Inverse heat transfer approach for nondestructive estimation the size and depth of subsurface defects of CFRP composite using lock-in thermography

An inverse heat transfer approach is developed to characterize the size and depth of subsurface defects in CFRP composite materials through the reconstruction phase profile of lock-in thermography (LIT) obtained by finite element simulation. This work mainly focuses on the application of hybrid method that integrates simulation annealing algorithm (SA) and Nelder-Mead simplex search method (NM) for determination of the sizes and depths of subsurface defects within the CFRP laminate materials. For this purpose, an 808 nm laser is used for imposing a modulated heat excitation on the CFRP laminate specimen, and the thermal images are collected using an infrared camera. The hybrid method is employed to find the optimal solutions of the objective or cost function constructed by phase profile of LIT between an experimental configuration and numerical solution for a CFRP specimen. The experimental results show that the size and depth of subsurface defects are effectively obtained through inverse solving the constructed objective or cost function by the hybrid method. The estimated maximum errors for the size and depth are less than 5% and 4% for given subsurface defects by the proposed method, respectively.