| Rolling contact fatigue(RCF)is common in the rail of metro line,especially on curved track.Due to the restriction of urban construction,many small radius curves are often set in metro lines.When the train runs on a small radius curve,the wheel–rail interaction is more intense,and the wheel–rail creepage and creep force become higher,which leads to rail RCF to more prominent.In recent years,with the rapid development of urban rail transit in China,the phenomenon of rail RCF becomes more and more serious.It will make the rail to be ground and replaced frequently,significantly increase the line operation cost,and worsen the wheel–rail interaction,threaten the running safety of train.Therefore,the research on rail RCF on small radius curve of metro is of great significance to reveal the initiation mechanism of rail RCF cracks,mitigrate the growth rate of rail RCF damage,and ensure the operation safety of train on small radius curve.Aiming at the RCF damage in low rail on a small radius curve of a metro line,through the combination of field test and numerical simulation,this thesis investigates the initation causes of low rail RCF,analyzes the influence of vehicle and track related parameters on low rail RCF,and puts forward mitigation measures.The main work and conclusions of this thesis are as follows.(1)Based on the field investigation of a metro line,the distribution characteristics and laws of RCF oblique cracks in low rail on small radius curve are mastered.The field investigation results show that the RCF oblique cracks in low rail on small radius curve are mainly distributed near the center of the rail top,the angle of cracks is about 135o,showing continuous distribution characteristics,and mainly appear on the circular curve.(2)A rail RCF mechanism analysis model is established,which includes a metro vehicle system dynamics model,the surface fatigue index based on the shakedown map and the damage function based on the wear number.The damage function is modified based on the field investigation results,ignoring the assumption that RCF damage is calculated only when the direction of rail longitudinal creep force is opposite to the direction of vehicle operation,and directly using the original wear number T? without any treatment to calculate the RCF damage of curved low rail.The simulation results of the modified damage function are basically consistent with the field test results,which verifies the accuracy of the modified damage function.It shows that when using the damage function to simulate and analyze the RCF damage of curved low rail,the direction of rail longitudinal creep force cannot be required to be opposite to the direction of vehicle operation.(3)Based on the established rail RCF mechanism analysis model,the measured wheel/rail profiles,vehicle and track parameters are taken as the input of simulation and calculation.Combined with the distribution characteristics and laws of on-site cracks,the causes of low rail RCF are analyzed from four aspects,i.e.,the direction of resultant creep forces,the wheel–rail contact positions,the surface fatigue index and the RCF damage distribution.The simulation results show that when the vehicles pass through the small radius curve at a uniform speed,the angles of resultant creep forces of low rail are mainly distributed around 45o and 225o.The wheel–rail contact points are concentrated in the top area of low rail.The surface fatigue index of rail in this area is larger than 0,which indicates that the material in the contact region shows the ratcheting effect.And the total damage value of rail calculated according to the modified damage function model is also larger than0,which means the initiation of RCF damage.With the continuous passing of vehicles,the worn wheels interact with the worn rails frequently,the RCF damage of low rail continues to accumulate until it exceeds the toughness limit of material.Finally,the fatigue oblique cracks with the angle of about 135o perpendicular to the direction of resultant creep forces are formed in the top area of low rail.(4)Using the established metro vehicle system dynamics model and the modified damage function model,the effects of curve superelevation,friction coefficient and rail cant on the RCF in low rail on small radius curve are investigated,and the corresponding mitigation measures are put forward.The simulation results show that according to the actual situation of small radius curve,properly increasing the curve superelevation and the friction coefficient of low rail,and maintaining the low rail cant at 1/70 can mitigrate the formation and development of RCF in low rail on small radius curve.Increasing the curve superelevation and the friction coefficient of low rail will aggravate the wear of high rail,it can be solved by reducing the friction coefficient on the side of high rail.(5)Using the established metro vehicle system dynamics model and the modified damage function model,the effects of primary longitudinal positioning stiffness,wheel profiles and vehicle running speed on the RCF in curved low rail of metro line are investigated,and the corresponding mitigation measures are put forward.The simulation results show that according to the actual situation of the curve of metro line,on the premise of ensuring the stability and safety of vehicle operation,properly reducing the primary longitudinal positioning stiffness,adopting DIN5573 wheel profile and reducing the running speed of vehicle are conducive to mitigrate the formation and development of RCF in curved low rail of metro line. |
PDF Full Text Request