Vibro-acoustic modulation (VAM)-inspired structural integrity monitoring and its applications to bolted composite joints

Vibro-acoustic modulation (VAM) – one of the prevailing nonlinear methods for material characterization and structural damage evaluation – is based on the effect of modulation of a low-frequency vibration (pumping vibration) on a high-frequency acoustic wave (probing wave). In this study, the contact acoustic nonlinearity (CAN), associated with changes in the solid-solid interface of a bolted joint under VAM, due to bolt loosening, is explored analytically and experimentally, on which basis a VAM-inspired approach is developed for monitoring structural integrity of bolted joints. Numerical simulation based on a theoretical model with structural nonlinear contact stiffness is implemented, to achieve insight into CAN induced by a loose bolt. A quantitative correlation between vibro-acoustic nonlinear distortion and degree of bolt loosening is ascertained. The developed approach is applied to detect bolt loosening in a composite bolted joint and to evaluate the residual torque of the loose bolt quantitatively. Take a step further, the VAM-based nonlinear approach is compared against an elastic wave-based linear method, underscoring a higher sensitivity to bolt loosening. From early awareness of bolt loosening to continuous evaluation of the bolt loosening progress, this VAM-based approach has provided a cost-effective framework for monitoring the health and integrity of a composite bolted joint.