Research On Thermal Fatigue Life Evaluation Of Forging Brake Disc Used For High Speed Trains

The brake disc is one of the key components of rail vehicle braking load, and the crack propagation is the main form of brake disc’s service failure. Thermal fatigue crack is mainly occurred on the friction surface of brake disc. During the long-term using of brake disc, the initial defects of brake disc and the tension-compression stress cycle caused by temperature gradient during the frequent braking can induce initiation crack on surface of brake disc, and extend to critical dimension gradually. The interaction between multiple cracks on brake disc can suppress or promote the crack growth, which provides reference for the evaluation of the brake disc’s life.The thesis is supported by the railway ministry science and technology research and development project "testing research and assessment of fatigue life on forged steel brake disc of high-speed train group" and "research on fatigue crack of EMU brake disc materials under thermal-cold cycling force".Taking the brake disc as research object, the load spectrum of brake disc was carried out on Wuhan-Guangzhou line, and the braking pressure load spectrum of brake disc’s forging steel was obtained during service process. By means of the finite element software, the thermal stress field and residual stress distribution of brake disc are obtained.On the basis, the stress intensity factor of surface crack is analyzed.Combination of 1:1 bench test and finite element calculation method, the multi crack mechanism is in-depth studied on brake disc.Based on the research results, the service life of forged steel brake discs is researched based on fatigue damage theory and fracture mechanics. The main work involves:(1)The crack of forged steel brake disc was analyzed from the material fracture. The radial crack is mainly form on the brake disc surface, while the circumferential crack is less. The anatomy of disc crack shows that the long crack is formed by the connection of multiple semi elliptical surface cracks.(2)The monotonic tensile properties of forged steel were reseatched at room temperature and high temperature.And the elastic plastic hook relationship of forged steel was established.The fracture toughness, low cycle fatigue crack growth rate and the threshold were obtained from the sampling test of brake disc.(3)Based on the simulation results, the distribution of contact pressure was obtained. The crack which was produced at high temperature boundary between brake disc and pad was presented. Three dimensional finite element model of the brake disc was established. Then, the sequential coupling numerical methods were adopted. The stress and strain response of brake disc under different braking conditions was simulated.The tension and compression cycle, which is formed by the residual stress of bad braking and thermal stress, is an important reason for the crack propagation.(4)The stress intensity factor of brake disc was analyzed under different braking conditions, such as different shape ratio, deep ratio, crack length and crack angle. The results show that when the load condition is given, the stress intensity factor distribution of crack front has certain rules. With the crack growth, crack shape tends to be flat.(5)Based on the 1:1 platform test and simulation results, the interaction law of crack growth under multiple cracks is revealed. And the interaction between muliti cracks varies with the number of cracks, the crack spacing, the crack length ratio and the crack distance. The more parallel cracks, the samller crack spacing, the slower cracks expansion speed; The bigger crack shape, the smaller crack distance of crack tip on collinear cracks, the faster cracks expansion speed; Circumferential crack has little influence on radial crack.(6)The crack initiation and propagation life under different braking conditions are calculated based on fatigue damage theory and fracture mechanics. It is found that there is a good time to prolong the service life of brake disc if a bad braking occur occasionally in the early stage.By calculating the cumulative damage of the measured line on Wuhan-Guangzhou railway line, it is found that the combination of various braking methods in the actual running process can effectively reduce the fatigue damage caused by the brake disc braking. According to the interaction between cracks, the stress intensity factor of the crack front is modified. Finally the method of evaluating brake disc’s fatigue life of high speed train is proposed.