Determination of instantaneous curvature and twist by digital shearography

Shearography is a whole-field, noncontact optical technique that allows the direct measurement of first-order derivatives of deflection on spatial coordinates, depending on the measurement setup. In many cases, the curvatures and twists of an object provide more interesting parameters, as they are directly related to the induced stresses when an object is subjected to external loads. We describe the use of digital shearography for the measurement of these stress-related parameters through phase retrieval when an object is undergoing continuous deformation. A sequence of shearograms is captured by a high-speed camera during the deformation. To avoid the problem of phase ambiguity, either a spatial or temporal carrier is introduced. A comparison of spatial and temporal carrier is also presented. The obtained three-dimensional matrix is then analyzed by Fourier and windowed-Fourier transform in a spatial and temporal domain and a high-quality spatial distribution of the deflection derivative, curvature and twist are extracted at any instant.