Discriminate the substrate crack under sprayed coatings using ultrasonic infrared thermography

Abstract

It has always been a challenge for most non-destructive testing (NDT) methods to discriminate the crack defects in substrate beneath sprayed coatings, owing to the interruption to testing signal by coating cracks. Ultrasonic infrared thermography (UIT) was employed to solve this problem in the current work. Coating crack and substrate crack were regarded as heat sources lying at different depth when excited by ultrasonic vibration. Coherent interference of thermal waves emitted from coating crack and substrate crack was introduced theoretically, and phase deviation (PD) of mixed thermal wave on surface was draw out as the discriminating feature for the deeper heat source, scilicet the substrate crack. Finite element modelling (FEM) results suggested that PD was more intensive in lower frequency range than that in higher frequency range. The proposed PD feature was validated experimentally with a plate specimen in which coating crack and substrate crack generated artificially under controlled monaxial cycle tensile load, experimental results were in qualitative agreement with FEM results. In addition, both FEM results and experimental results indicated that PD was insensitive to the heating intensity of crack defects under ultrasonic excitation, which was deemed to be a main unrepeatable parameter in UIT.