Linear and nonlinear guided wave based debonding monitoring in CFRP-reinforced steel structures

When carbon fibre reinforced polymer (CFRP) is employed to reinforce the damaged steel structures, the debonding problem existing at the interfaces between two materials can be fatal for the retrofitted structures. As a result, identifying the debonding from its earliest stage can be helpful, which leaves sufficient time for further maintenance actions. Based on the current research findings, the linear guided wave based detection technique can provide a good measure for identifying the existence of gross damage, but it is not as sensitive as the nonlinear guided wave method when detecting the micro damage. In order to develop an effective way to continuously monitor the debonding growth from its initiation, both linear and nonlinear methods were investigated and compared in terms of debonding evaluation in this paper. A fatigue test was carried out on a CFRP-reinforced steel structure, in which a small piece of Teflon film was artificially inserted, to facilitate the debonding development. The correlation coefficient (CC) as well as the damage index (DI) between the benchmark signals and signals after certain cycles of fatigue test were established using both methods. Furthermore, the probability based imaging method was applied to individually show the possible debonding areas based on the data from each method. The finding demonstrated different damage detection sensitivities between the linear and nonlinear guided wave based methods and verified the feasibility of monitoring the whole debonding process by combining these two methods, namely, the nonlinear method for debonding initiation and linear method for debonding growth monitoring.