The roles of conductivity in eddy current distributions in nondestructive testing of isotropic and anisotropic materials

The investigation of eddy current (EC) distribution is of paramount importance in enhancing damage detection and evaluation in eddy current testing (ECT) of conductive materials. The conductivity of material plays a decisive role in EC distribution, yet the specific mechanisms by which conductivity influences EC distribution remain unclear. This study employs the finite element (FE) method to explore EC distributions in materials with various conductivity characteristics under pancake-coil excitation, accompanied by a comprehensive analysis of the underlying mechanisms. In isotropic materials, as conductivity rises, the in-plane distribution of EC becomes increasingly concentrated beneath the coil windings. This phenomenon is elucidated through the proposed three-coil model. Concurrently, the rate of EC attenuation approaches that observed under uniform-field excitation. For carbon fibre reinforced polymer (CFRP), it has been reported that the EC has a faster rate of attenuation in the normal direction in unidirectional CFRP than in isotropic material and the EC density exhibits a nonmonotonic variation characteristic in the normal direction in multidirectional CFRP. The mechanisms analyses for the phenomena are proposed in this paper. These findings and analyses fundamentally elucidate the effects of conductivity on EC distributions, offering valuable insights for the identification and localisation of potential defects.