Self-loosening Analysis of Bolted Joints

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.