MEASUREMENTS OF THE ANGULAR AND FREQUENCY DEPENDENCE OF ACOUSTIC SURFACE WAVE SCATTERING FROM SURFACE CRACKS.

The objective of this work is the experimental definition of the interaction of acoustic surface waves with cracks to form the basis of defect characterization techniques and to test and guide theoretical developments. Results are presented on a simple approach to estimate the size of small surface cracks. The scattered radiation patterns of surface cracks irradiated by acoustic surface waves are interpreted to provide estimates of crack size. The technique is demonstrated for cracks as small as 100 mu m in radius with an accuracy of about 10%. The key features are the positions and spacings of nulls in the angular and frequency dependence of the backward scattered surface wave intensity. A simple model based on optical diffraction theory is presented and demonstrated on cracks (made with the indentation technique) in commercial hot-pressed silicon nitride studies at 100 MHz and on crack-like flaws in commercial aluminum studied at 2-10 MHz.