Temporal phase measurement methods in shearography

Shearography is a double-exposure speckle interferometry technique that is sensitive to the displacement gradient, a parameter closely related to the surface strain. For a statically loaded object double-exposure may be extended by phase-stepping to yield a wrapped phase map, which after further processing gives the displacement gradient field. For a dynamically loaded object this technique is not suitable because the phase is not stationary during the phase-stepping process. The other commonly used technique for phase analysis is based on the introduction of either a spatial or temporal carrier frequency. A temporal carrier is more suited to shearography as a spatial carrier is both difficult to implement experimentally and the optical differentiation can amplify distortions in the carrier fringes. Temporal phase unwrapping and Fourier transform based approaches have previously been used to process sequences of temporal interferograms in speckle interferometry. In this work an out-of-plane displacement gradient sensitive shearography configuration is used. The temporal phase-stepping is performed by moving the reference mirror in the interferometer and Fourier transform based methods are used to analyse the sequence of interferograms. The instrument was applied to the analysis of the displacement gradient variation during a heating and cooling cycle of a composite panel over a time of approximately 10 seconds.