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. 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. Experiments were performed on coated samples with artificial cracks. The crack contrast was analysed under coatings of various thickness. The contrast in amplitude and phase is decreasing with coating thickness. The effect of infrared transparency of a model coating was shown in experiment.