Effect of a Crack on the Displacement Current Field of Non–electrically Conductive Materials via Electromagnetic Induction Testing

The range of application of eddy current testing (ECT) has been recently extended to non-electrically conductive materials, in which case it is called electromagnetic induction testing (EIT) given that EIT detects changes in the electromagnetic field of the displacement current. Although EIT has been reported for non-destructive characterization of non-electrically conductive materials, its detection principle is still unclear. We used finite element analysis (FEA) and experiments to evaluate the effect of cracks on the displacement current field in non-electrically conductive materials for crack detection using EIT. FEA was performed on electrically and non-electrically conductive materials with slits simulating cracks. The FEA results showed that crack detection differed between materials, as eddy currents bypassed the crack, while displacement currents passed through and formed a current path. Furthermore, the electric field intensity and displacement current induced in the non-electrically conductive materials varied significantly in the cracked area compared with the uncracked area. Experiments were conducted to detect cracks in carbon fiber reinforced thermoplastics (CFRTPs) and glass fiber reinforced plastics (GFRPs), which have isotropic electrical properties in the in-plane direction. In the CFRTP, the electromagnetic field varied significantly even at locations far from the crack, whereas it changed only slightly near the crack in the GFRP. This result demonstrates that for non-electrically conductive materials, EIT can identify cracks by detecting localized changes in the displacement current flowing through the cracks. Our findings can help clarify the principle of crack detection in non-electrically conductive materials, thereby extending the application of EIT to these materials.