Unidirectional Shear Horizontal Wave Generation With Side-Shifted Periodic Permanent Magnets Electromagnetic Acoustic Transducer.

Abstract

Periodic permanent magnet (PPM) array electromagnetic acoustic transducers (EMATs) can efficiently generate and receive shear horizontal (SH) ultrasonic waves. Conventional PPM EMATs typically generate waves which simultaneously propagate both forward and backward. This can complicate the received signals and make it difficult to locate the position of scatterers. Unidirectional generation of ultrasounds can be achieved if two ultrasonic sources are separated by a predefined distance and are excited with the proper delay. Relying on this principle, EMATs have been previously designed aiming to generate other modes of ultrasonic waves. The main challenge when extending this conception to an SH-wave EMAT is how to restrict each coil to its specific magnet array. We present the concept of a unidirectional SH EMAT consisting of two racetrack coils and two interlaced PPM arrays, that are slightly shifted sideways, in such a way that the generated wavefronts still properly interfere. The design was fabricated and experimentally evaluated in an aluminum plate generating the SH0 guided wave mode. The forward to backward generated wave ratio is above 20 dB and well agrees with finite element simulations.