Comparison of intrinsic and acquired nonlinear effects in photothermal detection of delaminations in composites

Photothermal techniques are based on the harmonic detection of thermal waves at the surface of a material. Several factors may render the thermal wave response nonlinear. Such are intrinsic characteristics as the strong dependence of the thermal properties on temperature and/or acquired features as defects that may open and close (‘kiss bonding’) at the excitation frequency. Several theoretical and experimental studies have been performed to estimate and evaluate the existence of such overtones in the response and the potential exploitation for nondestructive testing. This work is focused on composite materials. Based on precedent experimental studies, this theoretical analysis shows the comparison of the rival effects in the generation of overtones. A finite element model was developed to solve the ‘intrinsic source case’ in comparison to an analytical approximative simulation for the case of overtones originating from the oscillation of a defect (delamination) size. Both models are reduced to a single dimension.