Study On Mechanism Of Wheel-Rail Rolling Contact Fatigue And Damage

With increases of train speed and axle load, railway transportation put forward a series of challenges to science and research. Many key problems of science and technology need to be urgently solved. Wheel-rail rolling contact fatigue and damage is one of the most serious problems. The solution to the problem is very important to the train running safety, decreasing of the transportation cost and developing high speed train.Firstly the classes of wheel-rail rolling contact fatigue and the studies on it in the past and now in the world are reviewed in detail. Based on the vehicle-track coupling dynamics theory, wheel-rail rolling contact theory, material wear model, heat transfer, plasiticity theory and finite element method, the initiation and development mechanisms and key impacting factors of some typical phenomena of wheel-rail rolling contact fatigue, such as wheel-rail braking damage, impact damage, wear and rail oblique crack, are investigated numerically and by experiment. Many measures are put forward to preventing and lightening the above damages. The main research and results are given as follows.Temperature rise and thermal stress due to wheel/rail friction play an important role in the failure of wear, flatting and checking. In this thesis, a thermal-mechanical coupling contact model of wheel/rail in sliding contact or braking is put forward to calculate the temperature rise and thermal stress with finite element method. The non-steady heat conduction between the contacting surfaces of the wheel/rail, heat-convection and radiation between the wheel/rail and an ambient are taken into consideration. The contact pressure distribution is gotten by contact element method. The numerical results show that the contact element method is effective. The thermal effects only exist in the thin layer of wheel/rail, and its effect decreases as the depth increases. The total stress levels near wheel-rail surfaces become larger due to the thermal load. Higher sliding speed leads to stronger thermal effects.A three dimensional dynamic elastic-plastic finite element model of the traditional rail joint with height difference is established, which considers the contacting interactions between the wheel and the rails, the rails and the joint bars, the joint bars and the bolts, the bolts and the rails. The influences of train speed, axle load and height difference of rail joint on the contact stresses and strains at rail head are investigated. The results obtained show that when the wheel rolls over the joint rails with height difference, the impact action between the wheel and rails is strong, which easily causes crushing and crack of rail head. The train speed and height difference have greater effect on rail stresses and strains than the axle load does.The thesis detailedly analyzes the relation between the wheel-rail contact traction and rail oblique crack. Spatial coupling dynamics models of rolling stock and track for Guang-Shen Line are established. The rolling stock types causing rail oblique crack are determined on the basis of the location of wheel-rail contact point and the magnitude and direction of the contact traction obtained from the vehicle curving dynamic analysis. The effect of rail weld irregularity in Jing-Guang Line on rail oblique is also studied. The numerical results show that Blue-Arrow power car and "SS8" locomotive car play the most important role in the initiation and evolution of rail oblique crack. The theory results are validated by experiments in which the displacement and its direction are measured when the trains pass through the tracks with rail oblique cracks. Through investigating the influences of curved track super-elevation, rail cant, curve radius, rail gauge and railpad stiffness on wheel-rail contact force and location, the optimal results of track structure parameters are obtained. The results provide theoretic basis and guidance for preventing and lightening the formation and development of rail oblique crack.The present thesis utilizes a numerical method to analyze the effect of railway vehicle curving on the wear and contact stresses of wheel/rail. The numerical method considers a combination of Kalker's non-Hertzian rolling contact theory, a material wear model, a vertical and lateral coupling dynamics model of the vehicle and the curved track and the three-dimensional contact geometry analysis of wheel-rail. The developed numerical program can consider a feedback process between the rail wear and the transient coupling dynamics of four wheels of a same bogie and the track. Through the detailed numerical analysis, it is found that the difference between the normal loads of the left and right wheels of the wheelset increases linearly with increasing the vehicle curving speed. The material wear volume per length along the rail running surface has a tendency to grow. However, the maximum normal contact stress fluctuates largely with the increasing curving speed. The increase of the maximum contact stress depends greatly not only on the normal load but also on the profiles of the wheel/rail. Increasing the track super elevation efficiently lowers the normal load difference between the left and right wheels of the wheelset, the contact stresses and the wear. Increasing the rail cant leads to the great growth of the low rail wear of the curved track, the reduction in the outside rail wear. The results are very useful in the maintenance of the track.A three-dimensional elastic-plastic stress analysis model for wheel-rail rolling contact is established. The model is applied to a case study of a section of Guang-Shen Line with rail oblique crack. Under the condition of different rail hardness, the residual stresses distributions and the magnitude and direction of the material plastic deformation are calculated. Besides, the effects of rail cant, rail gauge and curve radius on the static contact stresses of wheel-rail are investigated using Kalker's non-Hertzian rolling contact theory and its numerical program CONTACT. It is found that the rail cant has a greater effect on static contact stresses of wheel-rail than rail gauge and curve radius. The results provide a powerful tool for the optimal design of track structure.The princible and function of many types of rail grinding techniques are stated. Through the analysis of the operation character and damage type of Guang-Shen Line, the thesis introduces that the preventive grinding and asymmetric grinding of curved rail head should be applied to Guang-Shen Line. Based on the period when the rail obligue crack initiates and annual volume of the Guang-Shen Line, it is proposed that the preventive grinding interval of the curved track is about 6 months. The asymmetric grinding of curved rail head can make the wheel-rail contact point move toward rail centre and apart from rail shoulder. Accordingly, the contact stress between the wheel and rail decreases and the rail obligue crack can be eliminated and lightened.