The use of induction heating for thermographic inspection of composite laminates

Abstract

In the present paper, an active thermographic technique is proposed for the inspection of a unidirectional GFRP panel with integrated artificial internal defects and an aeronautical CFRP panel with real small production defects.Different inductor geometries were considered by varying the length and the number of spires, finally allowing to identify the optimal solution. Several tests allowed to identify the optimal experimental setup for defect detection as a function of the location of the defect.The acquired thermal maps were processed in Matlab with a suitable analysis algorithm for the determination of the absolute and normalized thermal contrasts. Another important consideration concerns the possibility of obtaining a qualitative indication on the defect depth. Although the thermal contrast depends on the choice of the intact and defective area, the maximum value of the absolute contrast was higher for the more superficial defects than for the deeper ones. Furthermore, the absolute contrast was correlated with the size of the defects. In the study of CFRP panel, the defects characterization was conducted in a comparable way to evaluate the contrasts with a suitable choice of undamaged reference area.The comparison of the proposed technique with the traditional pulsed technique has demonstrated the real potential of application on SMArt GFRP and aeronautical CFRP elements with the same level of reliability as traditional thermographic techniques. However, the induction technique has shown better ability to detect small real defects more accurately in CFRP panels. However, the limitation of the power generator and the size of the inductor could limit the detectability of defects.