Enhancement of lift-off performance and conversion efficiency using a copper backplate for a spiral coil EMAT in generating and receiving shear waves

Abstract

The application of Electromagnetic Acoustic Transducers (EMATs) into metal materials has been hard to implement for the technical problems of low transduction efficiency and high sensitivity of lift-off variation. The process of transmission and detection of a shear wave in a spiral coil EMAT is established based on a 2-D axisymmetric model using finite element method. A new equivalent reception circuit for an EMAT receiver is provided, and it takes the open-circuit voltage of a receiving coil, the coil equivalent impedance, impedance matching network, and the input impedance of a preamplifier into consideration. On this basis, the conversion efficiency and lift-off performance of the EMAT transmitter and receiver are investigated with respect to coil equivalent impedance, coil conductor diameter, impedance matching parameters, plate-to-coil distance and backplate thickness. Results indicate that compared with the specified EMAT without a backplate at a lift-off of 2 mm, the conversion efficiency and lift-off performance of the transmitting EMAT with a plate-to-coil distance of 1.5 mm can be respectively improved by 103.5% and 2.4 dB/mm, and the conversion efficiency and lift-off performance of the receiving EMAT with a plate-to-coil distance of 1.5 mm can be respectively enhanced by 85.5% and 1.6 dB/mm.