Ultrasonic guided waves are now routinely used in non-destructive evaluation. In plate-like structures, three fundamental modes can propagate, namely A0, S0 and SH0. Most of the guided wave literature has thus far focused on the use of A0 and/or S0 because these modes are easy to generate in plate-like structures using standard piezoceramic transducers. Yet, at low frequency, A0 and S0 are dispersive. The consequence of dispersion is that signal processing becomes complex for long propagation distances. SH0, on the other hand, has the particularity of being the only non-dispersive guided wave mode. Omnidirectional transduction of SH0 requires a rotational surface stress which cannot be easily generated using standard piezoceramic transducers. This paper presents a transducer concept based on piezoceramic patches assembled to form a discretized circle. The external diameter of the discretized circle was chosen to be half the SH0 wavelength at the desired centre frequency. Finite element simulations using the Comsol Multiphysics environment showed that in a 1.6 mm aluminium plate the modal selectivity of the transducer was more than 25 dB at 100 kHz. A full transducer was built for experimental validation. The experimental modal selectivity was in the region of 20 dB.
Pierre Belanger and Guillaume Boivin, "Piezoceramic omnidirectional transduction of the fundamental shear horizontal guide wave mode," Proc. SPIE 9804, Nondestructive Characterization and Monitoring of Advanced Materials, Aerospace, and Civil Infrastructure 2016, 98040M (Presented at SPIE Smart Structures and Materials + Nondestructive Evaluation and Health Monitoring: March 22, 2016; Published: 8 April 2016); https://doi.org/10.1117/12.2217478.
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