Study On Initiation And Propagation Of Short Crack For Low Cycle Fatigue At High Temperature And Complex Stress States

With the development of science and economy, the applications will be broader, of which are various power mechanisms and equipments under high temperature and high pressure, and the problem of safety will be more important. In engineering application, about eighty percent of invalidation is caused by fatigue failure. In the reliability and security evaluation of used machinery, present work is based on the classic fracture mechanics, and model of long crack propagation rate is used, effect range of which is the original state under long crack propagation. It is indicated by the experiments, for some materials, it takes most of the fatigue life that is short crack period, compared with long crack, short crack behavior is obviously unusual, which can't be explained by the traditional theories. There are a lot of advantages of study on fatigue short crack, for example, finding the physical mechanism of fatigue damage, explaining the study results of classic fatigue theories, all of which can afford academic information for the reliability and security evaluation of used machinery, life forecast and structural safety study under interactive fatigue-maintenances.In this paper, the short crack for low cycle fatigue at high temperature and complex stress states is studied. Through combined experiment technique and computer simulation, the material 16MnR under various stress state is experimentally studied, the propagation, coalescence and interference influence of short crack are observed by experiment interrupted and replication method. The average crack length and crack distributing angle are statistically analyzed. Through the simulation program of evolution of collective short fatigue cracks based on Monte Carlo method, evolution of collective short fatigue cracks under high temperature of material 16MnR is simulated. Comparing the simulation results with the experiment results, the simulation crack length is statistically analyzed. The propagation paths of leading and subaltern cracks are followed, and the short crack propagation rate is analyzed. What the experiment observation and simulation analysis indicate are as follows: in the beginning of the life, initiation of collective short fatigue cracks is the main form of fatigue damage, which can be taken for equality. High temperature fatigue damage results from the short crack along the grain boundary, which is effected by massing and growing of holes in the grain boundary. Stochastic initiation and propagation of crack go along vertical direction of the stress axis, and crack distributing depends on crack angles. In the last of the life, propagation and coalescence of short cracks are the main form of fatigue damage, the process...