Lamb wave excitation by Hertzian contacts with applications in NDE

Excitation of Lamb waves in solid prates by point-like Hertzian contacts for material characterization and nondestructive testing is investigated. A 2 dimensional model using normal mode theory is used to predict the relative excitation efficiency of the lowest order Lamb waves in anisotropic solid plates. Hertzian contact transducers with PZT-5H piezoelectric material and quartz; buffer rods are realized to operate in the 200 to 500 kHz range for experimental verification. Single mode operation with the lowest order antisymmetric Lamb wave (A(o)) mode is achieved in various plates at in agreement with theoretical predictions. The technique is applied for material characterization on single crystal silicon samples and defect detection in composite plates. The phase velocity anisotropy of the A(o) mode is measured with signal-to-noise levels exceeding 65 dB. In (111) cut silicon plates the absolute phase velocity is measured with +/-0.05% accuracy. The phase velocity anisotropy and effects of delamination in layered composite plates are calculated using the surface impedance approach. The experiments on graphite/epoxy composite plates agree with these calculations and show the potential of the method for defect detection with high resolution.