Study on self-loosening mechanism of bolted joint under rotational vibration

Highlights

• Proposing the theoretical prediction for bolted joint self-loosening on the free boundary conditions of bolt head and nut

• Two forms of loosening failure are revealed: bolt head loosening; nut loosening.

• The three-dimensional self-loosening failure mechanism map is drawn to more intuitively reveal the failure mechanism.

• Fine finite element mode of bolted joint considering the helical thread and the hexahedral element is established to verify the new theory.

• Complete slipping in the contact surface is the main reason for self-loosening under the rotational vibration.

Abstract

The self-loosening failure of bolt connection will bring a series of problems. Sakai first proposed the self-loosening theory of fixed nut and free bolt head under rotational vibration. Based on this theory, this paper proposed a more extensive self-loosening theoretical criterion under the boundary condition of free nut and free bolt head under rotational vibration. The theoretical analysis shows that there are two forms of self-loosening failure under the new boundary conditions, which are the bolt head self-loosening failure and the nut self-loosening failure. Combining with the finite element analysis, it is shown that the proposed self-loosening theory is in good agreement with the simulation results. It is concluded that the self-loosening mechanism of the first case is caused by the complete slipping of the bolt head bearing surface and the thread surface. In the second case, the self-loosening is caused by the complete slipping of the thread surface and nut bearing surface. Local slipping does not cause loosening failure, which is different from the loosening mechanism under transverse vibration. Finally, the influence of several factors on bolt loosening is studied. The results show that the vibration amplitude and initial preload have great influence on bolt loosening, while the vibration frequency has little effect on bolt loosening.

Introduction

Bolted joints are widely used in modern large equipment for their simplicity, convenient disassembly and high reliability, such as docking between spacecraft, main shaft of wind turbine, petrochemical pipeline and so on [1]. However, when the connecting structure is subjected to the change of external cyclic load, there will be some problems that endanger the safety of the structure, such as loosening failure, fracture failure and so on, which even may lead to catastrophic accidents endangering the safety of person and property. The types of external excitation load that affect the loosening of bolt connection mainly include transverse load, axial load, rotational load and so on.

The earliest study of Junker [2] in 1969 proposed that transverse load was more likely to cause loosening than axial load, which axial vibration would only cause the cyclic increase and decrease of bolt tension, but not cause the continuous decrease of bolt tension. Therefore, lots of studies on bolt loosening focused on transverse load, while the researches on axial load mainly focused on the loosening caused by plastic deformation and fretting wear. Liu [3], [4], [5] studied the bolt loosening under axial load and found that the bolt loosening is divided into two stages. The first stage is the rapid decrease of tension caused by the removal of the asperities on the contact surfaces and the plastic deformation. The second stage is the slow decrease of bolt tension caused by fretting wear. Wright [6] put forward the theoretical formula of the relationship between the bolt tension drop caused by fretting wear and the number of cycles in 1952. Schmitt's [7] research showed that the decrease of bolt tension in early stage under axial load is mainly induced by fretting wear. Yang [8] pointed out that the loss of tension force under axial load is proportional to the plastic deformation of the first cycle.

In terms of transverse load researches, the mechanism of bolt loosening under the transverse vibration is due to the local slippage accumulation on the thread surface [9], [10], [11], [12], [13]. Chen [14] found that the process of bolt self-loosening is similar to the form of "creep slip" under the lateral load by analyzing the motion speed of points in the outer ring of the bolt head, which indirectly indicates that the mechanism of loosening is the accumulation of local slip. Recently, Ding and Gong [15], [16] proposed modified Iwan models to theoretically analyze the mechanism of local slippage accumulation causing self-loosening. They found that the force along the radial direction is the main cause of local slippage accumulation of thread surface, upon which some novel anti-loosening thread were designed to verify the theory. Their research [17] also pointed out that the reduction of tension in the initial stage is due to stress release and stress redistribution by using finite element analysis and experimental study of different bolt thread with helix angle and without helix angle. On the other hand, Kasei [18] concluded that the driving force for the decrease of bolt tension force is the restoring force of elastic torsion deformation of bolt rod in the case of small vibration. Shoji’s [19] study showed that the wedge shape of the thread inclined to the bolt is the main factor of loosening and eccentric nut can effectively prevent self-loosening failure. Moreover, Zhang [20] using the user subroutine UMESHMOTION in ABAQUS studied the wear profile evolution of a threaded surface under transverse load. As for the fretting wear under the shear load, the views of the papers [21], [22], [23] are that the increasing of preload alleviates the fretting wear of thread through decreasing the relative slip, and the necessary thread coating can improve the anti-loosening ability by increasing the surface hardness and reducing the thread wear.

However, there are few studies on the loosening mechanism under rotational vibration. Sakai [24] proposed the theoretical formula of the loosening condition and the critical rotation angle of loosening under rotational vibration in 1978 and he also pointed out that the tension force would decrease caused by fretting wear when the critical rotation angle is not reached. In 2012, Yokoyama [25] adopted Ansys finite element analysis software and experiments to verify the correctness of Sakai's theory, but the premise of Sakai’s theory is under the condition of fixed nut and free bolt head. The self-loosening behavior of MJ and Spiralock threads [26] and hysteresis curves of the resultant torque versus the torsion angle [27] was studied under the boundary condition of fixed nut and free bolt head. In addition, Fujioka [28] and Qin [29], [30] also studied the bolted connection loosening of rotary structure and the influence of loosening on the joint stiffness and dynamic response of rotor.

Sakai [24] loosening theory is under the boundary condition of fixed nut and free bolt head, so this paper puts forward the theoretical criterion of loosening in the case of free nut and free bolt head. According to the theoretical prediction, there are two failure modes under the new boundary conditions, which are the bolt head self-loosening and the nut self-loosening failure. The three-dimensional failure mechanism map is depicted to reveal the loosening mechanism of bolted joint under the rotational vibration. Then, the secondary development of ABAQUS is employed to establish three-dimensional fine hexahedral finite element model to validate the correctness of the proposed theory. Finally, the effects of rotational vibration amplitude, vibration frequency and initial preload on bolt connection self-loosening are analyzed.