Shear Wave Imaging of Surface and Near-Surface Flaws in Ceramics

A 50 MHz shear wave transducer has been built on a silicon nitride buffer wedge, and operated with a modified C-scan imaging system to form a high frequency shear wave nondestructive testing system for ceramics. We have shown that coupling between the wedge and a sample at 45 degrees incidence angle is adequate for imaging purposes, and can be maintained even on fine-ground surfaces (about 1 micron rms surface roughness), while scanning at linear speeds of at least 25 mm per second, using sucrose solutions of 50% to 60% concentration as the couplant. We have shown that it is possible to build a system which will detect flaws of order 20 microns in size with .4mm resolution to a depth of 7mm, while scanning about 1 square inch per minute. Advantages of this method over the commonly used longitudinal wave techniques include efficient coupling of the sound into the sample, absence of a front-surface echo, and enhanced depth resolution due to the shorter wavelength of shear waves