Insensitive effect of crack length in non-electrically conductive materials on the displacement current field in electromagnetic induction testing

Abstract

Electromagnetic induction testing (EIT), an emerging extension of eddy current testing (ECT), has enabled the evaluation of non-electrically conductive materials by utilizing displacement currents generated by applying a high-frequency alternating current voltage. However, the fundamental detection mechanisms, particularly for crack characterization, remain inadequately understood. In this study, the effect of crack length on the displacement current field in EIT was investigated through finite element analysis (FEA) and experiments. FEA was performed to calculate eddy and displacement current fields in both electrically and non-electrically conductive materials with cracks of different lengths. The FEA results showed that the eddy current field changed significantly even for short crack lengths, whereas the displacement current field changed significantly only when the crack was positioned directly beneath the driver coil. In the experiments, different crack lengths were introduced into carbon fiber reinforced thermoplastics (CFRTPs) and glass fiber reinforced plastics (GFRPs), both exhibiting in-plane electrically isotropic properties detectable by EIT. In CFRTPs, cracks could be measured regardless of their length, whereas in GFRPs, they could be measured only when they extended beneath the probe. These findings indicate that the displacement current field in non-electrically conductive materials is relatively insensitive to crack length, highlighting the importance of probe positioning when EIT is performed. These insights can aid in optimizing non-destructive evaluation protocols for composite structures in various field applications.