Experimental study on behavior of time-related preload relaxation for bolted joints subjected to vibration in different directions

The effect of preload relaxation caused by the mechanical vibration on loosening of bolted joints with self-locking capability has been widely observed. This paper proposed a combination testing strategy involving tightening and relaxation to elucidate the loss mechanism of bolt preload. A logarithmic relationship between torque coefficient and tightening torque is reported based on the results of fastening tests. Two types of specimen were designed to evaluate the preload relaxation under axial and transverse vibration. A time-varying model is proposed for predicting the residual preload of bolts. An orthogonal test involving tightening torque, amplitude, and frequency was conducted. The results show that the relaxation evolution of bolted joints subjected to vibration is similar and can be divided into two stages. •A combination testing strategy of tightening and relaxation was proposed to improve the credibility of experimental result.•A logarithmic relationship of the torque coefficient and tightening torque was first reported through the fastening tests.•The relaxation evolution process of bolted joints subjected to axial or transverse vibration was similar and divided into two stages.•A time-varying model for predicting the residual preload of bolted joints was proposed.