FULL FIELD TENSILE STRAIN MEASUREMENT BY SHEAROGRAPHY

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 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. partial deriv u/partial deriv x, and the out-of-plane component, e.g. partial deriv w/partial deriv 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 and sequential illuminations) are introduced in the measuring device. The shearograms for each illuminating direction are evaluated by applying the phase shifting technique. The result by subtracting the phase maps of the two shearograms yields a new fringe pattern depicting the pure in-plane strain component and the result by adding the phase maps corresponds to the pure out-of-plane component. The theory and nondestructive tensile testing are demonstrated in this paper. Its applications for determining the strain distribution of the homogeneous and inhomogeneous material are presented.