Self-loosening Mechanism Of Single/Double Bolted Joints Under Dynamic Shear Load

Bolted joints have been used extensively in many engineering structures because of theirsimple structure,low cost as well as ease of assembly and disassembly.Self-loosening is an inherent and inevitable failure form of bolted joints under time-varying loads,especially under shear loads.However,in view of poor repeatability and great dispersion in experiments as well as the complexity of loosening behaviour,there is no unified understanding of the self-loosening mechanism.Therefore,the study of loosening mechanism and the influence of various factors on the loosening process has the important significance in the theories and the projects.Based on the fatigue testing machine,this study independently designed two sets of bolt loosening test devices by modifying the test device,and successfully simulated the loosening process of single/double bolted joints under dynamic shear load.The loosening process of single/double bolted joints under dynamic shear load has been investigated under many different experimental parameters,and the loosening behavior of single bolted joints of different lubrication conditions under shear load are systematically investigated.Combining the microscopic analysis using optical microscope?OM?,scanning electron microscope?SEM?and electron energy spectrum?EDX?,this paper reveals the loosening characteristics and loosening mechanism of the single/double bolt connection structure under shear load are revealed.The main conclusions obtained in this study are as follows:???Self-loosening mechanisms of bolted joints subjected to dynamic shear loadThe effects of many factors,such as initial tightening torque,shear load amplitude,thread fit and different materials of fasteners on the loosening behaviour of single bolted joints are systematically investigated.The study reveals that the loosening mechanism of single bolted joints under dynamic shear load are the slip between the contact interface and the fretting wear on the thread surface.The study finds that It is found that the loosening process of single bolted joints under dynamic shear load presents three stages.The research shows that the factors,the shear load amplitude,the initial tightening torque,the class of thread fit,loading form and the material of bolts/nuts,have a significant influence on the loose behavior and damage behavior of bolted joints.The relevant conclusions are as follows:?1?Under the same initial tightening torques,when the shear load amplitude is low?F_e?6 kN?,the self-loosening process can be generally divided into three stages,including a stable stage,a rapid declining stage,and a slow declining stage,the loosening degree increases linearly with the increase of load amplitude.The damage of the thread surface exacerbates with the increase of load amplitude and the damage mechanism is abrasive wear,fatigue wear and slight oxidation wear.When the shear load amplitude is large,the loosening process shows only exhibits two stages including a stable stage and a rapid declining stage,and eventually it becomes completely loosening,the damage of the thread surface is slight and the damage mechanism is abrasive wear.?2?Under the same shear load amplitude,the loosening degree of bolted joints increases with the increase of the initial tightening torques.When the initial tightening torque is large,the self-loosening process can be divided into three stages and the damage of the thread surface decreased with the increase of load amplitude.When the initial tightening torque is low,the self-loosening process can be divided into four stages and the connection structure is completely loosening,the damage of the thread surface is slight.?3?Under the same initial tightening torques,the loosening process for three thread fit combinations under dynamic shear load shows three stages of characteristics and the loosening degree decreases significantly for fit combinations from loosest to tightest.In addition,the microscopic damage shows that the damage of the thread surface for three thread fit combinations is roughly similar.The main wear mechanism is abrasive wear and fatigue wear.Under tight fit precision?4h/6H?,the fretting damage on the thread surface is the slightest.?4?Under the same initial tightening torques,due to the high friction coefficient between contact threads,bolted joints made of stainless steel have a poor anti-loosening property,while bolted joints made of QT steel have a significant anti-loosening performance,however,the dispersion of the loosening degree is poor.Under the same preload,bolted joints made of QT steel have better anti-loose performance and reliability than the bolted joints made of 1045steel.While,bolted joints made of stainless steel is not easy to slip because of its high friction coefficient,the fretting damage on the thread surface is slight and they have better anti-loosening performance than the bolted joints made of 1045 steel.???Loosening behavior of bolted joints subjected to dynamic shear load under various lubrication conditions.The loosening tests of three typical coated?PTFE,MoS₂ and TiN?bolted joints under dynamic shear load are carried out under two test parameters?the same initial tightening torque and the same preload?.The loosening behavior of bolted joints with two typical lubricating greases?MoS₂ grease and 85%lithium grease?and lubricating methods?local lubrication and full lubrication?under shear load are systematically investigated.The research results show that:?1?Under the same initial tightening torque,PTFE coated and MoS₂ coated bolted joints have a higher preload due to their low friction coefficient at the interface.Therefore,the contact interface is not easy to slip,the fretting damage on the thread surface is slight and PTFE coated and MoS₂ coated bolts have better anti-loosening performance.Conversely,due to the high coefficient of friction between the contact threads,the preload for TiN coated bolts is lower than that for the bolted joints of the other two coating under the same tightening torque,which easily lead to loosening.Under the same preload,three kinds of coating fasteners have the anti-loosening performances compared with uncoated bolted joints.PTFE coated bolted joints has the best anti-loosening performance,MoS₂ coated follows.TiN coated bolted joints gains a certain anti-loosening performance under the same preload,but the increaseis not obvious compared with uncoated bolted joints.In addition,it is not conducive to pre-tightening because of its high friction coefficient.?2?The damage of the thread surfaces of PTFE?MoS₂?coated bolted joints exacerbates with the decrease of initial tightening torque.Under a lower initial tightening torque,the damage of PTFE?MoS₂?coated bolt is more serious and the main wear mechanism is abrasive wear,delamination and slight oxidation wear which indicates that fretting wear is one of the loosening mechanisms of coated bolted joints.?3?When bolted joints adopts grease?MoS₂ grease and lithium-based grease?in the anti-loosening field,local lubrication reduces the threshold value and complete lubrication greatly reduces the threshold value,thereby reducing the anti-loosening performance of bolted joints.On the other hand,when the shear load is low?the amplitude is below the threshold value?,using local lubricating relieves the fretting wear of the thread surfaces,reduce the loosening degree of bolted joints.???Comparative study on loosening behavior of different bolted joints.The effects of many factors,such as the way of tightening bolt,forms and levels of loading, cyclic numbers and wire thread insert on the loosening behaviour of double bolted joints are investigated systematically.The generality and difference of loosing behavior of single/double bolted joints under shear load are summarized.?1?The loosening process of single/double bolted connection can be generally divided into three stages and the loosening mechanism of the two bolted joints is closely related to fretting wear on the thread surface.The loosening degree and and its dispersity increases with the increase of the shear load amplitude.?2?Using double bolted joints can reduce the loosening degree,however,it aggravates significantly the scatter of the loosening degree.?3?This research summarizes five anti-loosening measures,which provide theoretical guidance for anti-loosening of bolt or bolt groups under various shear loads.