Characterization of θ-bonding interface and delamination evaluation in composite structures using asymmetrical-frequency ultrasonic technique

Adhesive bonding is an important form of joint in composite structure. Joint quality of bonding interface directly affects the overall performance of the composite structures. Therefore, it is usually required to conduct 100% non-destructive testing and evaluation (NDT&E) on bonding quality. The normal incidence ultrasonic longitudinal wave reflection method is an effective detection technique. When there is a θ angle between the bonding interface and the surface of the part to be tested, a θ-bonding interface is formed. Because of the lack of ideal ultrasonic incident angle at the θ-bonding interface, it is difficult to detect and evaluate quality of θ-bonding interface. An asymmetric-frequency ultrasonic (AU) method was studied for the NDT&E of θ-bonding interface in carbon fiber laminate structures. An ultrasonic detection system with different frequency modes was developed, and θ-bonding specimens were designed and manufactured. The time-domain behavior and cross-sectional imaging characteristics from different frequency mode ultrasonic waves at θ-bonding interfaces are studied. The experiments demonstrated that the quality and time-domain resolution of the echo signal under AU mode are obviously better than those of the symmetric-frequency ultrasonic (SU) method, and the imaging quality and spatial resolution of the AU cross-section are obviously improved. According to the imaging gray-scale distribution characteristics, the interface behavior of θ-bonding interface region and its size, θ-bonding interface topography, θ-bonding interface and disbands in it, θ-bonding interface thickness, and the change of geometric features around θ-bonding interface region can be effectively identified. The minimum deviation between the ultrasonically estimated and mechanical measured average thicknesses reaches to 0.10 mm for 4.5 mm thick carbon-fiber composite specimen. The tilted bonding interface has no effect on disbands detectability using the AU technique. Thus, it provides an effective nondestructive characterization and reliable defect evaluation method for θ-bonding interface and impact pits in composite structures. A very good practical application has been achieved in composite structures.