Nonlinear Resonance-Based Monitoring of Bolt Loosening in a Slender Mast-Like Structure

Bolted joints in slender civil structures such as public lighting masts may undergo progressive changes in clamp force, contact condition, and energy dissipation under vibration, fretting, and environmental exposure. While many assessment approaches rely on static inspections, linear modal indicators, or nonlinear wave-based methods, this work investigates whether nonlinear resonance measurements can provide informative monitoring features for connection-condition changes in a mast-like bolted structure. A reduced-scale structure representative of a mast base connection was tested using bonded PZT transducers integrated on the accessible mast tube. A nonlinear resonance protocol combining increasing-excitation measurements with conditioned decreasing-excitation measurements was applied to progressively degraded joint states induced by controlled bolt back-off. Resonance frequency and quality factor were tracked as functions of excitation level. The results reveal a baseline resonance downshift, increasing excitation-level dependence of resonance frequency and quality factor, and separation between forward and conditioned reverse trajectories, indicating hysteresis and loading-history dependence. To support structural-level interpretation, a framework based on effective stiffness decomposition into reversible and hysteretic contributions is introduced. Within this framework, the measured quantities are interpreted as protocol-dependent observables of the jointed structure rather than direct intrinsic interface properties. The study shows that conditioned nonlinear resonance measurements provide informative monitoring features for connection-condition changes induced by controlled loosening. The main contribution lies in combining a conditioned nonlinear resonance protocol, a modal-scale interpretation framework, and bonded PZT transducers to interrogate a poorly accessible bolted connection through a structural mode.