Response of acoustic imaging systems using convergent leaky waves to cylindrical flaws

Abstract

Characterization of near-surface properties of materials by acoustic means is most convenienth done by surface-acoustic-waves (SAW) that are spatially confined to the surface. There are several techniques available to excite focused SAW's on the surfaces of non-piezoelectric materials for imaging purposes. A conventional acoustic microscope lens excites focused SANI's on the object surface, but with a low efficiency. SAW can be excited with a high efficiency using conical wavefronts as obtained from a conical axicon. Such methods can be used in a scanning arrangement to generate images of surface inho-mogeneities. In this paper, an analysis of imaging systems making use of convergent SAW's is given using angular spectrum approach and T-matrix formulation. Inhomogeneities on the object surface are assumed to be circular cylinders whose axes are perpendicular to the surface. The response of imaging systems to such defects is found as a function of defect position with respect to the focus point. Theoretical and experimental results are compared and are found to be in good agreement. © 1989 IEEE