Constrained thickness-shear vibration-based piezoelectric transducers for generating unidirectional-propagation SH₀ wave

Excitation of a pure guided wave with a controllable wavefield is essential in structural health monitoring (SHM). For example, a unidirectional-propagation guided wave can significantly reduce the complexity of signal interpretations by avoiding unwanted reflections. However, few transducers are currently capable of exciting a pure unidirectional-propagation guided wave, which cannot satisfy the emerging demands from the field of SHM. In this work, the thickness-shear vibration characteristics of the piezoelectric PZT wafer bonded on a waveguide are investigated by theoretical modeling and numerical simulations. It is found that there is a phase difference between the electric-excitation signal applied on the PZT wafer and the mechanical response signal of the bottom surface of the viscoelastic adhesive layer that connects the PZT wafer and waveguide. Moreover, such a phase difference can be adjusted by changing the equivalent width of the PZT wafer. Based on this finding, two piezoelectric transducers with different shape configurations are proposed to excite the unidirectional-propagation SH₀ wave (the fundamental shear horizontal wave). Finite element simulations and experiments are conducted to verify the performances of the two unidirectional transducers. Results show that the two transducers can excite a pure SH₀ wave and enhance the wave energy along a single direction. No time delay is required to excite the proposed transducers. Due to their simple configurations, the developed unidirectional SH₀ wave transducers will have great potential applications in SHM.