Detection of surface cracks in metals under coatings by induction thermography

The effect of non-metallic polymer coatings on the induction thermographic signal from metal surfaces with cracks was studied. Experiments were performed on coated samples with artificial cracks. Analytical calculations for the backward thermal wave propagation were performed both in the frequency and in the time domain. Numerical simulations were used to show the thermal patterns of cracks under coatings. In the experiments, the crack contrast was analysed under coatings with various thickness. The contract in amplitude and phase is decreasing with coating thickness and they were compared with optical excitation methods. The effect of the infrared transparency and reflectivity of a model coating were shown in experiments.