COMPUTER INTERPRETATION OF HOLOGRAPHIC AND SHEAROGRAPHIC FRINGES

Abstract

Double-exposure holography and shearography have shown to be two useful techniques in metrology and nondestructive testing. In metrology, the direct measurement of displacement (in holography) and displacement-gradients (in shearography) enables accurate computation of states of strain and stress. When applied to nondestructive testing, both techniques are particularly useful to the detection and evaluation of debonds in laminates. Debonds are readily revealed from anomalies in the fringe patterns; these anomalies also enable accurate estimation of both the size and depth of the detected debonds. To enhance the commercial viability of these techniques, it is desirable to automate the analysis of fringes with the use of image-processing devices and computers. In this project, a computer-aided fringe interpretation system using image-processing techniques was developed to analyse holographic and shearographic fringes. With this system, fringe patterns are first stored in an imaging-board via a CCD videocamera, then processed using the written software and finally the processed results are displayed. When applied to metrology, out-of-plane displacements and displacement gradients for fully-clamped square plates under uniform pressure or subjected to central point loads can be determined from holographic and shearographic fringes respectively. When applied to nondestructive testing, the size and depth of square and circular debonds in laminates can be accurately estimated for specimens tested under vacuum stressing. Although the present routines deal only with the above simple test cases, simple modifications of the developed routines will enable the assessment of specimens and debonds of other configurations.