Research On Contact Fatigue Damage Of Rail Based On Thermo-Mechanical Coupling

With the development of heavier axle loads, greater traffic density and higher train speed, rolling contact fatigue damage of wheel and rail becomes an increasing problem in recent years. And thermal damage is one of the most important failure forms of wheel and rail. The spalling of wheel/rail tread produced by the thermal damage leads to fierce vibration of vechicle and track structures, noise, discomfort for passengers, reduction of the use life of structural parts and then threats railway transportation safety. Therefore, study on wheel-rail frictional thermal damage is very important from the practical and cognitive points of view. According to the characteristics of wheel-rail rolling contact, the present thesis takes the truck/locomotive wheels and 60 kg/m rail as an example. And based on wheel-rail rolling contact theory, heat transfer theory and fracture mechanics, the thesis studies the frictional temperature and stress fields, crack growth behavior of rail during wheel-rail rolling-sliding contact by using thermo-mechanical coupling finite element method.Firstly, a thermo-elasto-plastic plane strain finte element model of rail is established by using the general-purpose FE code ABAQUS. In the numerical model, the heat-convection between the rail and ambient and temperature-dependent material properties are taken into consideration. The movement of boundary condition is used to simulate the movement of wheel-rail contact patch. The temperature field, the stress field, residual strains and residual stresses of rail are studied in the thermo-mechanical coupling case and only mechanical case. The effects of wheel-rail coefficient of friction, wheel load, wheel-rail creepage on the temperature rise, residual strains and residual stresses in the rail are investigated. The numerical results show that:1) The maximum Von Mises equivalent stress of rail occurs subsurface at about 0.2 mm. The influencing depth of the residual stresses and the residual strains is about 10 mm. The residual stress near the rail surface with thermal effect is larger than that without thermal effect.2) The temperature of the rail near the surface increases with increasing the friction coefficient and the axle load. The residual stresses near the rail surface increase with increasing the friction coefficient.3) The effect of elastic creepage on wheel-rail frictional heat should be not neglected. During wheel-rail rolling-sliding contact, the circumferential and axial residual stresses generated by the thermal load in the surface layer of rail appear to be tensile. The thermal load caused by small creepages can reduce the residual compressive stresses generated by the mechanical load.4) When the creepage become larger, the thermal response has a significant influence on the residual stresses and residual strains. As the creepage reaches a certain value, the circumferential and axial residual stresses in the thermo-mechanical case become tensile stresses, while in the mechanical case they appear to be compressive stresses. Secondly, a thermo-mechanical coupling plane strain finite element model incorporating an oblique rail surface crack is established. The effects of the frictional coefficients of wheel-rail contact and crack surfaces and the crack orientation on the crack growth behavior are investigated. The results reveal that the shearing mode appears to be predominant for the surface crack growth for both the only mechanical case and the thermo-mechanical coupling case. Compared to the only mechanical case, the shearing mode crack for the thermo-mechanical coupling case is more likely to grow. Reducing the wheel-rail contact surface frictional coefficient, increasing the crack-face frictional coefficient and avoiding the small angle of crack can restrict the rail surface crack growth.At last, the effect of multiple cracks interaction on the crack growth behavior in rail is caculated during wheel-rail full slip contact. Due to the interference effect of two cracks, the stress intensity factor K1 increases with an increase of the distance between two cracks for both only mechanical simulation and thermo-mechanical coupling simulation. The effect of the distance between two cracks on the crack growth for only mechanical simulation is significant. Compared with a single crack, the multiptle cracks interction can reduce the possibility of crack growth. In fact, only five or seven cracks seem to reveal the interaction of more cracks.