Research On Simulation Prediction Method Of Loosening Life Of Bolted Joint Based On Thread Wear

During the loosening process caused by thread wear,the clamping force decreases slowly and continuously without the rotation between the bolt and the nut.It is difficult to check this loosening since no backing-off of the bolt occurs.If the variation of clamping force with loading cycles,namely the bolt loosening curve,can be accurately predicted,the loosening life of bolted joint corresponding to different residual clamping forces can be obtained.This can be used to guide the interval for repair or replacement of bolted joint and the optimisation of load,structure and material parameter of bolted joint.However,it is difficult to analyse the loosening problem of large-size bolts through experimental methods due to the limited loading capacity of the testing machine.Instead,it is possible to investigate this loosening problem of bolts using simulation methods.Currently,although fretting wear has been proven to cause bolt loosening,the evolution of thread wear has not been considered in the simulation models of bolted joint.Therefore,developing a simulation model for predicting the loosening life of bolted joint caused by thread wear has become the industry demand.In this study,bolt loosening tests were performed under transverse loading,in which the fretting damage at threads was observed and the clamping force was measured.Then,simulation models with consideration of thread wear were developed,and the bolt loosening process was simulated.Based on the model,loosening process was simulated for bolted joint with different load,structure and material parameters,and then these parameters were optimised.This study consists of the following 4 parts.The results and conclusions obtained are as follows:(1)Bolt loosening tests subjected to transverse loadingIn order to grasp the law of bolt loosening caused by thread wear under transverse loading,and provide experimental verification basis for subsequent simulation modelling,bolt loosening tests were performed under transverse loading,in which the fretting damage at threads was observed and the clamping force was measured.The results show that the redbrown wear debris is produced on the thread surface under transverse cyclic loading.Severe oxidation occurs in the process of thread wear.The wear mechanism of thread surface is complicated including oxidation wear,delamination wear and fatigue wear.These observation results are consistent with the typical characteristics of fretting wear.With the increase of loading cycle,the fretting wear becomes more and more severe,and the clamping force decreases,but the decreasing rate gradually slows down.Besides,in the conditions with a larger preload,a smaller transverse displacement amplitude,a larger thread pitch,and a higher surface hardness of thread,the fretting wear becomes less severe,and the reduction of clamping force becomes smaller,thus the anti-loosening ability of bolted joint is improved.(2)Numerical model of bolt loosening based on thread wearBased on the structural deformation of bolted joint after the application of preload,a stiffness model of bolted joint with consideration of thread wear was proposed.The loosening process caused by thread wear was simulated by using numerical iterative calculation with this stiffness model.The results show that,with the increase of cumulative slip distance,the predicted wear depth at threads gradually increases,and the clamping force gradually decreases.The thread wear reduces the original deformation amount of each component of bolted joint,thereby causing the release of deformation force,and then leading to the reduction of clamping force.The evolutionary trend of the wear and clamping force is consistent with the test results.However,since the numerical simulation model simplifies the structure of bolted joint,this method can only qualitatively simulate the loosening process caused by thread wear.(3)Finite element model of bolt loosening based on thread wearTo quantitatively simulate the loosening process caused by thread wear,a finite element model of bolt loosening with consideration of thread wear was developed.The wear depth,contact variables,and clamping force were calculated using this model.The results show that,with the increase of loading cycles,the predicted wear depth at threads gradually increases,and the clamping force gradually decreases.The thread wear causes a change in the distribution and magnitude of contact stress,and an increment in sliding distance along the radial and circumferential direction on the thread surface.It can further change the distribution and magnitude of wear depth in this two directions.The predicted results agree with the experimental results with respect to the evolutionary trend of wear depth and clamping force.Because the accumulation of wear debris is not considered in the FE model,the predicted wear depth and the reduction of clamping force is larger than experimental results.Based on the predicted bolt loosening curve,the loading cycles corresponding to a certain failure criterion for bolt loosening,namely loosening life,can be obtained.(4)Optimisation of load,structure and material parameters of bolted joint based onfinite element modellingBased on the developed finite element model of bolted joint,the preload,transverse displacement amplitude,thread pitch,and surface hardness of thread were optimised from the perspective of fretting wear,so as to improve the anti-loosening of bolted joint.The results show that,the increment of the preload,thread pitch,the surface hardness of thread,and the decrement of transverse displacement amplitude,can alleviate the thread wear through reducing the relative slip between the threads of the bolt and the nut,thereby improving the anti-loosening ability of bolted joint.The predicted results agree with the experimental results with respect to the variation of wear degree and clamping force with load,structure and material parameters.This verifies the feasibility of applying the developed finite element model to the optimisation of relevant parameters of bolted joint.In summary,a simulation model with consideration of thread wear was proposed in this study.This model can be used to predict the loosening life,and optimise the load,structure and material parameters of bolted joint.In the following work,the accumulation of wear debris will be considered in this model,which can further increase the predicted accuracy.