The measurement of through thickness plate vibration using a pulsed ultrasonic transducer

Abstract

The use of pulsed ultrasound and digital spectral analysis to measure the through thickness vibration characteristics of plates has been investigated. It has been shown that, even at nominally normal incidence, some shear modes of the plate are excited in addition to the longitudinal modes. By modeling the propagation of an ultrasonic pulse through the plate, it has been shown that this is due to the mode conversion of the edge waves from the probe, which are always at a nonzero angle of incidence to the structure. However, not all the shear modes of the plate are excited in this way, and both the pulse propagation model described here and classical analyses can be used to predict correctly which shear modes will be observed. The results show that the use of pulse excitation and digital spectral analysis can give very accurate results and, since the testing time required is small, the technique is much more attractive for use in nondestructive testing than the conventional methods employing sine wave excitation.