Crack Propagation Analysis Of Wheel/rail Rolling Contact Fatigue On High Speed Railway

High-speed railway has played a significant important role for improving the economic growth and promoting social progress in China,after the first high-speed railway,named "Beijing-Tianjin Inter-city Railway",begun to operate in 2008.Meanwhile,the operational safety of vehicles is increasingly valued due to the growth of high-speed railway mileage,passenger capacity and operational speed.However,rolling contact fatigue(RCF)occurred on the wheel-rail,always as a potential danger for the running safety,has become an urgent problem to be solved.So,the deeply research for the initiation and propagation mechanisms of wheel-rail RCF cracks and the rational preventive measures are proposed is the great significance for guiding engineering practices.In the wheel-rail system,the wheel and the rail are in dynamic rolling contact,and the amplitudes and directions of the contact force are equal and opposite,respectively,i.e.,the force and the reaction force,resulting in the wheel-rail RCF can often occur on either the rails or the wheels.This thesis focuses on two typical damage patterns,RCF occurred on the rail surfaces and local rolling contact fatigue(LRCF)occurred on the wheels,were observed in high-speed railway.The research methods include the simulated calculation based on fracture mechanics and twin-disc rolling fatigue test based on the proportional test in laboratory.The research progresses and limits about the related field in recent years are reviewed in the first chapter.At present,the moving load method is still used by most of the crack propagation analysis models at home and abroad,i.e.,the moving Hertz contact patch,as the crack load boundary,is applied between wheel-rail contact surfaces,its defect is that the transient contact load changes caused by material discontinuities are not considered during wheel rolling over cracks.A multi-oblique crack propagation analysis model is established using ANSYS/LS-DYNA in the second chapter,and the crack load boundary is applied by the 3-D wheel-rail rolling contact algorithm broken through the limitations of steady-state contact load boundaries used in previous models.For chapters 3 and 4,the above model is used to simulate the transient process when the wheel with 300 km/h operated on multiple oblique identical cracks,based on the assumptions that the cracks are equally spaced and parallel,and the crack faces are completely the same.According to the field observations,the crack propagation angle is 30° typically,its length-to-depth ratio is fixed at 5,and the crack length and crack spacing are 10~20 mm and 5~20 mm,respectively.The main conclusions are as follows:(1)The cracks can reduce the tangential and normal contact stiffness between the wheel and rail,and the decreased amount will increase with the growth of the number of cracks;(2)The effect of the incompressibility of the liquid on the crack opening propagation is not considered,i.e.,the cracks have not been opened during the loading process,leading to the simulated RCF cracks are mainly propagated in the sliding type(type II)mode;(3)The nodal force of the crack tips for the multiple cracks is lower than a single crack under the same element size,and the relative displacement between the crack faces near the tips is larger due to the reduction of the contact stiffness,which resulting in stress intensity factors(SIF)of the crack tips increase with the increasing number of cracks;(4)For multi-cracks with a feature size(length)of 15 mm,the 3-crack model is sufficient to accurately simulate the interaction between the cracks when the crack spacing is greater than 5 mm;(5)When the crack spacing is larger than the crack feature size,the SIF results between the multi-crack model and the single crack model is less than 7%,i.e.,the multi-crack model can be simplified to the single crack model.In recent years,the wheel LRCF has attracted widespread attention,and their initiation causes have been identified as wheel indentation,but it is limited to qualitative description and has not been reproduced in the test.In the fifth chapter,the GPM-30 rolling contact fatigue test bench was used to carry out the twin-disc rolling fatigue test.Different types of initial artificial damage were applied to the contact surface of the wheel disc by different processing methods.The process of initiating LRCF from indentation is reproduced on the specimen cut from the real wheel.The test results show that the initial damage using Vickers hardness tester and EDM will not initiate LRCF due to their small and missing plastic deformation,the deep indentation applied by the plastic working of the tap on the machine tool may result in the initiation of LRCF with a critical depth of approximately 0.2 mm.In summary,the initiation of LRCF is determined by the plastic deformation and the sharp geometry in indentation which introduced during the indenting.Finally,the main work of the thesis is summarized and a prospect is made.