Size estimation of coating disbonds using the first derivative images in pulsed thermography

Abstract

It has been a challenging task to estimate the size of thermal barrier coating disbonds by pulsed thermography, because thermal images are naturally blurred by lateral heat diffusion around the edge of defect. In the present work the first derivative images were employed to weaken the effect of lateral thermal diffusion, in a similar way as for Full Width at Half Maximum (FWHM) based on thermal images. Theoretical analysis introduced the correspondence of thermal profile and the first derivative profile along diameter of an in-plane defect, it’s found that half maximum points of two side dips in the first derivative profile is much closer to the real defect edge than the half maximum points of positive peak in temperature profile. Experimental results suggested that FWHM-1st based on the first derivative images generated more precise estimated values of coating disbonds than FWHM did. In addition, Evolution of FWHM and FWHM-1st in cooling process also revealed that it’s the early images in regular regime of transient heat conduction give more precise estimation of disbonding size instead of traditionally considered early images just at the final instant of optical pulse.