In traditional meander line coil electromagnetic acoustic transducer （MLC-EMAT） structures， the bias magnetic field is usually set to be along the normal direction of plate surface. However， since the particle vibration of the antisymmetric Lamb wave is always dominated by out-of-plane components， using bias magnetic field perpendicular to plate surface is kind of inefficient. In this paper， the performance of both the normal bias magnetic field EMAT （NB-EMAT） and the parallel bias magnetic field EMAT （PB-EMAT） for transmitting and receiving A0 mode Lamb waves are thoroughly studied. The mechanisms of these two structures are elaborated. First， the finite element models of both structures are established. The magnetic fields of these two EMATs are numerically calculated and the results are compared with experiments. Then， the Lorentz force distributions excited by the two EMATs are compared to prove the feasibility of improving the excitation efficiency of MLC-EMAT by selecting the direction of bias magnetic field. Furthermore， the excitation efficiencies of NB-EMAT and PB-EMAT are quantitatively analyzed and compared in simulation software. Results show that the excitation efficiency of PB-EMAT is 108% higher than NB-EMAT. Finally， several groups of comparative experiments are conducted to verify the conclusion obtained through numerical calculation. Experimental results show that by simply replacing the tradition NB-EMAT with PB-EMAT， the excitation efficiency can be greatly increased by more than 50%. If PB-EMATs are used as both the receiver and transmitter， the excitation efficiency can be further increased by 113%.
Zongmin et al. (Mon,) studied this question.