A new diamond-shaped periodic permanent magnet electromagnetic acoustic transducer for side-lobe suppression of shear horizontal ultrasonic waves

Abstract

Periodic permanent magnet electromagnetic acoustic transducers (PPM EMATs) effectively generate shear horizontal (SH) ultrasonic guided waves in metallic media. Depending on their construction parameters, PPM EMATs can generate ultrasonic waves towards unwanted directions, due to the presence of side lobes in their radiation pattern. In this paper, a new PPM-like EMAT is proposed with the objective of reducing the side-lobe level in the radiation pattern of PPM EMATs. This was achieved by altering the magnets’ dimension in the PPM array following a diamond-like shape, reducing their width as one gets further from the centre of the transducer. The radiation behaviour of the EMAT was evaluated by means of an apodization factor, defined as the aperture reduction ratio. The side-lobe level showed a maximum reduction for an apodization factor of approximately 2. Four different configurations of the new PPM-like EMAT design and two reference conventional PPM EMATs were fabricated and experimentally evaluated on an aluminium plate. Experimental radiation patterns were compared to analytical model results showing good agreement. Relative to the conventional PPM EMAT, the new apodized diamond-shaped PPM EMAT presented a side lobe suppression of up to approximately 8 dB.