Advances on the side-shifted dual periodic permanent magnet electromagnetic acoustic transducers design for unidirectional generation of shear horizontal ultrasonic guided wave

Shear horizontal (SH) ultrasonic guided waves can be generated by periodic permanent magnet (PPM) electromagnetic acoustic transducers (EMATs) and are attractive for non-destructive evaluation. Conventional PPM EMATs generate SH waves in two main, opposite, directions, which can hinder signal interpretation. One can generate waves in a single direction through the interference mechanism of two spatially separated wave sources. This principle cannot be applied to a conventional PPM EMAT due to the coil and magnet arrangement. A variation of PPM EMAT, namely, the side-shifted dual PPM EMAT, overcame this limitation and generated SH waves in a single direction. The original design presented unwanted backward propagating side lobes and required fine alignment in its fabrication procedure to minimise unwanted wave mode generation. This paper reviews the advances in the side-shifted dual PPM EMAT design and in signal processing techniques that yielded improvements to its unidirectional radiation pattern and unidirectionality.