Evaluation of defects in composite plates under convective environments using lock-in thermography

Lock-in thermography is a technique which is increasingly being used for the evaluation of subsurface defects in composite materials such as carbon-fibre-reinforced polymers (CFRPs) in aircraft structures. Most CFRP structures have a finite thickness and non-destructive inspection is performed in a natural ambient environment. In this paper, a photothermal model is developed in order to investigate the behaviour of thermal waves in homogeneous plates and layered plates with finite thicknesses under convective conditions. The model is then utilized to predict the phase differences produced by multi-layer subsurface defects and optimum inspection parameters. The theoretical results are compared with the experimental results. The detectivity of lock-in thermography for CFRP is also presented in this paper.