Analysis on the relationship between EMAT transmitting efficiency and the electrical conductivity of the coating on material surface

The conductive or ferromagnetic material with coating layer can be tested by electromagnetic acoustic transducer (EMAT) without pre-treatment. The electro-acoustic converting occurred on the surface of the specimens is mainly concentrated within the skin depth of the eddy current. Thus, the electromagnetic property of the material surface has great influence on the efficiency of EMAT. The eddy current densities induced in the coating of specimens and Lorentz force are calculated with finite element software ANSYS when EMAT is applied to the 20 steel-based specimens coated with different electrical conductivity materials. With shear wave EMAT, the specimens are tested and the relationship between the amplitude of the first echo wave signal and electrical conductivity of the coating is analysed. The theoretical analysis and experimental results show that under the fixed excitation condition, the higher the electrical conductivity of the coating material, the larger the induced eddy current density and Lorentz force are; the increase of the electrical conductivity of the coating is beneficial to the electromagnetic acoustic excitation and the acquisition of higher voltage amplitudes of the echo waves signals.