Optimization Design of High-Purity Shear Wave Electromagnetic Acoustic Transducer with Butterfly Coil
Dong Ming1,2, Li Hanghui1, Ma Hongwei1,2, Chen Yuan2, Cao Xiangang1,2
1. School of Mechanical Engineering Xi’an University of Science and Technology Xi’an 710054 China; 2. State Key Laboratory of Mine Electromechanical Equipment Intelligent Detection and Control of Xi’an University of Science and Technology Xi’an 710054 China
Abstract：The electromagnetic acoustic transducers (EMATs) are electromagnetically coupled ultrasonic transducers which can generate and detect ultrasonic waves on electrically conducting media. Compared with spiral coil, butterfly coil EMAT has the highest eddy current and pressure at the central axis, which has significant advantages in detection of inner defects and thickness measurement. The butterfly coil EMAT is easy to generate shear waves and longitudinal waves in an aluminum block. The echo of longitudinal wave will be judged as a defect echo, which brings a big challenge to defect identification. To improve the detect ability, an optimization design method for high-purity shear wave electromagnetic acoustic transducer with butterfly coil is prosed. Firstly, the spatial magnetic field distribution of the permanent magnet was taken into account, and the relationship between the Lorentz force and the type of excitation ultrasonic waves at different positions of the butterfly coil EMAT was analyzed. The propagation characteristics of the ultrasonic waves were examined in a 2D finite element simulation model, and the causes of multiple echoes were analyzed. The echo of the longitudinal wave will be judged as a defect echo, which can lead to inaccurate detection results. Secondly, a butterfly coil with varied size was proposed, where the wire width and spacing of the butterfly coil varies with the position, simultaneously, the transduction efficiency was altered. The longitudinal waves were suppressed while shear waves were enhanced, and high-purity shear waves were achieved. The influence of butterfly coil parameters on the excitation efficiency and the amplitudes ratio of shear and longitudinal wave was studied, and the optimized parameters were determined through finite element simulation. Finally, the equal-size and varied-size butterfly coil EMAT were prepared for the comparison experiment involving shear wave excitation and defect detection. The experimental results showed that when the magnet-to-coil width ratio reached approximately 1.92, the longitudinal wave was weakened by 66.1%, while the shear wave was enhanced by 36.3%. Consequently, the shear-to-longitudinal amplitude ratio increased from 5.8 to 23.1. A flat bottomed hole with a diameter of 5 mm was inspected using the optimization EMAT transducers, the amplitude of the longitudinal echo reflected by the bottom and the converted echo have been suppressed, and both echoes were almost submerged in the noise signal. The amplitude of the shear echo reflected by the flat bottomed hole has been enhanced, making it easier to detect defects. The varied-size butterfly coil EMAT designed in this paper can weaken the longitudinal wave and enhance the shear wave, thereby improving the reliability of shear wave electromagnetic ultrasonic internal defect detection. The following conclusions can be drawn from the analysis: (1) Butterfly coil EMAT is easy to generate shear waves and longitudinal waves in an aluminum block. The receiving signal include echo of longitudinal wave, shear wave and converted wave. The longitudinal wave echo will be judged as a defect echo, which brings a big challenge to defect identification. (2) Reducing the wire width and spacing of the butterfly coil in the middle part can improve the energy transfer efficiency and enhance the shear wave amplitude, while increasing the wire width and spacing of the butterfly coil in the side part can weaken the energy transfer efficiency and weaken the longitudinal wave amplitude. The ratio of shear wave amplitude and longitudinal wave amplitude varies with the magnet-to-coil width ratio.
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2024, Vol. 39 
