Research On Thermal Damage And Structure Design Of Aluminum Matrix Composites Brake Disk For High-speed Train

Lightweight was one of the key technologies for the high-speed trains. Unsprung weight can be reduced by using SiCp/A356 brake disc, so as to reduce the weight of high-speed trains. Comparing to iron and steels, SiCp/A356 composites has larger linear expansion coefficient and lower strength at high temperature, which increase thermal damage tendency and limit speed range of SiCp/A356 brake discs. Based on finite element simulation, thermal damage and structure design of SiCp/A356 brake disc for high speed train were studied in this paper through theoretical analysis and experimental study. On the basis of the results, the structure optimization design of the brake disc was carried out in order to increase the speed of its application.The mechanical and thermal physical properties of the 20 wt% SiCp/A356 composites at different temperatures were tested in this paper. And the property-temperature curves were obtained. Through the method of the finite element analysis and experiments, micro-damage mechanism of SiCp/A356 composites was investigated systematically. Micro-damage mechanism obtained by the finite element simulation was consistent with the mechanism from the fracture analysis. It can be concluded that the crack initiation mechanism of the composites at room temperature was mainly tear of the matrix and fracture of SiC particles mainly; while the mechanism of the composites at high temperature was SiC particle debonding from the matrix and tear of the matrix. Finite element simulations of the monotonous tensile tests were carried out for different percentage of particle in the composites at room temperatures, and the same percentage of particle in the composites at different temperatures. The simulation results were consistent with the experimental results. It can be concluded that when the content of SiC particles in the composites was no more than 20 wt%, after the uniform assumption for the SiCp/A356 composites was valid during finite element analysis, which established the foundation for the finite element analysis of the SiCp/A356 brake disc.With regard to FEM of disc brake, three typical finite element models for the braking simulation in this paper were established. Based on the temperature data obtained by infra-red temperature test imaging system during 1:1 dynamo test, their characteristics and applicability were studied. On the basis of the part-disc indirect coupling method, the full-disc indirect coupling model was put forward by changing the flux input method. And the multi-contact elastic-plastic thermal-mechanical coupling model of disc brake was established in order to investigate the thermal-mechanical coupling problems. This provides the methods for the research on the thermal damage and structural design of the brake disc.In the aspect of thermal damage mechanism study of SiCp/A356 brake disc, the forming mechanism, influencing factors and preventive measures of three typical thermal damages including hotspots, thermal-cracks and cracking were investigated through the combination of the finite element simulation, the theoretical analysis and the test results in this paper. The results showed that hotspot was caused by the change of structure and the oxidation due to the high temperature caused by surface friction on the brake. Crack firstly initiated at grain boundary due to higher internal stress at the edge of high temperature field caused by surface friction. The cracks propagated along the grain boundary due to the radial and circumferential tensile stress and finally created thermal-cracks. Non-uniform deformation was caused by non-uniform distribution temperature field in the brake disc. When the deformation was constrained by the structure and mechanical constraints of the brake disc, the thermal stress was introduced. If the thermal stress was bigger than the strength of materials at corresponding temperature, it led to the crack of the brake disc. In addition, based on the thermal stress research of brake disk, five kinds of constraints which might lead to thermal stress were summed up.Considering whether the thermal stress led to brake disc cracking, the structure design of the SiCp/A356 brake disc was studied by Finite Element Method (FEM). The effects of the brake disc components on thermal stress were analyzed. A structural components combination method and general principles of structure design of the brake disc were put forward. Based on these researches, a brake disc structure was designed which satisfied the application requirements of emergency braking at 280 km/h.