Research On Stiffness Model And Stiffness Analysis Of Three-Dimensional Braided Ceramic Matrix Composites

Three-Dimensional(3-D) braided ceramic matrix composites(CMCs) are newly developed structural materials with excellent high temperature properties, and have begun to take the place of high temperature alloys and been more and more widely applied in hot components of aviation and aerospace. Because the microstructure of 3-D braided CMCs are very complex, the research on mechanical properties of CMCs is immature, the mechanical properties of fiber will decrease during the unique preparing process, and interface layer sedimentated around the fibers and cavities in the matrix make the problem more complicated, few research focus on stiffness properties of 3-D braided CMCs. Therefore, it's very important to research on this aspect.The main research works include:1. The simulation method of 3-D track and column braiding performs and braided composite structures is proposed and simulation system of 3-D track and column braiding is exploited. Based on the braiding principle of track and column braiding, the concept of braiding plan is introduced, mathematic collection and permutation theory is adopted to establish the mathematic model, then the design program to calculate the spatial location of braided yarns is exploited, the thread path is described by Bézier curve. The simulation system of 3-D track and column braiding is exploited by Visual C++ 6.0 and UG NX2.0 simulation software.2. The microstructure model of 3-D braided CMCs is developed and brought forword based on the improved three-cell model. The model is composed of interior unit-cell, two types of surface unit-cell and corner unit-cell. The helix fiber bundles is considered by analyzing fiber bundles paths in detail, the interface layer around the fiber is considered by describing the effect of interface layer on the fiber micro-dimension, and the fiber jamming conditions in different regions are considered by introducing average yarn packing factor in each unit-cell in the model. Then the mathematical relation between braiding technics parameters and microstructure of the unit cells are deduced.3. Tension and compression stiffness model of 3-D braided CMCs is proposed. The effect of cavities and in-site properties are also taken into account in the model. Firstly, effective elastic properties of unidirectional composite roles in the four unit-cells is derived by the components'elastic properties, then elastic tensors of composite roles on various orientations are transformed from each local coordinate system to the composite global coordinate system, stiffness properties of each unit-cell can be gained. At last, the effective elastic properties of CMCs can be obtained by the volume average of elastic properties of the unit-cells. Engineering elastic constants of 3D braided C/SiC are deduced. Comparison of the calculating results with the available experimental data shows the applicability of the proposed model and the method.4. Using the proposed tension and compression stiffness model, the influences of fiber volume fraction and braiding angle to stiffness properties of each unit-cell are discussed by the meso-mechanical method, the influences of fiber volume fraction, braiding angle, braiding structure column number, thickness of interface layer and cavity volume fraction to stiffness properties of the composites are discussed, the influences of elastic modulus of matrix and fiber to the stiffness properties are analyzed too.5. Bending stiffness model of 3-D braided CMCs is proposed. Firstly the total composite is divided into multilayer structure comprised of interior layer and exterior layer, the stiffness properties of each layer can be deduced, thus the bending properties of different braiding parameters can be deduced by material mechanics theories. Bending stiffness modul of 3D braided C/SiC is deduced and comparison of the calculating results with the available experimental data shows the applicability of the proposed model and the method.The influences of microstructure parameters and components mechanical behavior to bending properties of the composites are discussed by the meso-mechanical method.