Research On The Self-Loosening Mechanism Of Bolted Joints Under Axial Excitation

Because of the advantages of simple structure, convenient use, low cost and the like, bolted joints are widely used in many engineering structures. As basic fastening pieces, they have direct influences on the safety and reliability of the structural system. Self-loosening is one of the main failure modes of bolted joints in vibration environment, Self-loosening mechanism has been widely studied by many researchers, in view of its importance and complexity. Due to the differences of the machining accuracy and the surface roughness, reproducibility of the experimental results is poor and the dispersity is large. It is difficult to find the rules of self-loosening, and agreement on self-loosening mechanism has not been achieved. Thus, it is very important to carry out research on bolt loosening, which can not only deepen the understanding of the self-loosening of bolted joints, but also provide theoretical guidance for the selection of anti-loosening method of bolted joints in many fields.Based on a new bolt loosening test rig developed in this study, the loosening process of bolted joints has been successfully reproduced, and loosening tests of the joints of bolt/aluminum part and those of bolt/steel nut have been carried out under various tightening torques/preloads, amplitudes of the axial excitation and other parameters. Three coatings (PTFE, M0S2, TiN) are utilized to treat bolts, and their effects on the anti-loosening performance are studied. For the bolts coated with MoS2, a reasonable preload is obtained using the theoretical method, and its performance of loosening prevention is also investigated. Based on the dynamic analysis, the damage of threads is analyzed using optical microscope (OM), scanning electron microscope (SEM) and energy dispersive X-ray (EDX), and combined with the finite element analysis, the loosening mechanism of bolted joints has been revealed in details.The main accomplished research tasks and obtained results are as follows:(?) The loosening mechanism of bolted joints under axial excitationBased on the analysis of the dynamic response and the damage of threads, and combined with the finite element analysis, it is found that the loosening mechanisms of bolted joints are the plastic deformation of the structure and fretting wear between contact surfaces.(1) The loosening process of bolted joints can be divided into two stages:the axial load of the bolt drops rapidly due to the plastic deformation of the structure and the removal of asperities on contact surfaces in the first stage; because of the ratchet effect of material, plastic deformation increases slowly, and fretting wear between contact surfaces is the main reason of the decrease of the axial load of the bolt in the later stage.(2) It is found that the loosening behaviour of bolted joints is strongly dependent upon the tightening torque/preload, the amplitude of axial excitation and number of cycles. With the increase of the tightening torque/preload, the ratio of the residual axial load of the bolt to preload increases under the same axial excitation, and the damage of threads reduces gradually. The main wear mechanism of the contact surfaces between threads transforms from delamination and adhesive wear into abrasive wear. With the increase of the amplitude of axial excitation, the ratio of the residual axial load of the bolt to preload decreases under the same tightening torque/preload, and the damage of threads increases gradually. The main wear mechanism of the contact surfaces between threads transforms from abrasive wear into delamination. Because the abrasive dust piles up on the contact surfaces and then leaves, the axial load of the bolt appears to firstly increase and then decrease during the experiment, but it presents the overall decrease trend. For the bolted joints (bolt/aluminum part) with a lubrication of MoS2, the ratio of the residual axial load of the bolt to preload gets large under the same axial excitation and the damage of threads gets slight compared with the unlubricated bolted joints (bolt/aluminum part). The main wear mechanism is abrasive wear.(II) The loosening mechanism of bolted joints coated with various coating under axial excitation(1) In the first stage of loosening process, for the bolted joints coated with PTFE/MoS2, due to the low coefficient of friction, there are few asperity on the surfaces of threads, then the change of axial load of the bolt is small. On the contrary, the change of axial load of the bolt coated with TiN is large in this stage.(2) In the second stage of loosening process, due to the poor adhesion property of PTFE coating, the damage of the coating on threads is serious, and the change of axial load of the bolt coated with PTFE is large in this stage. The frictional stress on the threads of the bolt coated with MoS2 is small due to the low coefficient of friction. Coupled with the good adhesion property of MoS2, the damage of the coating on threads gets slight, and the change of axial load of the bolt is small in this stage. For the bolt coated with TiN, the damage of the coating on threads is slight due to its excellent wear resistance, and the change of axial load of the bolt is also small in this stage.(3) For the bolt coated with MoS2, under the same equivalent stress as that of the uncoated bolt at the thread root, the preload increases by nearly 20%. The frictional stress and the dissipated friction energy per unit area decrease, suggesting that the damage of threads gets slight. On the other hand, the cumulative plastic deformation caused by axial excitation at the roots of threads decreases because of the increase of the contact area. The anti-loosening performance can be greatly improved.(?) Accurate modelling and numerical analysis of bolted jointsAn accurate finite element model used to simulate the bolted joints is built up using Matlab and ABAQUS packages, and a theoretical method is used to verify the reliability of the finite element model. The research results are summarized as follows:(1) It is found that the first thread carries about 30% of the total load, and the first three threads carry about 70% of the total load. The first root of the thread has the greatest stress concentration, where is a risk of fatigue fracture. With the increase of the number of the bolt threads, the stress concentration reduces first quickly, then slowly. In addition, the stress concentration at the bottom of the root of the nut threads is smaller than that of the bolt threads.(2) The dissipated friction energy per unit area in the radial direction is largest near the crests of threads, suggesting that the damage in this area is serious. In addition, the damage is slight near the roots of threads, because there is no relative sliding in this area. With the increase of the order of working threads, the dissipated friction energy per unit area in the circumferential direction presents the overall decrease trend, but it does not decrease monotonously. This suggests that the damage of threads gets slight in whole and is not continuous in the circumferential direction.(3) With the increase of the preload, the frictional stress increases slightly, but the slip amplitude and the dissipated friction energy per unit area decrease significantly, leading to slight damage of threads. In addition, the cumulative plastic deformation caused by axial excitation at the roots of threads decrease because of the increase of the contact area. Consequently, the ratio of the residual axial load of the bolt to preload decreases. With the increase of the amplitude of axial excitation, the frictional stress and the dissipated friction energy per unit area increase, leading the damage of threads to get serious. In addition, the cumulative plastic deformation caused by axial excitation at the roots of threads increases. Consequently, the ratio of the residual axial load of the bolt to preload to decreases.(4) The cumulative plastic deformation caused by axial excitation at the roots of threads and the dissipated friction energy per unit area on the thread surface can be reduced by improving the bolt preload and decreasing the friction coefficient between threads and that between bolt head and the clamped body. The anti-loosening performance can be improved.