Hertzian contact transducers for nondestructive evaluation

Hertzian, dry,pointlike contacts are used to couple ultrasonic energy efficiently to plate wave modes in anisotropic solid plates. A new transducer configuration using a piezoelectric PZT-SH transducer and a quartz buffer rod is realized for this purpose. In particular, the lowest order antisymmetric Lamb wave (A0) mode is excited and detected in anisotropic plates at a frequency of 200 kHz and with signal-to-noise levels exceeding 65 dB. Time delay measurements and FFT techniques are used to obtain dispersion and anisotropy curves of phase velocity in anisotropic plates. The accuracy of the method is limited mainly by the distance measurements. In (001) silicon plates the agreement between theory and experiments is obtained with an accuracy of +/- 0.08%. Anisotropic phase velocity measurements are also performed for uniaxial graphite/epoxy composite plates and the results are compared with the theoretical model to derive the elastic constants of the composite. Through accurate phase velocity measurements, it is possible to detect delamination defects in composite plates with high resolution operating at 200 kHz. Using the surface impedance approach, a model is also developed to predict the effects of delaminations on phase velocity and the results are verified by experiments. (C) 1996 Acoustical Society of America.