Ultrasonic point-source/point-receiver measurements in thin specimens

This paper reports on the application of the ultrasonic point-source/point-receiver technique (PS/PR) to thin, anisotropic specimens. By using a scanned source and viewing a large number of signals measured at adjacent source/receiver configurations, one obtains a so-called scan image which represents the detailed spatial and temporal characteristics of the elastic wave field in a specimen. The measurement system uses either a focused, pulsed laser beam operating as a dipole source or a small aperture, piezoceramic shear transducer serving as a monopolar source. Detection of the signals is with a sensitive piezoceramic sensor that responds to the lateral or shear motions of the specimen surface. Scan images were obtained in a Silicon wafer whose thickness was 625 m and in one- or two-ply, unidirectional, graphite/epoxy laminates whose thicknesses were approximately 145 and 275 m, respectively. The experimental data are analyzed using a simple plane-wave, plane-stress model that describes the propagation of quasilongitudinal and quasitransverse membrane waves in the plane of the plate. Good agreement is obtained between the experimental and theoretical group velocity curves of the membrane waves in the laminates. It is shown that the measured group velocity data of different wave modes can be inverted to recover the elastic constants of a material with excellent reliability and accuracy. The measurement system was also used to map out the group velocities in branches comprising the cuspidal region of the quasi-transverse group velocity curve