Development of an Omnidirectional SH0 Mode Electromagnetic Acoustic Transducer Employing a Circumferential Periodic Permanent Magnet Array

This work presents a new omnidirectional electromagnetic acoustic transducer (EMAT) employing a circumferential periodic permanent magnet array, which was proposed for generating and receiving shear-horizontal (SH) guided waves in an aluminum plate. The novel transducer consisted of 12 axially polarized sector magnets arranged in a circle, and the magnetic field directions of adjacent magnets were opposite. A spiral coil was arranged under the magnet array. A vertical magnetic field generated by the magnets and the circumferential eddy current generated by the spiral coil formed a Lorentz force in a plate acting in the radial direction of this new omnidirectional EMAT. By controlling the sizes of the sector magnets, the omnidirectional SH ₀ mode (fundamental SH guided wave mode) was generated. To maximize the output of the EMAT at the given excitation frequency, it was proposed to add acrylic spacers between the sector magnets and determine the size of the magnet based on the frequency bandwidth of the excitation signal while also optimizing the omnidirectional sound field. A three-dimensional (3D) finite element model of the EMAT was established using COMSOL Multiphysics to perform numerical simulations, and the effectiveness of the optimization method was verified. The effectiveness was verified by experiments. The simulations and experiments showed that the proposed EMAT could generate omnidirectional SH ₀ mode guided waves in an aluminum plate.