Development of a Torsional Guided Wave EMAT for Internal Inspection of Ferromagnetic Pipes

T(0,1) torsional guided waves play a significant role in pipe inspection due to their non-dispersive nature. They exhibit a greater sensitivity to axial defects compared to longitudinal guided waves. This article focuses on achieving non-contact detection of torsional guided waves in ferromagnetic pipes with small diameters. To address this, a novel plug-in magnetostrictive transducer designed for generating and detecting torsional guided waves is introduced in this study. The transducer predominantly comprises an arc-shaped coil, a rectangular permanent magnet, and a supporting skeleton, which has the advantages of a simple structure and a high signal-to-noise ratio. In this article, our primary endeavor involves commencing with numerical simulations to accurately model the distribution of both static and dynamic magnetic fields within the transducer. Simultaneously, we undertake an in-depth analysis of the transducer's operational principles, elucidating its mechanism for generating torsional guided waves. Then, the distribution and magnitude of the circumferential static magnetic field in the pipe are optimized by changing the size of the permanent magnet to enhance the signal amplitude of the guided waves. After that, the frequency characteristics of the transducer are experimentally investigated, and the results show that the optimal excitation frequency of the transducer is basically consistent with the design frequency. Finally, the ability of the transducer to inspect the defect is demonstrated, and the transducer is capable of detecting the axial defect with a depth equivalent to half the wall thickness of the pipe and a length of 8 mm.