Full-range bolt preload monitoring with multi-resolution using the time shifts of the direct wave and coda waves

A bolt may undergo a process from slight preload perturbation to significant preload reduction during its service life, it is a challenge to simultaneously detect both slight and significant bolt looseness based on the same waveform. Previous studies were prone to use direct wave or coda waves individually, which is suitable for a narrow range of bolt preload and cannot realize the full-range bolt preload reduction. To address this challenge, this article proposes a novel full-range bolt preload monitoring approach with multi-resolution using the time shifts of the direct wave and coda waves. Due to the difference in propagation paths, preload reduction induced time shift varies considerably between the direct wave and coda waves. By calculating the time shifts of the direct wave and coda waves, bolt looseness, including the slight preload perturbation and the significant preload reduction, can be simultaneously estimated. Based on the acoustoelastic effect and the wave path summation, a theoretical model is established, and the monitoring principles are presented. The analytical results show that the time shifts of the direct wave and coda waves are proportional to the preload variations. Validation experiments are carried out, and the results show that at specific preloading levels, the ultimate detectable resolutions of bolt preload (DRBP) using direct wave and coda waves are 1.68% and 0.32%, respectively. Therefore, the ultimate DRBP of the direct wave is 5.25 times that of the coda waves, indicating that the coda waves can be used as the "probe" with high resolution to detect tiny perturbations of preload, and the direct wave can function as a low-resolution "probe" to monitor significant preload deterioration. By calculating the time shifts of the direct wave and the coda waves, bolt looseness with different degrees can be monitored simultaneously.