Analysis Of The Coalition And Interference Of High Temperature Low-cycle Short Fatigue Crack Under Complex Stress Condition

With the development of world economy, there is more and more application of power machine and high-temperature high-pressure apparatus, which inevitably leads to more problems about safety reliability. In practical engineering, nearly eight percent of structural failure is caused by fatigue, so it is of much academic and engineering meaning to do research work on fatigue. It has been testified in many experiments that the fatigue fracture process is affected by the initiation, propagation, conjunction and interference process of short cracks, and sometimes the short-crack phase occupies more than eight percent of the total life. The macro fracture mechanics can not be employed to deal with short crack because of the randomcity of its propagation rate, so it is necessary to study the behavior of short crack in microscopic view. At the present time, the study on short crack is still on its preliminary stage, and much profound research needs to be done.In this paper, the conjunction and interference of surface fatigue short cracks in high temperature under complex stress state was investigated, and the physical model describing the evolution process of short cracks was established though incorporating the study on the behavior of short cracks and known regularity, then the visualization of the initiation, propagation, conjunction and interference process of short cracks was realized with the use of Monte Carlo method. The work that has been done in this paper is summarized as following:1. The engineering background and significance of the study on the high temperature strength are dissertated, and a variety of long and short crack models being studied in the structural fatigue field are discussed systematically.2. The experimental study on the conjunction and interference of surface short cracks of 16MnR under complex stress state and high temperature was done. The conjunction and interference of short cracks were observed and analyzed successfully through experiment intermittence and the application of copy-surface technology, and then the evolution characteristic and the damage mechanism were also acquired.3. The simplified Monte Carlo method of micro structure of metallic material is established based on the fact that the solidification of metal is Monte Carlo process. By means of 'random distribution, growing with constant rate and point-by-point scanning', the intergranular map was formed, and the approximate linear shape of intergranular structure was also mappedthrough the identification and picking out of triple junction, which was the preparation for the consequent numerical simulation.4. The physical model describing the initiation, propagation, conjunction and interference process of fatigue cracks was set up with the Monte Carlo method, and the obstructive effect of the micro structure, the interference effect among short cracks and the conjunction of them were all taken into consideration. The model can simulate the whole process of the initiation, propagation, conjunction and interference of fatigue cracks, and visualize the micro physical process of material fatigue damage.5. Through the analysis of simulation results and, the total length of short cracks on a certain area is brought forward as the key parameter related to material damage, and the relationship between that parameter and the relative life of material were presented as polynomial expressions and diagrams which can be used to estimate the life of material.6. The application of the model studied in the remaining life estimation and its prospect are also discussed.