Theoretical analyses of an alternating current electric flux leakage inspection method and experimental verification.

This work proposes an alternating current electric flux leakage (AC-EFL) testing methodology that can be applied to detect metallic material surface defects. The inspection principles and feasibility of this strategy are described in detail on the basis of electric field (E-field) theory. Theoretical analyses using a three-dimensional (3D) finite element method (FEM) are presented to explain the working principle of this technology. A disturbed E-field caused by defects can be decomposed into a non-conservative (or rotational) E-field that arises from a time-variant magnetic field and a conservative (or irrotational) E-field that results from free charge. 3D FEM simulations were employed to separately analyse the rotational and irrotational E-field distribution inside and outside an AC-carrying aluminium rod with a surface crack. The simulation results demonstrated that, for metallic materials with high conductivity, the conservative E-field due to free charge could be neglected when the frequency of the exciting voltage is high compared to the non-conservative E-field produced by the time-varying magnetic field. Experiments applying a prototype coplanar capacitive probe were also conducted and the experimental results agreed well with those of the theoretical analyses, confirming the signal mechanisms of AC-EFL. [ABSTRACT FROM AUTHOR]