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.在涡流检测（eddy current testing, EC）中，集肤效应（skin effect）是制约亚表面缺陷（subsurface defects）检测的关键因素，而集肤深度（skin depth）则是探头设计与频率配置的重要参数。本文针对铁磁性金属（ferromagnetic metals）与非铁磁性金属（nonferromagnetic metals）中的集肤效应展开对比研究。众所周知，铁磁性金属的高磁导率（high permeability）会因更大的涡流损耗（eddy current loss, EC loss）而加剧涡流的衰减。本文则报道了铁磁性金属高磁导率对集肤效应的另一重作用。本文计算了饼形线圈（pancake coils）激励下两种材料内涡流在径向（radial direction）与深度方向（depth direction）上的分布情况。结果表明，高磁导率材料内的涡流在径向分布更为弥散。更为重要的是，铁磁性金属的高磁导率使得饼形线圈激励下的涡流集肤深度趋近于均匀磁场激励下涡流的标准集肤深度；而在两种激励方式下，非铁磁性材料的涡流集肤深度则存在显著差异。基于涡流扩散抵消理论（diffusion-cancellation theory），该差异可通过涡流的径向弥散效应予以解释。最后，本文对比了铁磁性Q235钢与非铁磁性黄铜（brass）内的涡流分布，并通过实验对仿真结果进行了定性验证。