Development of a multi-component shearography instrument forsurface strain measurement on dynamic objects

摘要

Quantitative surface strain measurement is demonstrated on dynamic test objects using a shearography instrumentcomprising four measurement channels. The measurement channels are formed using four observation directions and asingle illumination direction. Images from the four measurement channels are transported to a shearing interferometerusing fibre-optic imaging bundles after which they are spatially multiplexed onto the quadrants of a single CCD camera.This facilitates the simultaneous acquisition of data from the four measurement channels. A pulsed laser source wasused to effectively freeze the motion of the dynamic surface at two positions in its cycle. The phase variation caused bysurface deformation in the time between the two recordings was calculated using the spatial carrier technique. Theorthogonal displacement gradient components which characterize the surface strain of the object were calculated fromthe unwrapped phase maps from each measurement channel using a matrix transformation. Two test objects wereinvestigated. The first was a thermally loaded PTFE plate rotating at 610 rpm. Images were recorded a frequency of10 Hz, corresponding to the repetition rate of the laser. The second object was a speaker cone that was set to vibrate atfrequencies in the range of 1.9-4.5 kHz. Phase measurements were made from images recorded 1.6 μs apart using dualpulsed illumination in combination with a dual-framing CCD camera.