Surface strain measurement using multi-component shearography with coherent fibre-optic imaging bundles

Abstract

Shearography is a full-field interferometric speckle technique used to determine displacement derivatives. Measurement of surface strain is possible using shearography if six components of displacement gradient are calculated. This can be achieved using shearography instrumentation that incorporates at least three measurement channels combined with two orthogonal shear directions. This paper presents a laser shearography instrument that utilizes coherent imaging fibre bundles to port four spatially multiplexed speckle images to a single CCD camera via a shearing Michelson interferometer. The four images are spatially multiplexed onto the sensor of a CCD camera. Wrapped phase maps are derived from the recorded speckle interferograms using temporal phase stepping. The unwrapped phase maps are combined with the measurement channel sensitivity vectors using a matrix operation to determine the required displacement derivatives. Results from an out-of-plane displacement of a flat aluminium plate are presented and compared with a computational model. Results from a second test object that show in-plane and out-of-plane strain components are also shown.