Enhanced loosening evaluation of CFRP lap-bolted joints using out-of-plane-dominated resonant nonlinear ultrasonics

Carbon fibre reinforced polymer (CFRP) lap-bolted joints are widely used in aerospace and mechanical engineering, however, early bolt loosening is difficult to detect using conventional linear ultrasonic methods. Nonlinear ultrasonic techniques offer high sensitivity to variations at the contact interface, but their weak nonlinear responses are easily masked, limiting detection reliability. This study proposes a successive computing methodology (SCM) that integrates finite element (FE) modelling, ultrasonic analysis, and modal identification to enhance bolt loosening evaluation in CFRP lap-bolted joints using a novel out-of-plane dominated resonant nonlinear ultrasonic (OPR-NLU) method. A three-dimensional FE model is developed to simulate bolt loosening and ultrasonic wave propagation, and linear perturbation analysis is employed to distinguish OPR modes, in-plane resonance (IPR) modes, and non-resonant (NR) frequencies. Experimental validation is performed using a laser scanning vibrometer system. The results demonstrate that resonant excitation amplifies nonlinear ultrasonic responses, with OPR providing a stronger enhancement than IPR. The sensitivity for bolt loosening evaluation follows the order ‘OPR > IPR > NR’. The proposed method significantly improves the sensitivity and stability of early bolt loosening evaluation, contributing to a reliable approach for structural health monitoring of CFRP lap-bolted joints.