Optical detection of leaky-Rayleigh waves at air–solid interfaces

Abstract

Due to the difficulties associated with ultrasonic generation and detection in air in the megahertz region, air-coupled nondestructive evaluation has only recently been explored. In this work, a shear transducer is mounted directly over the edge of a solid specimen so that it can generate a bulk mode in the interior of the solid as well as a Rayleigh wave along the surface. A recently developed optical heterodyne interferometer proved to be sensitive to the leaky-Rayleigh wave in air more than a centimeter away from the solid surface. Results of velocity and attenuation measurements of leaky-Rayleigh waves for several solids will be presented. The technique can also be applied to plates to measure leaky-Lamb velocity and attenuation in air. This method can be used for low optical reflectivity solid surfaces: Thus it can be extended for noncontact nondestructive evaluation applications.