Research On Characteristics Of Hot Spots And Initiation And Propagation Mechanisms Of Cracks Of Brake Discs Used For High Speed Trains

Brake disc is an important component of the foundation brake system for railway vehicles. Hot spots and thermal fatigue cracks appear on the friction surface of brake discs during repeated braking process. Propagation of fatigue cracks on the friction surface may lead to the failure of brake discs after a long period of service. This study is mainly focus on the features of hot spots and initiation and propagation mechanisms of fatigue cracks. It shows a significant theoretical meaning and application value for ensuring operation safety and benefit for railway vehicles.In this study, steel brake discs which are used for high speed trains were chosen to be the object and a series of theoretical and experimental studies were conducted. The evolution of hot spots and fatigue cracks on the friction surface were investigated by experimental observations. Low cycle fatigue behavior and the failure mechanisms of brake disc steel were studied. Basing on the strain-stress response under different braking conditions obtained from finite element analysis, the law of crack initiation and crack growth was discussed. Initiation and propagation life of the thermal fatigue cracks on brake discs were evaluated. The main research work of this paper is listed below.The features of the temperature field and oxidized film in the hot spot region were analyzed. The microstructure transformation of the brake disc steel was investigated by experimental observation of a failure brake disc part. The formation mechanisms of the hot spots were found to be combined action of localized friction heat, surface oxidization and microstructure transformation.With a brake disc with initial cracks, repeated braking tests were conducted for 1000 times on the full scale dynamo test rig with different braking energy. The evolution of fatigue cracks of brake discs on a railway vehicle in use were investigated by tracking research in a year. The crack growth behavior under different braking conditions was discussed by using an image processing code which was developed for crack identification and statistics. It was found that the repeated braking cycles with high braking energy will accelerate the initiation of the cracks. The routine braking with relatively low braking energy causes the stable propagation of fatigue cracks on the friction surface. The fracture surface of the cracks on the friction surface shows a semi-elliptic shape. The increase in length of cracks is caused by the growth of a single crack or the coalescence of several cracks.Monotonic and cyclic tensile tests were conducted from room temperature to 800℃. The thermal elasto-plastic constitutive model of the brake disc steel was built. Cyclic stress-strain behavior and the relationship between cyclic strain range and low cycle fatigue life at different temperature levels were researched. The failure mechanism of the brake disc steel at different temperature level and loading conditions were compared by analyzing the microstructure transformation, crack growth behavior and the morphology of the fracture surface.Thermal cyclic tests were carried out to investigate the volume expansion caused by phase changing of brake disc steel in the hot spot region. A FE simulation method which coupled the thermal stress with phase changing stress during braking was proposed. According to the simulation results, contact condition between brake disc and pads shows a significantly impact on the distribution of residual stress after braking. The occurrence of hot spots or hot bands leads to a great increase in the residual tensile stress level near the overheated area. The high stress region that formed around the phase changing area induces the initiation and propagation of surface cracks and embedded cracks.Basing on the linear cumulative damage rule and the local stress strain method, the crack initiation life of thermal fatigue cracks on the friction surface was evaluated. By using the linear elastic fracture mechanics method which considered the small-scale yielding, the relationship between stress intensity factor (SIF) and grow rate of fatigue cracks was built and the crack propagation life of the brake disc was evaluated. Crack initiation and propagation life drops with the occurrence of the hot spots induced by non-uniform contact of friction pairs.