A New Dual-AC Magnetostrictive Shear Horizontal Guided Wave Transducer

Electromagnetic acoustic transducer (EMAT) typically employs rigid permanent magnets (PMs) or flexible magnetizing coils (FMCs) energized by high pulse currents to provide a bias magnetic field. However, the PMs result in a bulkier transducer, whereas the FMCs require accurate timing control of the magnetizing current. Based on traditional static bias magnetostrictive shear horizontal (SH) wave theory, this article designs a novel dual ac magnetostrictive SH wave transducer and investigates the design parameters influencing its operational efficiency. The working mechanism of the dual-ac magnetostrictive SH wave transducer is derived, followed by the development of a finite element model for the SH wave transducer under ac magnetization. The effects of parameters such as magnetizing current amplitude, frequency, excitation current amplitude, and wire length on the excitation wave amplitude are examined. It is observed that with increasing magnetization frequency, the wave amplitude first increases rapidly, then slightly decreases, and finally stabilizes, approaching the dc magnetization state. An experimental system is subsequently built, and the experimental results regarding the effect of magnetization frequency on the excitation wave amplitude are found to be in good agreement with the simulation results, validating the feasibility of the dual-ac magnetostrictive method for exciting SH waves. Further defect detection experiments are conducted on steel plates with hole and crack defects of varying thickness. The results show that when the magnetization frequency and current are set to 10 kHz and 30 A, respectively, only the proposed transducer can effectively detect a 3-mm hole defect. For a $20\times 1\times 2$ mm crack defect, the signal-to-noise ratio (SNR) of the new transducer increases by more than three times. When the plate thickness is increased to 3 mm, the SNR for both transducers decreases, but the SNR of the new transducer increases by more than 2.5 times compared to the permanent magnetized transducer.