Point-Focusing of Shear-Horizontal Wave Using Fan-Shaped Periodic Permanent Magnet Focusing Coils EMAT for Plate Inspection

摘要

Ultrasonic guided wave testing method is very effective in safety monitoring of complex structures, such as large-scale tank floor and industrial metal plate. The fundamental shear horizontal (SH) wave mode with the Lorentz force mechanism has the advantage that the propagation velocity does not change with the frequency. However, it is hard to recognize the dispersed ultrasonic signal accurately due to the bidirectional propagation characteristics of the SH guided wave. Therefore, a new transducer structure with a fan-shaped periodic permanent magnet (PPM) and centripetal conductor has been designed to achieve guided wave focusing and signal enhancement. Compared with the traditional transducers, the new-designed transducer has been proved to be more effective in energy focusing on the simulation and experimental verification. For the new structure of the transducer, the aperture angle, focusing radius, and coil rotation angle are studied in detail in this paper. The results show that, as the aperture angle and the coil rotation angle decrease, the signal intensity becomes higher at the focusing position. Moreover, as the focal radius increases, the amplitude of the ultrasonic signals on the focusing side and the divergent side will decrease, while the difference between the two sides will increase exponentially. In the defect detection in this paper, the reflected signal intensity of the new-designed fan-shaped PPM focusing coils electromagnetic acoustic transducer (EMAT) is approximately two times that of the traditional parallel PPM parallel coils EMAT. Therefore, the newly designed EMAT is of great significance for the signal enhancement.