Digital shearography for pure in-plane strain measurement on the object surface under three-dimensional strain conditions

摘要

Digital shearography is a coherent optical method in conjunction with the digital image processing. It allows the shearogram, which depicts directly displacement derivatives, in real time to be observed. Thus, it is suited well for nondestructive testing. However, its application for strain measurement has not been widely adopted in industry, because the shearogram includes usually both the in-plane strain, e.g. partial derivu/partial derivx, and the out-of-plane component, e.g. partial derivw/partial derivx. Although the out-of-plane component can be separated from the shearogram by manipulating the illumination, the pure in-plane strain component can be never determined exactly by adjusting the illumination angle. This paper presents the principle of shearography with two independent illuminating directions (usually the same but mutual illuminations) for the pure in-plane strain measurement. The shearograms for each illuminating direction are generated by applying the phase shifting technique. Thus, the phase maps of the two shearograms corresponding the two illuminating directions can be obtained. The result by subtracting the phase maps of the two shearograms yields a new fringe pattern depicting the pure in-plane strain component. The experimental procedure and its applications for determining pure in-plane strains are presented.