Baseline-free localisation of kissing bonds in bonded structures using nonlinear Lamb wave responses under dual-amplitude excitations

The detection of kissing bonds in bonded structures presents a persistent challenge primarily due to the closed interfaces, which often render conventional linear ultrasonic methods ineffective. Moreover, existing localisation methods typically require baseline data from intact structures, limiting their applicability in variable operational environments. To address these limitations, this study proposes a novel baseline-free nonlinear ultrasonic tomography framework for kissing bonds localisation. The methodology exploits the ‘breathing’ behaviour of closed interfaces during wave propagation. A nonlinear signal difference coefficient (NSDC) is introduced by correlating the nonlinear responses under dual-amplitude excitations along same propagation paths. The response from the low-amplitude excitation is used as an in-situ reference for the response from the high-amplitude one, eliminating the need for baseline data of an intact structure. The NSDC is integrated into a modified reconstruction algorithm for probabilistic inspection of damage (RAPID) with shape factor calibration for semi-quantitative characterisation of defect severity. Experimental validation on aluminium-epoxy bonded specimens with artificial kissing bonds demonstrates accurate localisation with a relative error of approximately 4.18%. This study provides a robust and practical solution for in-service detection of bonded structures without prior knowledge of the intact structural condition.