Study On Bolt Bearing And Loosening Mechanism Of Internal Combustion Engine Under High Temperature

Bolts are widely used because of their simple structure,convenient disassembly and low cost.With development of industry,the requirements for bolts are higher and higher,especially in aerospace and automobile industry.During the operation of internal combustion engine,high temperature and vibration load are often accompanied.As the cylinder head connector,the bolt must ensure the tightness of the cylinder during the operation of the internal combustion engine.In the high temperature environment,the material mechanical properties of bolts will change,and creep deformation will occur,which aggravates the failure process of bolts and reduces the reliability of bolted joint.Based on the bolt on internal combustion engine,aiming at the problem of thread load distribution and bolt loosening under high temperature creep,this paper focuses on three aspects: experimental test,creep model fitting and numerical simulation.The main purpose is to obtain the high-temperature mechanical properties and creep deformation characteristics of the material according to the high-temperature test of the material.Then fit the creep model with the creep test data to obtain the creep model parameters,input them into the finite element model,and carry out the finite element analysis of bolt bearing distribution and bolt loosening based on the creep model.The main research contents and conclusions of this paper are as follows:(1)Study on high temperature mechanical behavior of aluminum alloy and 45 steelThe high temperature tensile mechanical properties of aluminum alloy materials are tested,and the high temperature mechanical properties of 45 steel are studied.The results show that the change of temperature has a significant impact on the mechanical properties of aluminum alloy materials and 45 steel,and the yield strength and tensile strength of aluminum alloy materials and 45 steel are significantly reduced at high temperature.In addition,high temperature creep tests were carried out on aluminum alloy and 45 steel,and the effects of load and temperature on the creep deformation of aluminum alloy and 45 steel were studied.The creep model parameters are obtained by fitting the creep test data with the creep model,and then the creep model is applied to the analysis of bolt bearing and bolt loosening.(2)Study on the load distribution of bolts under high temperatureBased on the Time-Hardening creep model and considering the high temperature creep behavior of aluminum alloy materials,a two-dimensional axisymmetric finite element model of bolt bearing is established.The effects of linear elasticity,plasticity and creep on the load distribution of threaded teeth at 20℃,250℃,300℃ and 350℃ are analyzed.The results show that the load of bolt thread is mainly concentrated on the first three thread.In the elastic stage,the axial load of the first three thread accounts for more than 60%,but the deformation between the thread contact pairs has a great influence on the load.With the increase of temperature,the creep deformation increases,and the load of the first two thread teeth decreases.According to the load-bearing law of screw thread,the screw thread is optimized.The optimized bolt can effectively improve the load-bearing uniformity of screw thread;Finally,the load-bearing laws of threaded teeth under the two creep models are compared.The results show that the load-bearing uniformity of threaded teeth under the two creep models will be improved,and the load-bearing ratio of the last thread in the load-bearing distribution of threaded teeth calculated by the modified Graham creep model will be greatly increased.(3)Study on bolt loosening under transverse vibration and high temperatureFor the study of bolt loosening under high temperature creep,a fine finite element model of bolted joint with thread rotation is established.Then,based on the Combined time hardening creep model,the effect of high temperature creep on bolt loosening is studied by numerical simulation.In addition,various factors affecting bolt looseness are studied,such as transverse vibration amplitude,initial preload and friction coefficient.The results show that under high temperature and transverse vibration load,the bolt preload first decreases sharply,and then decreases at a stable rate.From 450℃,the creep deformation will increase by more than 150% when the temperature increases by 50℃.High temperature creep leads to the increase of bolted joint deformation,which makes the bolt preload decay faster.The excessive preload will increase the creep deformation,resulting in an increase in the attenuation percentage of preload.The larger the amplitude of transverse vibration and the smaller the friction coefficient,the more relative motion will be generated between the contact pairs of thread teeth,resulting in the faster attenuation of preload.