Comparison of Skin Effects in Ferromagnetic and Nonferromagnetic Metals in Eddy Current Testing

In eddy current (EC) testing, skin effect is a constraint factor of detecting subsurface defects and skin depth is an important parameter for probe design and frequency setup. This paper compares the skin effects in ferromagnetic and nonferromagnetic metals. It is well known that the high permeability of ferromagnetic metal makes the attenuation of EC faster due to the greater EC loss. In this paper, the other role of the high permeability of ferromagnetic metal on the skin effect is reported. The EC distributions in both the radial and depth directions in the two types of materials excited by pancake coils are computed. It shows that the EC is more dispersed in the radial direction in high-permeability material. More importantly, the high permeability of ferromagnetic metal makes the skin depth of EC under the excitation of pancake coil close to the standard skin depth of EC under the excitation of uniform magnetic field, whereas there is a big discrepancy in the skin depths of ECs under the two types of excitations. This difference is explained by the radial dispersion of EC based on the EC diffusion-cancellation theory. Finally, the EC distributions in ferromagnetic Q235 steel and nonferromagnetic brass are compared, and the simulation results are qualitatively verified by experiments.