Measurement of sensitivity of different wave modes to subsurface defects
Excitation of acoustic wave modes in a layered solid is investigated when the solid is immersed in a liquid. This is done by computing the reflection coefficient of acoustic plane waves at the liquid-layered-solid interface. Then, a method to evaluate the sensitivity of various modes supported in the layered structure to subsurface defects is introduced. The method involves insonification of the layered structure with a conical wave whose axis coincides with the defect. Since all the rays in the conical wavefront hit the planar interface at the same angle, it is possible to excite a single kind of mode in the layer. By adjusting the angle of inclination of the cone, it is possible to excite the modes selectively. Since the conical waves converge to a line focus at the cone axis, the excited mode will focus on the defect. By recording the reflected signal amplitude as the cone angle is varied, a curve is obtained from which it is possible to conclude the sensitivity of various modes to the subsurface flaw. The results of such measurements indicated that the generalized Lamb wave modes are more sensitive to subsurface defects than the Rayleigh waves. An imaging system which makes use of focused Lamb waves was built and the system produced images of very small subsurface defects.