Nondestructive evaluation of debonding in composites using air-coupled coda wave analysis and local defect resonance techniques

Low acoustic energy conversion efficiency is a major challenge for air-coupled ultrasonic technology. In the determination of the lift-off distance of air-coupled sensors, there is a balance between the acoustic energy attenuation and the difficulty of extracting defect information. In this study, an air-coupled local defect resonance (LDR) technique with coda wave analysis is proposed for the nondestructive evaluation of debonding in composites. A sensor consisting of 19 elements was used to simultaneously excite and receive ultrasonic waves. Air-coupled LDR experiments were conducted on the two types of composite structures. The effects of sensor lift-off distance and coda wave analysis on the performance of the LDR technique were investigated. It was found that the sensor lift-off distance and the coda wave analysis had a significant effect on the defect detection capability of the LDR technique. For composites, the optimal sensor lift-off distance was found to be between 3.5λ and 5.5λ, where λ is the wavelength. Compared to multiple reflection echoes, the coda waves are more suitable for identifying the damage in composites. The proposed non-contact ultrasonic technique effectively reduces the required incident acoustic energy and can be used for efficient detection of debonding in composites.