Multi-resolution bolt preload monitoring based on the acoustoelastic effect of ultrasonic guided waves

During the long-time service of a bolt, its preload may suffer slight perturbations or significant reductions. It is a dilemma to monitor preload changes at high resolution and full scale. Approaches for bolt preload monitoring with multiresolution should be developed. In this paper, a simple and effective multi-resolution bolt preload monitoring approach using ultrasonic guided waves (UGW) is proposed. A linear relationship between the time-of-flight (TOF) variation of multi-reflected waves and preload is derived to theoretically reveal the multi-resolution properties of UGW. The variations of TOF before and after the slight preload perturbations are extracted by using a global evaluation method. Experimental results show that the signal-to-noise ratio (SNR) of the 1st, 2nd, and 3rd-reflected UGWs is larger than 20 dB. The resolution of the 2nd-reflected UGW is higher than that of the 1st-reflected UGW and lower than that of the 3rd-reflected UGW. The ultimate detectable resolutions of bolt preload (DRBP) of the 1st and 3th-reflected UGWs are 0.9% and 0.5%, respectively. By using the 1st and 3th-reflected guided waves, the bolt looseness with different degrees can be monitored simultaneously.