Thermal-mechanical Coupling Fatigue Crack Propagation Analysis Of Train Brake Disc Considering Residual Stress

A high-speed train has the advantages of environmental protection,economy,and high efficiency.With the increasing mileage of operation year by year and the improvement of China’s railway speed,higher requirements are put forward for the braking system to ensure the safety of train operation.Basic braking is the last barrier to ensuring the safe operation of a high-speed train.In the process of train braking,the interaction between the brake disc and friction block will transform the huge kinetic energy of the train into heat energy,which will make the temperature of the disc body rise rapidly and make the stress field of the brake disc have complex changes.The yield strength of the material will decrease with the increase of temperature,and the huge thermal stress will make the material yield,resulting in plastic deformation.After the disc is cooled to room temperature,the plastic deformation of the material will generate residual stress and affect the distribution of the stress field of the brake disc.When the train is braking several times,the cracks on the disc gradually expand.In severe working conditions,the cracks may run through the brake disc and threaten the safe operation of the train.Therefore,the study of the thermal-mechanical coupling fatigue cracks propagation law of brake discs is of great significance to ensure the safety of train operation.At present,a lot of research has been carried out on the thermal-mechanical coupling response of brake discs of high-speed trains.Considering the complex contact state between the brake disc and friction block,some scholars use the uniform heat source method to simulate the process of friction heat generation.However,in the actual braking process,the change of disc contact state will affect the distribution of the temperature field and stress field in the friction ring area of the brake disc.Therefore,in this thesis,the difference in contact stress in different friction areas of the disc and the plastic behavior of the brake disc material at high temperatures are considered,and the thermal-mechanical coupling calculation of the brake disc is carried out by applying the method of non-uniform heat flux.Meanwhile,the effects of residual stress and different braking conditions on the thermal-mechanical coupling response of brake discs were studied.Finite element software ABAQUS,FE-SAFE,and FRANC3 D were used to analyze brake discs’ surface crack propagation behavior,and the fatigue life of brake discs was evaluated.The main conclusions of this article are as follows:1.A method of applying non-uniform heat flux was proposed,and the proposed method was verified by a multi-mode vehicle braking performance simulation test bench and measured residual stress data.The thermal-mechanical coupling calculation of the brake disc under different braking conditions is carried out.The results show that the stress state of the brake disc is mainly determined by the circumferential stress state,the maximum equivalent plastic strain and the maximum temperature appear almost at the same time,and the residual stress is mainly determined by the peak value of temperature and thermal stress.Then the thermalmechanical coupling simulation of the brake disc under different combined braking conditions was carried out.The analysis shows that the residual stress generated in the initial braking condition will reduce the variation range of the circumferential stress of the brake disc in the subsequent braking process,and make the stress response of the brake disc tend to be under tension state,and then deteriorate the thermal-mechanical coupling response of the brake disc.2.FE–SAFE was used to calculate the fatigue crack initiation life and location of the brake disc,XFEM was used to determine the direction of the disc surface crack propagation,and ABAQUS and FRANC3 D were used to simulate the disc surface crack propagation behavior.The results show that the initiation location of the disc crack is on the side of the friction radius deviating from the center of the circle,the surface crack propagates along the radial direction and perpendicular direction of the disc,and the propagation life of the disc crack is mainly determined by the early crack propagation.In the emergency braking condition,the stress intensity factor and the growth rate of the crack front end and the crack rear-end are greater than that of the crack bottom,and the growth rate of the crack front end increases sharply at the later stage of the crack growth,which will cause the disc surface crack to reach the limit length in a short time and make the brake disc fail.3.Considering the influence of residual stress and different braking conditions,fatigue crack initiation and propagation of brake discs were analyzed.It is found that initial residual stress has little influence on the fatigue crack initiation life of the brake disc when emergency braking is the follow-up braking condition,while the initial residual stress has a great influence on the fatigue crack initiation life when ramp braking or routine braking is the follow-up braking condition.For the crack initiation stage,the fatigue initiation life of the brake disc is longer when the initial residual stress is small and the subsequent braking condition is conventional.In the crack propagation stage,the initial residual stress is the main factor affecting the fatigue crack propagation.