Surface strain measurement of rotating objects using pulsed laser shearography with coherent fibre-optic imaging bundles

Abstract

In this paper, surface strain measurements made using a pulsed laser shearography system from a thermally loaded test object, rotating at 610 rpm, are presented. The shearography instrument described here has four measurement channels consisting of four observation directions and a single illumination direction. The use of multiple channels, combined with orthogonal shear directions, enables the measurement of the six orthogonal components of displacement gradient required to determine the surface strain. Fibre-optic imaging bundles are used to transfer images from the observation positions to the shearing interferometer and CCD camera. Spatial multiplexing of the images onto the CCD camera allows for simultaneous acquisition of data from the four views. The repetition rate of a pulsed Nd:YAG laser and the framing rate of the CCD camera were synchronized with the rotation rate of the test object, which enabled the capture of images recorded on subsequent revolutions of the object. Analysis of the deformation-induced phase change between recordings was performed using the spatial carrier technique. The orthogonal displacement gradient components were obtained from the unwrapped phase measurements using a matrix transformation based on the sensitivity vector of each of the four measurement channels.