Bolt load looseness measurement for slip-critical blind bolt by ultrasonic technique: a feasibility study with calibration and experimental verification

Preload is important for the performance of bolted connections, especially for high strength bolt like slip-critical blind bolt (SCBB). There have been relatively few studies focused on detecting the looseness of blind bolts prior to this research. This article proposes a method based on the acoustoelastic effect to monitor the change in the preload in the bolt and detect the relaxation from the initial preload. The technique is suitable for such blind bolted connection because it only needs to connect with one side of bolted connection, unlike some other bolted connection monitoring methods. Considering that for SCBB, the traditional acoustoelastic technique cannot be applied because it needs the unstressed state of the bolt as baseline. The relationship between looseness of bolt load and change of travelling time is deduced. The measuring objective is then changed to the looseness of bolt load, instead of the bolt load itself. The practical processes of calibration, real-time monitoring and periodical detection are proposed, considering the application on real construction site. The tests on different configurations of SCBBs prove the reliability of the ultrasonic technique based on change in time-of-flight.