Excitation characteristics of circumferential SH guided waves in steel pipes generated by EMATs with few-cycle PPM

Circumferential Shear Horizontal (CSH) guided waves provide an effective method for detecting defects like axial cracks and corrosion in pipes. Periodic Permanent Magnet Electromagnetic Acoustic Transducers (PPM EMATs) are typically used to generate CSH guided waves. However, there is an offset problem to which little attention has been paid. The offset problem refers to the offset of the center (position of maximum energy) of the operating region caused by the variation in the peak frequency of the spatial spectra of PPM with 2 or fewer cycles. Furthermore, the excitability of guided waves is one of the factors that needs to be considered when selecting the excitation parameters of EMATs, but there are still some studies that have not sufficiently addressed this issue. In this paper, the offset problem and the excitability of CSH guided waves were investigated. Firstly, by obtaining the operating regions corresponding to PPM with different cycles, the cause and influences of the offset problem were studied. The results show that the offset in the peak frequency of the spatial spectrum of PPM is the fundamental reason causing the offset problem, and it not only leads to incorrect prediction of the excitation efficiency of guided waves but also affects the selection of the excitation parameters of EMATs. Secondly, finite element simulations and experiments were performed to assess the influence of the excitability on the excitation efficiency of the CSH0 and CSH1 modes in pipes. By analyzing the simulation and experimental results of 2-cycle PPM, as well as the simulation results for PPM with 1 to 5 cycles, the impact of the excitability on the CSH1 mode was confirmed from two perspectives. The final conclusion indicates that an accurate prediction of the amplitudes of CSH guided waves with different modes is only possible through a comprehensively consideration of the operating region of EMAT and the excitability of guided waves.