Monitoring of crack propagation in fibre-reinforced-polymer (FRP) strengthened steel plates using guided waves

The application of Lamb waves to monitor fatigue crack propagation and to detect fatigue crack initiation in carbon fibre reinforced polymer (CFRP) strengthened steel plates is investigated. A literature review of recent developments and advances of Lamb wave-based structural health monitoring (SHM) applications is presented. The characteristics of Lamb waves, current signal processing and feature extraction technique, and damage-detection techniques are studied. The mechanism of nonlinear Lamb waves, especially contact acoustic nonlinearity (CAN), is investigated, along with latest numerical and experimental achievements and findings of nonlinear Lamb wave-based SHM. This study consists of two parts, monitoring fatigue crack propagation and detecting crack initiation. Two methods are proposed accordingly. The first uses linear Lamb waves to monitor the fatigue crack propagation. 3-dimensional Finite element (FE) models are developed in Abaqus to assess the proposed linear method, along with Lab-based experimental validation. Based on the results, it is concluded that Lamb waves are sensitive to the concealed fatigue cracks, and Lamb wave-based SHM is a promising technique to monitor fatigue crack propagation. The second method employs nonlinear Lamb waves to detect the initiation of fatigue crack because of their remarkable sensitivity to small-scale damage. The principles of wave nonlinearity are presented and the mechanics of CAN is discussed. The proposed nonlinear method is first assessed in steel plates. The effects of CFRP laminates on Lamb waves are investigated and the signals captured in steel plates with/without CFRP laminates are compared. The simulation and experimental results are presented and discussed.