Diffraction of a rectangular beam by a crack edge.

Ultrasonic transducers, operating at microwave frequencies, radiate highly directional wave fields or beams. These beams are used to probe solids for defects such as cracks. To model this process, the compressional and shear waves reflected from a semi-infinite crack and diffracted from its edge when it is struck by a two-dimensional, compressional beam are calculated asymptotically. A beam whose initial cross section is rectangular is considered. The crack is assumed to lie in the nearfield or in the nearfield-farfield transition region of the incident beam. Thus the incident beam is only slightly divergent when it strikes the crack edge, and it ensonifies only a finite region of the crack surface. But this region acts as if it were a secondary aperture. Therefore, of particular interest in the calculation are the results showing how the reflected beam diverges as it propagates away from this secondary aperture and arrives at the aperture of the incident beam. [Work supported by NSF.] [ABSTRACT FROM AUTHOR]