Development of surface point-focussing ultrasonic transducer using PVDF

An ultrasonic contact transducer generating point-focused Rayleigh waves along the flat surface of the specimen was developed as an improved technique for detecting, sizing, and imaging surface cracks and subsurface defects. The cone-type shoe was designed so that the incident angles of rays of longitudinal waves generated at any point on the top convex surface of the shoe were constant and all the Rayleigh waves were propagated along the surface or the subsurface of steel and point-focused by circular geometry. The developed surface point-focusing transducer showed narrow beam width (6 dB width=1.2 mm) and high echo-amplitude (17.5 dB higher than the conventional angle probe using PVDF). The simplified model for echo-amplitude showed good agreement with measured results