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.

Calibration and correction procedure for quantitative out-of-plane shearography

“…The result of Goto and Groves [31] showed very large variation of the shearing distance (exact range for the shearing distance variation was not given explicitly). In a similar manner, using a modified DIC algorithm and sequential blocking of mirrors, Francis et al [36] experimentally measured a shearing variation for a shearography setup which included the Mach-Zehnder instead of the Michelson interferometer. The reported shearing variations across the image were as large as 10% (±5% from the average value).…”

Shearography technology and applications: a review

“…Carrier fringe patterns can then be produced through the subtraction of speckle patterns recorded from the two pulses. An alternative design involves the use of a Pockels cell and polarization optics to direct the beams along separate paths via the two lenses L1 and L2 [76] 3 . One of the drawbacks of this approach is the difficulty in obtaining a high enough carrier frequency to fully separate the phase-dependent terms from the background term in frequency space whilst maintaining good contrast carrier fringes.…”

“…The variation of the condition number against angular separation is shown in figure 21(a) [76]. The red bars labeled configuration 1-3 mark the locations of the three configurations shown in figure 20.…”

Applied optics to engineering photonics: a retrospective

“…Figure 29 shows how the injection current modulation, polarization state and phase within the interferometers are related. More recently we have incorporated the imaging fiber bundles described in Section 3.1 to implement an instrument based on multiple observation directions and used this with continuous wave [55] and pulsed [56] laser illumination, which could not have been fiber delivered due to the high peak energies produced by short pulsed lasers, to measure dynamic strain events.…”