Debonding Defect Detection by Applying Pulsed, Lock-in and Linear Frequency Modulation Thermal Excitation Methods in the Inspection of Fiber-Reinforced Metal Laminates

Fiber-reinforced metal laminates (FMLs) are characterized by excellent fatigue damage tolerance, impact resistance and easy processing, and they have been widely used in the aviation industry. Three active infrared thermography methods, namely, pulsed thermography (PT), lock-in thermography (LT), and linear frequency modulation (LFM) techniques, were used to detect internal debonding defects in FMLs. The application characteristics of the above three techniques were investigated. The simulation analysis was carried out by using the PT, LT, and LFM methods, and the LFM method proved to be optimal. Infrared thermography experiments were performed by applying pulsed, sine, and LFM excitation methods. In order to improve efficiency of defect detection, principal component analysis (PCA) was adopted to process experimental image sequences. The results show that the optimal defect identification results of FMLs can be obtained by the combination of LFM and PCA techniques.