High-frequency guided waves for defect detection in stiffened plate structures

The use of a special type of high-frequency guided ultrasonic wave (coupled Rayleigh-like waves) has been investigated with a view towards applications for the non-destructive testing of aircraft structures. Rayleigh-like waves transfer energy between the plate sides and have good sensitivity for the detection and localisation of small defects on both surfaces of a plate-like structure. Using a combination of evaluation in the time and frequency domain, the defect location and damaged plate side can be accurately determined. The waves can be excited and measured using standard Rayleigh wave wedge transducers in a pulse-echo configuration.

Due to the beating phenomenon, the Rayleigh-like wave can propagate past regions with surface defects or features. This allows for the remote detection of defects in areas where access is restricted by structural features, such as stiffeners and stringers. By selecting appropriate excitation frequency and position, a significant part of the energy can be transmitted behind the obstacle and reflected at the defect, allowing detection. This has been shown experimentally, where surface slots were detected behind multiple stiffeners by standard pulse-echo measurements. The measured signals were verified from Finite Difference (FD) simulations and the detection sensitivity for different defect locations and depths has been discussed.