Ultrasonic guided wave unidirectionality enhancement by unidirectional generation and unidirectional reception with electromagnetic acoustic transducers

Shear horizontal ultrasonic guided waves generated by periodic permanent magnet (PPM) electromagnetic acoustic transducers (EMATs) are convenient for non-destructive evaluation. Conventional PPM EMATs generate and receive waves that propagate in two main opposite directions, which can complicate signal interpretation. There has been a recent effort to design dual-EMATs that generate waves unidirectionally. A similar procedure can be applied for reception, predominantly obtaining signals that only contain waves that impinge on the dual-EMAT receiver from a single direction. In practice, unidirectional generation relies on producing identical stimuli by both sets of active elements of a dual-PPM EMAT, which depends on factors such as the degree of similarity of coils and magnet arrays. Here, combined unidirectional generation and unidirectional reception is investigated as a way of increasing unidirectionality in practice. Experiments were performed with a side-shifted dual PPM EMAT operating in the pulse-echo mode for generating the SH0 wave in an aluminium plate. Combined unidirectional and reception increased unidirectionality by at least approximately 10 dB.