Investigation of acoustic emission mechanisms in multi-bolt lap joint structure under harmonic excitation

Monitoring bolt looseness under complex vibration conditions is essential to ensure the safety and reliability of diverse engineering systems. Acoustic emission (AE) techniques have emerged as a promising tool for this purpose, yet their practical applications remain limited due to the complexity of AE signal interpretation and insufficient mechanistic understanding, particularly in multi-bolt assemblies. To address these challenges, this study investigates the mechanisms of AE generation in multi-bolt structures subjected to sinusoidal vibrational excitation (sweep and harmonic excitation tests) through a combined experimental-numerical approach. Sweep excitation tests reveal that AE responses are highly sensitive to structural resonance and exhibit a nonlinear, approximately exponential, dependence on dynamic response. Harmonic excitation tests further demonstrate that AE events occur repeatedly at specific vibration phases, suggesting the strong phase-locked correlations of AE events with structural motion. Finite element simulations also confirm that the variation of AE activity coincides with the evolution of the contact status. The above experimental and simulated results illustrate that AE bursts are likely to predominantly originate from micro-collisions at contact interfaces. Moreover, both numerical contact-status contour and surface morphology observations indicate that the micro-collisions resulted AE events are localized around the upper and lower sides of the contact patches of external bolts. In conclusion, this study establishes a mechanistic correlation among bolt torque, dynamic responses, interfacial contact states, and AE activity, offering a more comprehensive understanding of AE mechanisms in complex bolted assemblies.