Debonding detection in scarf repaired composite structures based on ultrasonic guided wave phase delay

With the increasing application of composite materials in modern aircraft structures, the demand for reliable repair techniques has grown significantly. Scarf repair is one of the most widely used methods; however, the integrity of the bonded interface is critical, as it is susceptible to debonding under cyclic loading or hygrothermal environments, leading to stress concentration and potential structural failure. This study proposes a debonding monitoring method for scarf-repaired composite structures based on ultrasonic guided wave phase delay. By combining a multi-frequency fusion probability-weighted imaging algorithm with the phase shift – debonding length relationship, the proposed method enables both accurate damage localisation and contour identification without relying on baseline signals. Finite element simulations and experimental validations are conducted to verify the approach. The simulation results yield an average damage centre localisation error of 2.8 mm and a contour recognition coverage (Rec) of 78%. Experimental results demonstrated an average localisation error of 3.4 mm and a Rec of 76%. Furthermore, the method exhibited strong robustness against temperature variations, with minimal performance deviation observed at an elevated temperature of 60°C. These results confirm the method’s feasibility, effectiveness, and environmental stability for real-time structural health monitoring of scarf-repaired composite structures.