An approach for defect visualization and identification in composite plate structures using air-coupled guided ultrasound

Composite materials are today widely used in engineering applications because of superior strength-weight ratios offered by them as well as high structural performance and corrosion resistance. However defects such as fibre breakage, matrix cracking, de-bonding and delaminations in composites impact their structural integrity and reliability negatively and NDE techniques to rapidly identify such defects are valuable. Ultrasonic guided waves have over the years emerged as attractive tools for scanning of large structures and recently they have been considered for rapid inspection of plate and pipe installations. Air-coupled ultrasound for generation of Lamb waves is particularly attractive for composite applications in view of the non-contact inspection offered, as well as the possibilities for rapid mechanized scanning. In this paper we present damage identification and visualisation approaches for quasi-isotropic composite plate structures, based on air-coupled plate guided ultrasonic (Lamb) waves. In the implementation demonstrated, an 8-layered quasi-isotropic [0/+45/-45/90]s glass fibre reinforced plastics (GFRP) composite plate is interrogated using air-coupled pitch-catch guided ultrasound. Propagation of Lamb waves in the laminates and their interaction with delaminations of different sizes at various locations along the structure are studied using 3D finite element (FE) analysis. The visualization approach is validated using experiments, leading to quantitative predictions of defect parameters such as sizing, location and depth. The approach is also extended for the inspection of complex composite structural features such as I- and T-sections.