Modeling Of The Microstructures And Prediction Of The Overall Moduli Of SiC Composites Toughened By SiC Fibers

In view of their excellent mechanical behaviors,thermal stability,low expansions and many other properties in extremely high temperature environments,continuous fiber reinforced ceramic matrix composites(SiC_f/SiC CMCs)have been widely applied as the key components in the hot end of aircraft engines.In our country,synthesis of new SiC_f/SiC CMCs and development of advanced testing devices have been conducted only from very recent years,hence,researches in the fields of fundamental theories,raw SiC material development and other relevant subjects largely lag behind the developed countries,which greatly restrict the development of high performance aircraft engines of our own.So far,developing new kinds of SiC_f/SiC composites and improving the performance of existing ones commonly adopt the trial-and-error methods,which are largely dependent on the designers'experience and affected by the contingency.Also,it is a time-and money-consuming process.With the development of the high performance computers,micromechanics theories and optimization design methods,accurate evaluation of the macroscopic properties and parametric optimization design of micro-architectures of SiC_f/SiC composites have become the intensive topics of the current researches.Note that most studies on predicting the macroscopic properties of SiC_f/SiC CMCs simplifies the preforms at the microscopic scale as being consisting of SiC bundles of the polygonal cross-section and composed of multiple linear segments,which are essentially different from the practical microstructures in applications.Such simplications lead to the large discrepancy between the numerical predictions and experimental data.Besides,the current studies on the influence of the microstructures together with the embedded porosity of SiC_f/SiC composites have not been yet systematic.To deepen the understandings on the influence of microstructure constitutions and distribution of porosity on the macroscopic properties of SiC_f/SiC composites,the following researches were coducted in this study:(1)Development of an automatic approach for modeling the microstructures of continuous fiber reinforced ceramic matrix composites:a discretization strategy for dividing3D tetrahedral elements automatically is developed,and the configurations of fibrous tows,continuous fiber bundles,complex preforms are reconstructed automatically based on the relevant parametric descriptions and the anisotropic material parameters are assigned accurately.(2)Construction of the governing equations of boundary value problems and establishment of a computational method for evaluating the macroscopic moduli of continuous fiber reinforced ceramic matrix composites:the governing formulations,the strong form,weak form and discretize form of the SiC_f/SiC boundary value problems are constructed,and the anisotropy of SiC_f bundles is described accurately.Then,established a multiscale approach to predict the overall moduli of the SiC_f/SiC composites,and the analytical solution of a simplified model is employed to verifiy the computational approach.(3)Systematic discussions are made on the influence of the SiC fiber and fiber bundle constitutions on the overall moduli of continuous fiber reinforced ceramic matrix composites.The predicted results show that the volume fraction of fibers has a great influence on the overall moduli of the fiber scales,the material properties,volume fraction and waviness ratio of fibers bundle have a certain influence on the overall moduli of SiC_f/SiC composites.(4)A numerical strategy to described the porosity and to generate pores in SiC_f/SiC composites is proposed with the help of the porosity distributions of the fiber and fiber bundle scales observed experimentally.The validity of the elaborated strategy is illustrated by comparing the numerical predictions and the experimental data.The influence of the porosity of both the fiber and fiber bundle scales is discussed in detail.The prediction results show that the pores in fiber bundle scales have much greater influence on the overall moduli of SiC_f/SiC composite than in fiber scale.