Development of a shear horizontal wave electromagnetic acoustic transducer with periodic grating coil

With the non-dispersion characteristics, the lowest shear horizontal mode SH0 can be widely applied in the detection and monitoring of a defect in plate structures. In order to achieve SH0 mode excitation in aluminum plates, based on Lorentz force mechanism, a new directional SH0 mode EMAT based on periodic grating coil (PGC), called PGC-EMAT, is designed in the study. On the surface of the aluminum plate, the PGC produces the horizontal eddy current. The oppositely induced eddy currents are generated by the adjacent grating units. The current I of the multiple wires in each grating unit is the same. The rectangular magnet provides the static magnetic field perpendicular to the surface of the aluminum plate. In addition, the PGC-EMAT with different grating units of the periodic grating coil and coil layers are simulated by three-dimensional finite element simulation. The simulation results show that with the increase of grating units of the periodic grating coil, the tangential displacement of PGC-EMAT will increase, and the tangential displacement of double-layer periodic grating coil is larger than that of single-layer periodic grating coil when the number of grating units of the periodic grating coil is the same. The developed EMAT can excite and receive a single SH0 mode and the actual center frequency of the transducer coincides with the theoretical value. It was proved that the central frequency of the transducer was controlled by the distance between the adjacent grating units of the periodic grating coil. In order to increase the energy of the signals excited or received by PGC-EMAT, the number of cycles of grating units and the number of layers of the transducer were studied. The experimental results showed that there was a linear relationship between the number of cycles of the grating units and the energy of the excited and received signals. The energy of the excited and received signals of the double-layer PGC-EMAT was larger than that of the single-layer PGC-EMAT when PGC-EMAT was used as excitation transducer or receiving transducer. The simulation results were experimentally verified. The developed PGC-EMAT can be applied in defect detection.