| The continuous improvement of train operating speed,axle load and traffic density puts forward higher requirements for the safety operation of railway vehicles.Wheel/rail contact is the only way to realize train’s traction and braking,which has always been one of the most central problems of railway vehicles.The thermophysical properties in contact patches include the generation and transmission of friction heat and its influence on the tribological and mechanical properties in the wheel/rail contact patches,which is a multidisciplinary coupling problem and it has been a hot topic in the research.Although it has been studied for a long time,it still exists deficiencies as following: the heat transfer model used in most studies is inconsistent with the actual situation of wheel and rail;due to the small number of grids used to solve the heat transfer model,the results are not accurate;the difference of frictional thermal characteristics of different creep-traction models are unclear;the single thermal stress characteristics are rarely reported.In order to reveal the thermal-physical processes and their properties in contact patches systematically,the theses uses theoretical analysis and numerical analysis methord to carry out research as following: local and average properties of friction heat under different creep-traction models;small domain numerical method of temperature field and thermal stress field of contact patches;research on temperature field characteristics of wheel-rail contact patches;thermal stress characteristics of wheel-rail contact patches.The main contents of this theses and the main conclusions obtained are as following:1.The theses compared the geometric characteristics of the adhesive area and the sliding area of the contact patch,the local characteristics of creep rate,shear stress and normal stress,and the local and average characteristics of friction heat under the conditions of Carter,Vermeulen-Johnson(V-J)and Haines creep slip-traction models.The results show that the Haines model is superior to Carter model and V-J model in terms of the rationality of the contact spot geometry,creep rate,shear stress,normal stress,frictional heat local and average characteristics.The results show that the sliding area of the V-J model is larger than that of the other two models with the same Fx/f P.The sliding area of Carter and Haines modle is basically the same.The sliding area increases with the increase of Fx/f P,and has a large growth rate in the area where Fx/f P is close to 1.The average creep rate of the Carter model is the largest,the Haines model is the middle,and the V-J model is the smallest.When Fx/f P is close to 1,the average creep rate of the Haines model is close to the result of the Carter model.For the same Fx/f P,the average creep rate of Carter and Haines models is independent of axle load,but the average creep rate of V-J model is related to axle load.The friction power of the contact patch increases with the increase of Fx/f P,and has a high growth rate in the region of Fx/f P close to 1.The friction power of Carter and Haines models is related to the axle load,but the friction power of V-J is not.2.Numerical methods to improve the numerical analysis accuracy of heat transfer and thermal stress of contact patches are studied in detail,and a calculation method for small domain is proposed.The focus of this method is to determine the boundary conditions at the separation boundary between small domain and whole domain.It is found that energy conservation can be satisfied and very accurate solutions can be obtained by using heat flux interpolation at internal nodes to obtain the boundary conditions of heat flux.In order to verify this calculation method,the program of finite volume integral method is developed in the adaptive coordinate system.After the program is checked by an analytical example,the heat transfer of a moving heat source cylinder acting on the side is analyzed numerically by small domain and large domain methods.The results show that it takes great effort to obtain the analytical solution of moving heat source.As for the calculation examples selected in this theses,the number of grids in the whole domain is 15 times that in the small domain,the time step is the same,and the convergence time of the whole domain is 50 times that in the small domain.The small domain calculation method not only saves the calculation time greatly,but also guarantees the high calculation precision.3.Using the small domain numerical method and its program,a detailed study was carried out on the change rule of friction heat transfer of contact patches with time under different creep-traction models.The results reproduce the different temperature values,temperature distributions and temperature gradients in the contact patches of different creep-traction models.It is found that under the conditions of Carter,V-J and Haines creep slip-traction models,the temperature obtained by Carter model is higher than the other two models,and the temperature obtained by V-J model is the lowest.Similarly,the temperature gradient in each direction obtained by Carter model is also the largest.The results show that the higher the speed,the greater the traction,the greater the maximum temperature rise of the contact patches.For an EMU below 360 km/h(the axle load is 15 tons),the maximum temperature rise is less than 110°C when use Haines creep-traction model.The change rate of the temperature of the contact patches caused by the movement of the contact patches over time can reach 105 °C/s,and the temperature gradient in the movement direction of the contact patches can reach 105 °C/m.The large negative gradient appears in the median level of traction,and the positive gradient appears in the saturation area of traction.The transverse temperature gradient on the surface of the contact patches can reach 104 °C/m,but the temperature gradient in the direction of the wheel center can reach 106 °C/s.4.In this theses,a contact patches thermal stress model is established,and a small domain solution method and program for thermal stress under adaptive coordinates are realized on the basis of the developed numerical method of finite volume integral method of temperature field.It is found that the method and program of solving thermal stress in small domain proposed in this theses are scientific and reasonable,and can get more accurate thermal stress.Considering the limitation of calculation work,the paper carried out a detailed calculation study of the thermal stress in the contact patches under the Haines creep slip-traction model,and analyzed the stress characteristics in the contact patches under three conditions: only mechanical load action,only thermal load action,and combined action of mechanical load and thermal load.It is found that there is a large thermal stress in the contact area,the value of thermal stress is about 200 MPa,and it is basically distributed on the surface and the sub-surface of the contact patches.With the increase of depth,the value of the thermal stress decreases obviously.After the wheel surface is just out of contact,there is still considerable thermal stress,which forms residual stress.The thermal deformation of contact patches is small.The friction heat of wheel and rail has little effect on the distribution of normal stress and shear stress on the surface of contact patches.The results also showed that if von-Mises stress was used as the equivalent stress,the equivalent stress formed by thermal stress alone on the surface of the contact patches is small.No matter compared with the equivalent stress of thermal stress alone or the equivalent stress of mechanical stress alone,the equivalent stress of total stress varies greatly,and its basic performances are the existence of thermal stress weakens the equivalent stress of the highest mechanical stress on the one hand,the maximum reduction is about 100 MPa,and the existence of thermal stress enhances the equivalent stress of most areas of the contact surface on the other hand,the maximum enhancement range is 500 MPa.It is found that the internal driving force of the increase of equivalent stress comes from the temperature gradient along the movement direction of the contact patches,which means that,even if the temperature level is low,the role of thermal stress cannot be underestimated,and reasonable reduction of temperature gradient will be of great help to reduce the equivalent stress.Thermal stress has great influence on the equivalent stress in the direction of wheel surface depth. |
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