Driving mechanism on magnetostrictive type electromagnetic acoustic transducer for symmetrical vertical-mode Lamb wave and for shear horizontal-mode plate wave

Abstract

The properties of electromagnetic acoustic transducers (EMATs) for using the symmetrical vertical mode-(S0) Lamb wave and the shear horizontal fundamental mode-(SH0) plate wave were experimentally studied. The EMAT, consisting of a meandering coil and an electromagnet, can effectively transmit and receive the S0 mode Lamb wave and the SH0-mode plate wave in a thin metal plate. For ferromagnetic materials, such as steels, the magnetostriction is important for the transduction. The wave amplitude is not proportional to the static field strength, but is dependent on the magnetostrictive properties of the material. In this study, the amplitude and the phase of the S0 mode Lamb wave and SH0 mode plate wave generated by the EMATs are measured while changing the static field strength to the sheets. The amplitude of the S0-mode Lamb wave in a low carbon steel sheet initially decreases with a static field to the minimum value, then increases again accompanying the phase inversion by 180 ° and finally decreases without any phase inversion. The amplitude of the SH0 mode plate wave in a low carbon steel sheet initially increases to the maximum value with the static field, then decreases to the minumum value and increases again accompanied by a phase inversion of about 90 °. These data were discussed on the basis of the magnetostrictive curve to the magnetizing current of an electromagnet in low carbon steel sheets.