THROUGH-TRANSMISSION ULTRASONIC-IMAGING OF SUBSURFACE DEFECTS USING NONCONTACT LASER TECHNIQUES

Through-transmission ultrasonic NDT measurements have been performed with a non-contact laser combination system. High power (approximately 2 MW) laser pulses were used for the generation of longitudinal pulses in metallic samples. On reaching the far surface, ultrasound was detected with a 50 cm confocal Fabry-Perot interferometer. The interferometer analysed speckle from back-scattered laser light so that measurements could be made from samples with only a machine finish. Signal-to-noise ratios were sufficiently good for two-dimensional scans to be performed, producing optical images of artificial defects. For improved image quality, signals were normalised taking into account changes in sample surface reflectivity. Median filtering was also used. Results show that images of 4-mm diameter drill holes can be obtained with resolutions of +/- 0.5 mm.