Experiment And Constitutive Model Studies On Temperature Dependent Cyclic Plastic Deformation Behavior Of U75VG Rail Steel

With the rapid development of high-speed railway in China,more and more attentions have been paid to the safety and running quality of high-speed railway system.Once the braking force exceeds the maximum adhesion force between wheel and rail,the relative sliding of wheel and rail occurs when the high-speed train is braking,and the relative sliding friction between wheel and rail induces significant temperature rising.The temperature rising not only causes the thermal stress on the surface of wheel and rail,but also causes the degradation of mechanical properties of wheel and rail materials,which further affects the mechanical stress.The interaction between thermal stress and mechanical stress produces the thermal damage,which seriously threatens the operation safety and running quality of high-speed trains.In order to study the thermal damage in wheel-rail contact of high-speed railways,the monotonic tensile and cyclic tests of U75 VG rail steel at different temperatures are carried out to obtain the cyclic deformation characteristics.Moreover,the cyclic plastic model of U75 VG rail steel is established based on experimental observations.Finally,the coupled temperature-displacement elastoplastic analysis on wheel-rail contact in the condition of high-speed sliding is carried out by using the finite element software ABAQUS.The results provide a basis for evaluating the thermal damage mechanism of high-speed rails.The main contents of this work are summarized as follows:(1)Monotonic tensile tests,strain cycling tests and stress cycling tests of U75 VG rail steel are carried out within 25 ℃ ~ 600 ℃.Results show that the dynamic strain aging of U75 VG rail steel is significantly enhanced when the temperature is around 300℃.As a result,the strength performance of the steel near 300℃ is close to that at 25℃.The dynamic strain aging provides additional hardening in the initial stage of strain cycling and results in a cyclic hardening at 300℃.In the stress cycling,the steel exhibits remarkable ratchetting behavior,and its evolution closely depends on the loading stress level and temperature.(2)Based on the experimental strain cycling characteristics,ratchetting strain evolution and classical cyclic plastic constitutive model,a temperature-dependent cyclic viscoplastic constitutive model is established,and the influences of temperature and dynamic strain aging on the strain cycling features and ratchetting behavior are considered by introducing temperature-dependent terms into the isotropic hardening and kinematic hardening equations.The improved model is implemented into the finite element software ABAQUS by using implicit stress integral algorithm,and the monotonic tensile,strain cycling and stress cycling experiments of U75 VG rail steel at different temperatures are predicted reasonably.(3)In order to analyze the thermal damage mechanism of high-speed rails under coupled temperature-displacement analysis in high-speed railways,the threedimensional finite element model of wheel-rail contact is established by ABAQUS.The coupling effect between mechanical stress,thermal stress caused by nonuniform temperature rising and degradation of material properties of rail is studied by coupled temperature-displacement analysis in ABAQUS.The research results show that the stress and strain contours are similar to each other under coupled temperature-displacement and isotropic temperature cases due to the dynamic strain aging;the temperature rising on the surface of rails increases with the increasing axle load and friction coefficient,and the friction coefficient has a dominate role in temperature rising;in the meantime,due to the influence of dynamic strain aging,the mechanical properties of U75 VG rail materials with temperature rise within 300 oC will not be significantly weakened.