5. New Techniques and Phenomena
An acoustic wave pissing through a material has its velocity changed when stress is applied. This is due to changes in the third order elastic constant and the density of the material. By using a small diameter beam or a focused beam Incident on a metal and reflected from both its front and back surfaces, It is possible to measure the difference in phase of the two reflected waves; the beam Itself can be scanned over the surface of the material. Three kinds of measurements will be shown. The first relates the change of velocity of a compressional wave to the applied stress taken 'in an MTS testing system. The second shows a scan of the profile of the velocity change around a circular defect. The third is an image of the stressed region around a circular defect obtained with a scanned electronically focused system operating in a phase contrast mode.
Barnett, D.; Herrmann, G.; Hunter, J.; Johnson, G.; Kino, Gordon S.; Leung, W.; Selfridge, A.; Steele, C.; Shaw, J.; and Waugh, T., "Measurement of Stress Profiles by Phase Contrast Techniques" (1978). Proceedings of the ARPA/AFML Review of Progress in Quantitative NDE, September 1976–June 1977. 23.