3D Reconstruction Of Micropores In The Material Based On Stereology And Its Fatigue Life Prediction

Cast aluminium alloy, which exhibits many advantages including low mass, high specific strength, and low production cost, is widely used in various fields. Due to its process characteristics, material internal defects,such as micropores, are usually difficult to avoid. Fatigue cracks of the material initiate from micropores easily,especially the micropores on the surface or the near surface region. In order to predict fatigue life of the material more accurately, we need to get the 3D distribution of micropores in the material.In this thesis, the micropores cross sections in the casting Al-Zn 713 had been studied by using metallographic experiment and image analysis and processing technology. Using the equivalent radius to characterize the size of the micropores.To determine the minimum number of test specimens,the maximum and minimum equivalent radius of the micropores with change of specimen numbers were studied.Also the sample mean and the variance of equivalent radius of micropores were analyzed.Kernel density estimator method instead of traditional histogram method was proposed to deal with the experimental data.Thus the negative in stereology reconstruction was avoide and it could get smooth and continuous curves.The function and bandwidth of kernel density estimator on the effect of 2D distribution of micropores were studied. The 3D distribution is from the 2D density estimate in an iterative manner, so the precision of reconstruction was improved. Different number of groups and specimens on the results of 3D reconstruction of stereology were studied. It was found that the three dimensions size distribution of the micropores in the Al-Zn 713 materials obeys two parameter lognormal distribution. To test the reliability of new method, it generated a cube model included three dimensions size distribution of micropores and carried on a large number of random cross sections. It was found that the micropores distribution had a good agreement with the experimental results.The dangerous micropores which lead to fatigue crack initiation easily had been analysed. Material finite element analysis model considered micropore defects had been established. Based on the finite element analysis result, the micropores distribution and fatigue life model had been established using the modified Coffin-Manson formula. According to the three dimensions distribution of micropores obtained above, finally the material fatigue life curves had been determined. Established "metallographic experiment, kernel density estimator and stereology in 3D reconstruction, finite element analysis, fatigue life" method for the study of fatigue performance of different material in micropores defect providing a new way of thinking.