Longitudinal modes of a surface bonded piezoelectric wafer active transducer (PWaT) and their influences on PWaT resonances

Abstract

This work investigates the longitudinal modes of surface-bonded piezoelectric wafer active transducers (PWaTs) and how they influence the PWaT resonances. Unlike conventional one-dimensional bar structures that essentially have only one non-dispersive longitudinal mode, the bonded PWaT has two dispersive longitudinal modes, with one evanescent (non-propagating) wave mode below a cut-off frequency. Their propagation characteristics, such as wave numbers, attenuations, and mode conversion at the PWaT edges, clarify two research questions raised from the time–frequency analysis of the PWaT electromechanical impedance (EMI) signature and the associated broadband pitch-catch signal. The study also revealed that the PWaT resonances stem from the Fabry-Perot interference of the waves reflected at the two edges of the PWaT, providing a sound theoretical foundation that further explains the influence of the adhesive on the PWaT resonances. These theoretical discoveries will lead to physics-based interpretation of EMI and ultrasonic pitch-catch signals generated by surface bonded PWaTs, which could lead to new sensing mechanisms in future.