Lorentz Force Distribution with Local Curvature and Near-field Calculation of Racetrack Coil EMAT

Electromagnetic acoustic transducer (EMAT) has been widely studied and applied because it can generate ultrasonic wave in metal without coupling agent. It usually consists of one or more sets of permanent magnets and coils, the working principle is that Lorentz force in skin depth of conductive material caused by electromagnetic induction, and generating ultrasonic waves in the structure. When ultrasonic is propagating in solid, the sound pressure is not stable in near-field. If there are factors to be measured within this field, they may not be identified accurately. For EMAT, Lorentz force is used as the excitation mechanism, and its distribution on the curved surface is not uniform, which is comprehensively affected by local curvature of surface, EMAT structure and other factors. In this paper, the influence of structure parameters of racetrack coil EMAT and curved part on the Lorentz force distribution was studied, and the calculation model of near-field is established by Lorentz force distribution. In order to further verify the validity of the calculation model, the finite element model was established and the laws of near-field length, the size of EMAT and local curvature of curved parts are obtained. The racetrack coil EMATs that consistent with the theoretical analysis were made and the experiments were carried out. The relationships between beam focusing ability, sizes of EMAT and the radius of curved parts are summarized by calculating amplitude, the correctness of near-field calculation method is coherently proved by finite element verification and experiments.