Abstract
Due to the high magnetic flux density at the edge of the magnet, the narrow-magnet Electromagnetic Acoustic Transducer can excite the ultrasonic wave with a larger amplitude than the conventional EMAT, but the waveform of the former is distorted. This paper presents a new focused EMAT design with a narrow magnet, which can both enhance the ultrasonic A0-Lamb signal and correct the distorted waveform. A synthesis equation of the ultrasonic signals based on the contribution of the in-plane (IP) and out-of-plane (OP) components of the magnet field is proposed to analyze the mechanism of the waveform distortion. The distorted wave is corrected by an uneven split-wires arrangement strategy. The EMAT employs a sector-shaped narrow magnet and a set of focused meander-line coils containing uneven split wires, for generating a 1MHz focused A0-Lamb wave in an aluminum plate. A three-dimensional finite element model of the EMAT is established using COMSOL Multiphysics to perform numerical simulations, and the effectiveness of the EMAT design is verified. The experimental results show that the EMAT not only corrects the signal distortion due to the narrow magnet and suppresses its side lobes, but also enlarges the amplitude of the A0-Lamb wave by 55.6% under the same magnetic flux density when compared with conventional EMAT.
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