Air-coupled guided wave inspection of aerospace composites

• Main Author: Low, Jun Feng
• Other Authors: Alex Yan Qingyu
• Format: Final Year Project
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Materials
• Online Access: https://hdl.handle.net/10356/153047
• Institution: Nanyang Technological University

Composites are known to have highly tunable properties to meet the requirements for a wide variety of applications. Most notably, they have been heavily utilised in aircrafts. Non-invasive and non-destructive testing (NDT) is required to monitor the structural health of such vehicles. Currently, water-jet coupled ultrasound testing is used for the non-destructive testing of aircraft. However, the use of water-jet NDT to inspect ¬honeycomb structures in aircrafts gives rise to a danger of water ingression that may induce degradation within the composites. Therefore, the implementation of a non-contact NDT technique for the purpose of aircraft inspection is desirable. One proposed solution is to replace the use of water with a less intrusive couplant that is still able to conform well to surfaces. The potential solution is Air-Coupled Ultrasound Testing (ACUT) which uses atmospheric air as the couplant for ultrasound investigation. The main problem in using air as a couplant is the high energy loss between interface of energy transfer. Ultrasound inspection uses the interaction of acoustic waves with defects to determine and locate defects. High energy loss with the use of air as a couplant, hinders the transfer of useful information in the acoustic waves required for defect detection. Recently, progress has been made in the development of air-coupled ultrasound transducers to allow acoustic waves, that contain useful information, to be received. There is good potential for the use of Air-Coupled Ultrasound Testing in inspection of aerospace composites. This report aims to evaluate the use of air-coupled guided wave for inspection of aerospace laminates and honeycomb-sandwich structures. A 2D simulation is used to predict the influence of delamination and disbond on air-coupled guided wave signals. Linear scans using air-coupled ultrasound transducers are performed respectively on a 2-ply laminate that contains a delamination defect and a honeycomb-sandwich structure that contains a subsurface disbond. Experimental data is compared to the simulation and some agreement was observed. The results show that the presence of defects changes the amplitude of the leaky guided waves detected by the receiver. While defects can be detected and located using air-coupled transducers, further investigation is required for curved and complex composites that are also used commonly in aerospace applications.