Design of Staggered Halbach EMAT to Generate SH Guided Wave in Pipe Structures

Abstract

This research investigates the application of Staggered Electromagnetic Acoustic Transducer (EMAT), mainly focusing on the beam steering capabilities of Shear Horizontal (SH) Guided Waves in pipe structures. As a specific case of EMAT, Staggered EMAT introduces a lateral movement in the magnet rows so that the wave propagation occurs at an angle. A three-dimensional model of double row Staggered Halbach EMAT is developed and a finite element analysis is carried out in an aluminium pipe with high radius to thickness ratio. To obtain better signal-to-noise ratio (SNR), the size of the magnets and the number of magnets in a row are optimized by studying multiple cases. For all the configurations, the in-plane surface displacement is studied keeping the wavelength of the transducer constant. The corresponding frequency is derived from the dispersion curve for SH0 wave mode. The results shows that there is significant increase in the amplitude of the in-plane surface displacement with the increase in magnet height and the number of magnets in a row, ultimately leading to the signal with higher amplitude and better SNR. The study then analyses Staggered Halbach EMATs where the second row of magnets is displaced by the maximum value of half of the wavelength. The interference between the waves generated from the two magnet rows resulted in the directivity change of the guided wave, thereby achieving the steering of the SH wave by the maximum value of 27°. The finding of this study shows the enhanced beam steering capabilities of Staggered EMATs, particularly in the field of pipeline monitoring and can lead to more precise and effective detection of structural defects. Keywords: Staggered Halbach EMAT, Beam Steering, SH Guided Wave, Pipeline Inspection