Abstract
Digital shearography is a coherent optical method in conjunction with the digital image processing. It allows the shearogram, which depicts directly displacement derivatives, to be observed in real time 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 opposite illuminations) are introduced in the measuring device. The shearograms for each illuminating direction are evaluated by applying the phase shifting technique. By subtracting the phase maps of the two shearograms the result yields a new fringe pattern depicting the pure in-plane strain component. The result obtained by adding the phase maps corresponds to the pure out-of-plane component. This paper describes the relevant theory. Its application for measuring the strain distributions and the local strains is presented.
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