Direct-Write Piezoelectric Transducers on Carbon-Fiber-Reinforced Polymer Structures for Exciting and Receiving Guided Ultrasonic Waves

Advancements in the structural health monitoring (SHM) technology of composite materials are of paramount importance for early detection of critical damage. In this work, direct-write transducers (DWTs) were designed for the excitation and reception of selective ultrasonic guided waves and fabricated by spraying 25-
$\mu \text{m}$
-thick piezoelectric poly(vinylidene fluoride-co-trifluoroethylene) [P(VDF-TRFE)] coating with a comb-shaped electrode on carbon fiber-reinforced polymer (CFRP) plates. The characteristics and performance of the ultrasonic DWTs were benchmarked with the state-of-the-art devices, discrete lead zirconate titanate (PZT) ceramic transducers surface-mounted on the same CFRP plates. The DWTs exhibited improved Lamb wave mode excitation (A
₀
or S
₀
mode) relative to the discrete PZT transducers. Moreover, high signal-to-noise ratio was obtained by effectively canceling other modes and enhancing the directivity with the periodic comb-shaped electrode design of the DWTs, despite the smaller signal amplitudes. The enhanced directivity overcompensates for lower amplitude attenuation, making DWT a good candidate for locally monitoring critical stress hot spot regions in the CFRP structure prone to early damage initiation. It is shown that pairing a DWT sensor with a discrete PZT actuator could further achieve balanced performance in both wave mode selection and signal amplitudes, making this combination really attractive for ultrasonic SHM.