Broadband torsional guided wave magnetostrictive patch transducer with circumferential alternating permanent magnet array for structural health monitoring.

T(0,1) guided wave is non-dispersive and has been widely used in the long distance inspection and condition monitoring. Magnetostrictive patch transducer (MPT) is a new type of EMAT with high energy conversion efficiency, which can be used to generate and receive torsional guided waves in pipe samples. In this paper, a permanent magnet broadband torsional guided wave MPT composed of a linear coil EMAT array is proposed. The MPT was experimentally verified to be broadband and the bandwidth is 230.79 kHz when using a 5 mm wide linear coil. The wavelength corresponding to the center frequency of the transducer is about 0.6 times the width of the coil. Base on the design, through-hole defects with diameters of 3-6 mm in the 2-mm-thick aluminum plate were quantified by frequency sweeping detection, and the relative error is less than 7 %. Compared with the single-frequency guided wave signal reflected by through-hole defects, the variation pattern of the quantitative results using frequency sweeping can contribute to a more accurate prediction of the remaining service life of the test specimen.