NONDESTRUCTIVE EVALUATION USING REAL-TIME SHEAROGRAPHY

Abstract

This thesis begins with an account on the development of shearography as background. The conventional double-exposure shearography for static testing and the commonly adopted time-average shearography for vibration study have been credited for their particular contributions and evaluated for their shortcomings. Real-time shearography, though receiving only limited qualitative attention from researchers in the past, stands out as a most flexible shearographlc technique. This project aims to provide a sound theoretical basis for real-time shearography and to explore in depth its potential applications in nondestructive testing and vibration analysis. The development of a simple and effective technique for determining fractional fringe order is also another primary objective in this project. The principle behind the formation of high resolution "live" fringe patterns by real-time shearography is presented. It is shown that both linear and nonlinear photographic recordings yield instantaneous fringe patterns which depict instantaneous surface displacement gradient of the deformed object. It is found that real-time shearography, coupled with static stressing, produces fringe patterns of uniform fringe contrast whereas shearogram resulting from vibration stressing has fringe visibility which decreases with increasing surface displacement gradient. In either case, real-time shearography requires a simpler optical setup and eliminates the need for Fourier filtering during fringe read-out. Nondestructive testing using real-time shearographic technique hiss achieved significant success in detecting sub-surface delamination in composite plates. Under partial vacuum stressing, the depth of flaw can be calculated with high accuracy and consistency at any easily-readable maximum fringe order or convenient partial vacuum pressure. Using vibration stressing, the flaw depth in a component can be determined without having to determine the fringe orders. Vibration analysis performed on a cantilever plate specimen produces instantaneous fringe pattern which depicts the out-of-plane surface displacement gradient with respect to the image-shearing direction at various resonant modes. In the final phase of the project, the long-existing difficulty in determining non-whole-number maximum fringe order is addressed. A simple wedge translation technique using phase shifting approach in real-time shearography is developed for this purpose. The experimental results have high accuracy, with the wedge translation corresponding to a 2? radian phase shift deviating by less than 7 percent from the derived theoretical prediction for a range of wedge angle used. Moreover, the sensitivity of the technique can be controlled by varying the wedge angle. Recommendations for further study have been included. In nondestructive testing, future work should encompass the study of real structural components, paying particular attention to internal delamination without air inclusion. In vibration analysis, the shearographic study of specimen with complex geometry and arbitrary loading can be incorporated with finite element analysis. The wedge translation technique should he refined to enhance its resolution and for use in electronic shearography.