Strain distributions made visible with image-shearing speckle pattern interferometry

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 our-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. Copyright (C) 1996 Elsevier Science Ltd.