Determination of strain distribution by means of digital shearography

Abstract

Digital shearography is a coherent optical method in conjunction with digital image processing. It allows the shearogram, which depicts directly displacement derivatives, in real time to be observed and to be evaluated numerically. Strains are functions of the displacement derivatives. Thus, the shearogram contains the strain information, but usually it includes both the in-plane strain, e.g. ∂ u/∂ x, and the out-of-plane component, e.g. ∂ w/∂ x. In order to get the pure in-plane strain as well as the pure out-of-plane component, two linearly independent directions of illumination (usually the same but mutual illuminations) are introduced in the measuring device. The shearograms for each illuminating direction are evaluated by applying the phase shifting technique. The result of subtracting the phase maps of the two shearograms yields a new fringe pattern depicting the pure in-plane strain component, and the result of adding the phase maps corresponds to the pure out-of-plane component. This theory and experimental procedure are demonstrated in this paper. Its applications for determining the strain distribution of homogeneous and inhomogeneous materials are presented.