Strain distributions made visible with image-shearing speckle pattern interferometry

Abstract The measurement of in-plane strain with image-shearing speckle pattern interferometry has so far been impeded by the lack of an efficient method for suppressing the out-of-plane displacement derivative. This paper presents a simple optical set-up with three light sources arranged in a particular symmetry and an associated procedure which allows accurate isolation of all six displacement derivatives which can contribute to the formation of fringes in image-shearing speckle pattern interferometry. The method uses a phase-shifting technique and is demonstrated on the deformation of a rectangular tensile specimen with a central hole. The measured in-plane strain components are compared with the results of finite element analysis. High quality fringe patterns are obtained which reveal even localized strain concentrations. The optical set-up proposed in this paper is well suited for applications in an industrial environment, and the associated procedure for isolating the displacement derivatives can easily be automated.