Abstract: Conventional ultrasonic C-scan imaging is suitable for quantitatively determining the debonding of engineering components. However, this method using low-frequency transducer fails to detect the thin coatings due to the overlap of the echoes from the sequential coating surfaces. A new ultrasonic C-scan imaging method based on ultrasonic reflection coefficient amplitude spectrum (URCAS) is proposed for determining the coating debonding. The URCAS extremum and resonant frequency dependence of the reflection coefficient at the debonding interface between coating and substrate is investigated. Two URCAS characters:amplitude fluctuation △R and resonant frequency order n are extracted to reconstruct the C-scan images. An ultrasonic experiment is conducted on an epoxy coating specimen with the thickness of 0.3 mm. A debonding with a size of 10 mm×10 mm is embedded between the epoxy and aluminum substrate. The measured relative error of the debonding area through the C-scan images reconstructed by △R and n are 4.9% and 1.0%, respectively. The proposed ultrasonic method is valid for nondestructive characterization of the debonding of the coatings.

Key words: amplitude spectrum, coatings, interface debonding, resonant frequency, ultrasonic C-scan imaging