New Vibro-Acoustic Method Detects Bolt Loosening with 36.5 dB Sensitivity

A new paper from arXiv (2605.29950) introduces a vibro-acoustic method for detecting preload loss in bolted joints, a critical issue in rail-vehicle systems where loosening alters stiffness, damping, and nonlinearity. Existing monitoring techniques often fail to combine controlled shaker tests with sensing of nonlinear features. The team—Berkay Kullukcu, Robin Pianowski, and Dina Hannebauer—used a triaxial accelerometer and a microphone to capture acoustic responses from a bolted demonstrator under varying preload conditions (0%, 20%, 40%, and 80%).

The method employs single-tone excitation at the structure's natural frequency (130 Hz) and frequency-modulated (FM) sweeps from 125 to 135 Hz. Single-tone tests revealed additional high-frequency spectral peaks in the loose state, while FM excitation further distinguished preload levels. Harmonic band power ratios, normalized to the carrier frequency, quantified the nonlinearity: the loose state versus 80% preload showed a 17.5 dB difference for the second harmonic (l=2) and a 36.5 dB difference for the sixth harmonic (l=6). This approach offers a robust, non-invasive way to monitor bolted joint integrity, potentially improving safety and maintenance efficiency in rail systems.