parts. Second, an adaptive method aiming at improving the convergence criterion for Newton-Raphson iterative algorithm has been proposed and implemented. The test results have demonstrated the effectiveness of these improvements.