Evaluation of structural discontinuities in piezoelectric shear wave transducers using an immersion setup

The proper functioning of ultrasonic transducers depends on the presence of structural discontinuities. The performance of longitudinal wave transducers can be assessed by recording the spatial distribution of out-of-plane vibration at their surfaces. However, evaluation of shear wave transducers is challenging, as out-of-plane vibrations are typically absent. This paper proposes to detect defects in shear wave transducers by receiving waves generated by mode conversion at structural discontinuities. The distortions in the uniform distribution of shear oscillations caused by these discontinuities and the generation of propagating waves with a non-zero out-of-plane component were studied experimentally and numerically. In the experiment, high-frequency shear wave transducers made of lithium niobate were placed in contact with an immersion liquid. The waves radiated by the defects were recorded by a mechanically scanning focused immersion transducer with a similar frequency range. It was shown that defect-free regions do not radiate waves into the immersion liquid whereas the edges of horizontal compact cracks generate longitudinal waves with opposite phases, and vertical extended cracks generate leaky guided waves that propagate along the water-transducer interface. Visualization of the internal structure of these transducers using the same experimental setup in regular pulse-echo mode shows that the horizontal cracks are visible in the images whereas the vertical cracks are not detected. The experimental results were confirmed by numerical simulation of wave generation by defects in shear wave transducers, performed using Comsol software.