Development of an omnidirectional SH0 piezoceramic transducer

Abstract

Ultrasonic guided waves are now routinely used in non-destructive evaluation. In plate-like structures, three fundamental modes can propagate, namely A0, S0 and SH0. Most of the guided wave literature has thus far focused on the use of A0 and/or S0 because these modes are easy to generate in plate-like structures using standard piezoceramic transducers. Yet, at low frequency, A0 and S0 are dispersive. The consequence of dispersion is that signal processing becomes complex for long propagation distances. SH0, on the other hand, has the particularity of being the only non-dispersive guided wave mode. Omnidirectional transduction of SH0 requires a rotational surface stress which cannot be easily generated using standard piezoceramic transducers. This project investigated the use of piezoceramic shear plates cut into six trapezoids bonded to a plate in order to form a discretized circle. The individual elements of the hexagonal shaped transducer were synchronized to generate shear surface stress simultaneously. The external diameter of the discretized circle was chosen to be half the SH0 wavelength at the desired centre frequency. Finite element simulations using the Comsol Multiphysics environment showed that in a 1.6 mm aluminium plate the modal selectivity of the transducer was more than 30 dB at 100 kHz. The concept was then validated experimentally on a 1.6 mm aluminium plate. The 3D experimental displacement field was measured using a laser Doppler vibrometer system. The experimental modal selectivity was 20 dB.