Automatic Stress Analyzing System by Digital Speckle Photography Using 2-D Hybrid Method

The automatic stress analyzing system based on the digital image correlation and 2-D intelligent hybrid method was developed. The surface images of an object before and after the deformation were taken with a CCD camera and a long focal microscope. From these images, the displacement data corresponding to the 2-D mesh pattern were calculated by digital image correlation. The displacement data obtained from the experiment were including large error, so it is difficult to evaluate the stress and the strain with high accuracy using raw displacement data. We applied the 2-D intelligent hybrid method proposed by Nishioka et al. in order to evaluate the 2-D stress field. Then, the stress and the strain were estimated by the algorithm in the latter half of 2-D finite element method. It is very easy to draw the contour map of the stress or the strain by the post processor. The error of displacement analysis was still large in the present system. It is necessary to raise the accuracy of sub-pixel order deformation of digital image correlation for the purpose of actually aiming at practical use. Introduction In the analysis of structures which have some cracks, if the stress and the strain distributions can be checked visually on the spot, there is the remarkable merit industrially. Although strain gage is generally used for the stress analysis on the surface of an object, it is necessary to stick many gages for evaluating the stress in a domain correctly, and great time and a great labor are needed. Hence, it waits for development of the stress analysis system by an optical and non-contact measurement method. Then, speckle photography was developed as one of such approaches. To measure the displacement, the double-exposed specklegram of the specimen being deformed in between exposures is taken by speckle photography. The displacement at any point on the specimen was obtained by pointwise filtering of the specklegram using a thin laser beam. Many Young’s fringes patterns around the crack tip were taken and analyzed by the image-processing system [1, 2]. To date, the stress-intensity factors of homogeneous and dissimilar specimens have been evaluated with high accuracy by measuring the displacement along the crack-line on the specklegram [3]-[5]. Moreover, the stress-intensity factor was evaluated by using the asymptotic solution derived by Sun and Jih [6]. However, the speckle photography needs two-step processing which consists of taking specklegram and generating Young's fringes, so this technique is not suitable for real time measurement. In order to realize real time and automatic analysis, a system must process the analysis in a short time, and must avoid man's intervention. Therefore, digital image correlation (DIC) [7]-[9] was adopted to analyze the displacement on the surface of the specimen. In digital image correlation, the surface images before and after the deformation were taken by a CCD camera and a long focal microscope, and the correlation coefficient is calculated by a personal computer. From this coefficient, the displacement at any point of the image can be obtained. The displacement obtained by the experiment is not as smooth as that obtained by the finite element analysis. Therefore, the displacement data were smoothed by 2-D FFT filtering and least squares method [10]. However, it is difficult to analyze the stress and strain directly using raw displacement data Key Engineering Materials Online: 2004-08-15 ISSN: 1662-9795, Vols. 270-273, pp 800-807 doi:10.4028/www.scientific.net/KEM.270-273.800 © 2004 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific.net. (Semanticscholar.org-13/03/20,17:37:59) Title of Publication (to be inserted by the publisher) including the measurement error. Then, we paid attention to intelligent hybrid method proposed by Nishioka et al. [11, 12]. In this method, the strain and stress analysis error are decreased by the correction. The displacement data corresponding to 2-D mesh patterns were calculated by digital image correlation using two images before and after deformation for the intelligent hybrid method. The stress and strain were calculated with high accuracy by the intelligent hybrid method. This automatic stress analyzing system is able to carry out from the image acquisition to stress analysis successively. Principle Digital Image Correlation. 2-D digital image correlation allows the measurement of full-field, inplane displacement from digital images before and after deformation. This method is easy to use, accurate and fast. Two digital images of A {xi=1, 2, ....., N} and B {yi=1, 2, ......, N} were taken before and after loading. Most digital image correlation algorithms work by searching the gray value pattern in small local neighborhoods commonly referred to as subsets. A similarity measure is employed to determine the displacement of the subset center in a second image (under loading). The digital image correlation algorithm finds the subset center displacement by maximizing this similarity measure. Pick up small but finite subsets V1 and V2 from images before and after the deformation, and the correlation coefficient ρ is calculated by