Research Of Material Fatigue Life Prediction Method Based On Stereology

Cast aluminium alloy is widely used in various fields because of good casting performance, high specific strength, and low production cost. Due to its process characteristics, material internal defects such as micropore are usually difficult to avoid, they have important influence on fatigue crack initiation and fatigue life for materials and parts. The fatigue performance study based on pore defects is one of the current research hotspots in the field of fatigue. Focus on the micropore defects on the surface of casting aluminium alloy in this paper, material internal micropore 3D information has been reconstructed by using stereology method, then a material fatigue life prediction method has been researched, combined finite element analysis.In this paper, the micropore cross sections in the casting Al-Zn 713 has been studied by using metallographic experiment and image analysis and processing technology. Micropore cross section profiles and its regularity has been obtained. At the same time, the two dimensions size distribution of micropores has been calculated and its regularity is analysed.According to the two dimensions size distribution of micropores which is obtained from the experimental study of the micropore in the casting Al-Zn 713, the three dimensions size distribution of micropores has been reconstructed by stereology method, and validated further, It is found that the three dimensions size distribution of the micropores in the materials obeys two parameter lognormal distribution and the precision of the reconstruction result is reliable. In addition, the dangerous micropores which lead to fatigue crack initiation easily has been analysed based on the reconstruction result by using statistical method. The effect of micropores on the fatigue crack initiation has been studied when considering the geometry size and depth of micropore have influence on fatigue crack initiation.According to the three dimensions size distribution of micropores obtained above and the analysis result of the dangerous micropores, material elastic-plastic finite element analysis model considered micropore defects has been established. The stress and strain around the micropore are analyzed, then the crack initiation arc is determined, the plastic shear strain on the crack initiation arc has been calculated later. Based on the finite element analysis result, the material fatigue crack initiation life has been predicted using the modified Coffin-Manson formula. In the work of prediction, the material fatigue crack initiation life has been estimated when the micropore is under a variety of sizes, depth location, and different stress levels. Finally the material fatigue limit S-N curves, the range of crack initiation life, and the fatigue strength has been determined. The results are validated that they are in agreement with experiment date in other paper. The method“pore three dimensions size distribution reconstruction- finite element analysis- fatigue life analysis/prediction” researched in this paper is a new way can be used to study the fatigue life prediction.