Analyses On Dynamic Responses Of Polygonized Wheel Of High-speed Train Using Explicit FE Method

Out-of-roundness?OOR?is one of the key damages of high-speed wheels.It has gradually become a widely spread problem with the advent and rapid development of Chinese high-speed railways.The OOR can enhance the dynamic interaction of wheel-rail,resulting in increased vibrations and noise radiations of the vehicle-track system,further the reduction of the fatigue life of key components closing to the wheel-rail system,and even safety accidents under some extreme conditions.This thesis establishes a 3D transient rolling contact finite element?FE?model to systematically study the high-speed rolling contact behavior between wheel and rail in the presence of the wheel OOR,and the resulted high frequency vehicle-track dynamic interaction.Chapter 1 first introduces the high-speed wheel OOR briefly,followed by a detailed literature review on OOR,and the researches on the dynamic constitutive relations of wheel-rail materials are also reviewed.Up to now,the transient wheel-rail rolling contact behavior has not been studied in consideration of wheel OOR and nonlinear material behavior at high frequencies,so that this work develops a 3D transient FE model to fill the gap.A 3D FE model is developed using explicit FE method to study the transient contact behavior that wheel rolls over rail in the time domain,which's parameters are taken from a high-speed system in China.The numerical simulation by using it can output contact stress?stick-slip distribution and friction work and other contact results.The actual wheel-rail geometry?material nonlinearity?spin creepage and continuum vibrations of wheel-rail material are all taken into account.The rolling contact solutions under smooth condition indicate that this model can calculate the transient rolling behavior of wheel-rail accurately,which provides a research method to study the rolling contact between polygonized wheel and rail.The wheel OOR measured of high-speed trains are introduced into this model by modifying the coordinates of wheel surface nodes.With such a model,the high-speed rolling-sliding behavior and corresponding transient contact solutions of polygonized wheel are analyzed with different speeds?tractions?friction coefficient.It is found that the wheel OOR will cause sharp fluctuations of wheel-rail contact state.The shape and size of contact patches?contact forces?contact stresses and friction work variate significantly compared with smooth condition.On the premise of the same depth of polygonal wear,the fluctuation amplitudes under high order OOR conditons are greater than low orders,and high traction level will further enlarge their variation trend.In the case of large traction torque,excessive creep caused by too small friction coefficient can enhance wheel-rail wear.For the simulated wheel-rail system,the fluctuation amplitudes of friction work?tagential contact stresses and V-M?Von Mises?equivalent stresses of surface material all have extreme value at 300 km/h.Too large OOR depth leads to the instantaneous separation of wheel-rail.This phenomenon will appear under the condition of 18 order OOR of 0.4 mm at 300 km/h.Chapter 4 highlights the average strain rates of surface material of 0.25⁰.5 mm deep,considering the smooth wheel-rail and wheel macroscopic geometry irregularity?OOR?.It is found that the discrete scale such as element size and time step has an important effect on strain rate estimation.The code default uses an element size of 0.5 mm and a time step of0.32?s.The highest strain rate occurs on surface layer,and rate of the normal strain is the largest among all components,being 1.50 times as much as V-M strain rate.It occurs in the leading and trailering edges of the contact patch,whereas a local minimum occurs near the center of contact patch.At 300 km/h,the V-M strain rate of smooth wheel-rail reaches its maximum of 64.1 s^-1,and material elasto-plasticity has no effects.Macroscopic irregularities can increase the strain rates significantly,resulting the maximum V-M strain rate of 91.1 s^-1and 105 s^-1 in elasticity and elasto-plasticity respectively,and the elasticity results will be1.60¹.80 times when an element size of 0.25 mm is used.The maximum V-M strain rate increases monotonically with increasing friction coefficient,while traction coefficient is effective under rolling-sliding state.The maximum strain rate increases linearly with speed,and its value rises to 88.7 s^-1 for smooth wheel at 400 km/h.Chapter 5 focuses on the transient contact solutions between new wheel and new rail?i.e.,before shakedown?,for which an elasto-plasticity material model measured under different strain rates is employed together with a bilinear elasto-plasticity law.Compared with linear elastic material,the contact patch is enlarged under elasto-plasticity conditons because of the occurrence of plastic deformation,resulting in lower contact stresses and V-M equivalent stresses,and residual stresses and strains after rolling.Finally,conclusions are given and future work is discussed.