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 tra nsducer configuration using a piezoelectric PZT-SH transducer and a qu artz buffer rod is realized for this purpose. In particular, the lowes t order antisymmetric Lamb wave (A0) mode is excited and detected in a nisotropic plates at a frequency of 200 kHz and with signal-to-noise l evels exceeding 65 dB. Time delay measurements and FFT techniques are used to obtain dispersion and anisotropy curves of phase velocity in a nisotropic 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%. Anis otropic phase velocity measurements are also performed for uniaxial gr aphite/epoxy composite plates and the results are compared with the th eoretical model to derive the elastic constants of the composite. Thro ugh accurate phase velocity measurements, it is possible to detect del amination defects in composite plates with high resolution operating a t 200 kHz. Using the surface impedance approach, a model is also devel oped to predict the effects of delaminations on phase velocity and the results are verified by experiments. (C) 1996 Acoustical Society of A merica.