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

Abstract

Quantitative surface strain measurement is demonstrated on dynamic test objects using a shearography instrument comprising four measurement channels. The measurement channels are formed using four observation directions and a single illumination direction. Images from the four measurement channels are transported to a shearing interferometer using 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 was used to effectively freeze the motion of the dynamic surface at two positions in its cycle. The phase variation caused by surface deformation in the time between the two recordings was calculated using the spatial carrier technique. The orthogonal displacement gradient components which characterize the surface strain of the object were calculated from the unwrapped phase maps from each measurement channel using a matrix transformation. Two test objects were investigated. The first was a thermally loaded PTFE plate rotating at 610 rpm. Images were recorded a frequency of 10 Hz, corresponding to the repetition rate of the laser. The second object was a speaker cone that was set to vibrate at frequencies in the range of 1.9-4.5 kHz. Phase measurements were made from images recorded 1.6 μs apart using dual pulsed illumination in combination with a dual-framing CCD camera.