Scanned Imaging Techniques for Surface NDE

A phase sensitive laser probe in which the focussed spot is small, as compared with the acoustic wavelength, is capable of measuring the complex distributions of a SAW field along prescribed scan lines. Using the probe, it is possible, on a defect free sample, to measure the SAW velocity surface with an accuracy of a few parts in 10s. Such accuracy suggests that the technique is sufficiently sensitive to detect small changes in surface characteristics; the presence of a defect is revealed by perturbations in the relationship between various scans. The scattered radiation patterns from a surface crack irradiated by acoustic surface waves can be utilised to determine the defect size and location with improved accuracy. Results on deliberate and real cracks are presentedo Acoustic microscopy has proved to be a valuable tool for the NDE of complex IC deviceso With the advent of VLSI there is a clear need for an acoustic microscope showing sub-optical resolution. We report on the design of a high pressure gas acoustic microscope operating at room temperature which should provide a resolution below lOOnmo Finally, following the demonstration of a photoacoustic microscope at Stanford University, there has been a great deal of interest in the subject of photoacoustic microscopy. We describe the work at University College London which is based on the use of a conventional gas photoacoustic cell together with pulsed laser excitation. A finite difference approach has been used in order to study the effect of an inclusion buried under the surface on the photoacoustic signalo In the case where the thermal properties of the inclusion are known apriori, our one dimensional model indicates that it should be possible to obtain not only the depth of the inclusion beneath the surface but also its thickness.