Focusing of Surface Acoustic Waves on Nonpiezoelectric Materials: Geometrical Approach and Application to NDT

Abstract

Surface acoustic waves, whose energy remains confined to a small layer typically one acoustic wavelength thick, are used in various non destructive testing applications [1–3]. The detection of defects lying at a small depth inside the bulk of materials or evolving perpendicularly to their surface, which are not easily attainable by other methods, may then be performed. In non destructive testing domain, it is of great interest to use focused acoustic waves in order to increase sound intensity and to improve spatial resolution [3–6]. The detection of defects with their longer dimension perpendicular to the sample surface, like some cracks, need a focusing of surface acoustic waves. This operation allows to increase the sensitivity of the method. The traditional SAW generation systems may not be used for focusing Rayleigh waves on nonpiezoelectric substrates. For this purpose one must use geodesical lenses or thin layer coatings inserted along the SAW propagation path [7]. Recently some authors have worked on the focusing problem. They used a dispersed wide beam width giving a poor lateral resolution [8] or a PFDF transducer at low frequency [9,10]. A new ultrasonic system working at 30 MHz is reported here for the generation of focused surface acoustic waves on nonpiezoelectric materials. Using a simple geometrical model, the resolution and conversion efficiency of the system are estimated. Experimental results on ceramic samples with artificial defects are reported and the performances of the technique are analyzed.