Abstract. Bolted joint is one of the most widely used connections because of relatively simple structure, reliability, easy disassembly and maintenance. However, self-loosening of bolted joints under cyclic external loading is still an unsolved issue and need to make a further research. As is known to all, the self-loosening process can be divided into two distinguishable stages under cyclic transverse loading. The first stage is featured by a short and sharp clamping force reduction with no relative rotation between the nut and the bolt. The loss of the clamping force is due to the accumulation of local cyclic plastic deformation at the root of the engaged thread, which is account for no more than 10-15%. Therefore, the second stage is the primary cause of self-loosening. In this study, a further research on the second stage self-loosening of bolted joints is presented by the ways of combining theoretical analysis with the three-dimensional finite element simulation. The nature of the second stage self-loosening of bolted joints lies in: 1) the variation of the contact pressure between the engaging thread surfaces, 2) lateral micro-slip between the engaging thread surfaces. Both of them result in an apparent slip between the engaging thread surfaces of bolt and nut which finally leads to the self-loosening of bolted joints. In addition, the effects of preload, amplitude of cyclic transverse loading and the friction coefficient of engaging surfaces on self-loosening are studied by the finite element simulation. Moreover, a reasonable explanation of the trend about the self-loosening of bolted joints is given. The corresponding results are very important for the bolted design. It can be concluded that reasonable preload may improve the ability of bolted joints resisting to self-loosening.
Bolted joints is one of the most widely used connections due to relatively simple structure, easy accessibility and reliability. However, Bolted Joints will gradually lose the clamping force under cyclic external loading, especially transverse loading. In order to fully reveal the nature of early stage self-loosening of bolted joints, in view of the common bolted joints of grade 6.8, this paper firstly establishes the elastic finite element model, and analyzes the change of clamping force under different pre displacement. The results show that the clamping force is proportional to the pre displacement. Then, in view of the common bolted joints of grade 6.8 under transverse cyclic loading, this paper establishes the elastic-plastic finite element model, and simulates the process of early stage self-loosening of bolted joints under different lateral cyclic load amplitude, different initial preload, and different clamping surface friction coefficient. It can be seen that the early stage self-loosening of bolted joints under lateral cyclic loading is due to the accumulation of local cyclic plastic deformation at the root of the engaged thread. The bigger lateral cyclic loading amplitude leads to the faster self-loosening. The greater preload will result in a greater remaining clamping force. There is almost no relationship between the friction coefficient of clamping surface and the self-loosening of bolted joints. Reasonable preload can effectively improve the bolted connection relaxation.
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