Eddy current pulsed thermography with an inductive heating layer (ECPT-IHL) for subsurface defect detection in GFRP materials

Due to its induction heating mechanism, eddy current pulsed thermography (ECPT) has been applied to detect defects in conductive materials instead of non-conductive materials, such as glass fiber-reinforced polymer (GFRP). In order to extend the application of ECPT for nondestructive testing (NDT) of non-conductive materials, this study proposes a novel method named eddy current pulsed thermography with an inductive heating layer (ECPT-IHL). This method introduces a temporary inductive heating layer to the surface of the non-conductive materials as an auxiliary heat-generating component. Firstly, the detection mechanisms of ECPT-IHL in transmission detection mode (TDM) and reflection detection mode (RDM) are analyzed, respectively. Then, the feasibility of ECPT-IHL is verified by conducting experiments on GFRP specimens with flat-bottom back-drilled hole (FBH) defects under TDM and RDM modes. In addition, the influence of different inductive heating layers (including Q235 steel plate, 430 stainless steel foil, and flexible graphite sheet) on the defect detection effect in the two detection modes is investigated to optimize the detection performance. Finally, the detection capability of ECPT-IHL is evaluated by detecting FBH defects of different depths, and a feasible and reliable method is proposed to characterize the defect depths in TDM based on the extracted temperature difference. Notably, due to the translucence of GFRP materials, the abnormal temperature response caused by the coupling of heat radiation and heat conduction is also discussed. The results above show that ECPT-IHL successfully extends the application of traditional ECPT to non-conductive materials and is effective in detecting minor defects in GFRP laminates.