Enhancement of the mode purity of shear horizontal mode of a thickness-shear transducer through design changes

In ultrasonic guided waves, arrays of thickness-shear piezoelectric transducers are often used to generate Lamb and shear-horizontal waves in plates and longitudinal/torsional waves in pipes. The shear-horizontal modes (torsional modes in pipes) are particularly useful for guided wave testing. Although the ultrasonic output of such transducers are well known both numerically and experimentally, few results are available in the literature regarding the influence of geometry, electrode layout and materials on the ultrasonic output of the transducer: in particular, the influence of those parameters on the mode purity of the generated shear horizontal mode in plates requires further investigation. Numerical simulations with finite element modelling (Comsol Multiphysics) have been conducted on a thickness-shear transducer on a plate to understand the influence of these parameters. The study has been conducted both in frequency domain and time domain: the former was used to calculate the frequency response function of the transducer-waveguide system while the latter was used to verify the proportionality between different modes. Different configurations of the transducers have been designed and tested numerically, and the in- and out-of-plane displacements generated are compared for all the three configurations. The effect of geometry and electrode layout are at first assessed in terms of purity of the shear horizontal mode; the most performing configuration is then further modified to enhance the amplitude and the signal to noise ratio of the generated mode. Design changes can then be predicted and suggested.