Multi-Scale Analyses Of Damage Evolution In Woven Composite Materials

Woven fabric composites are constructed by weaving two fiber tows into each other. The interlacing of fiber bundles makes the composite materials possess several advantages, for example, increase of the intra- and inter-laminar strength, damage tolerance, and impact resistance, and ability of making near net shape structural components. Consequently, woven composites, as a structural material, have received more and more attention recently. The materials have been widely used in the areas of aeronautical and aerospace engineering, bio-medical engineering, and automotive industry. For more effective use and design of woven composites, it is desirable to investigate deeply their mechanical properties.In this dissertation, two-scale models, the microscopic repeated unit cell (RUC) model for yarn and the macroscopic repeated unit cell model for woven composite, are presented, and 3D finite element analyses are performed to predict the effective stiffness, the effective strength, and the damage evolution process of plain woven composites and the 3D woven composites. The micro-RUC model for the yarn is built based on a hexagonal array of fibers. The properties, such as elastic constants and strengths, of different fiber/matrix systems with various fiber volume functions are obtained based on FE analysis together with appropriate failure criteria of the two basic constituents of the composite (fiber and matrix). Modifications to the existing theoretical formulas for calculations of stiffness and strength of yarns have been made based on the comparisons between the predicted results and data given in the literature.A macro-geometry model, named three-dimensional curve model, is established to characterize more accurately the inter-structure of plain woven composites. The numerical results of yarn obtained on the microscopic repeated unit cell (RUC) model are adopted as the properties of impregnated tows in the macroscopic analysis. The responses of the composites under tensile, compress and shear loadings are obtained by using 3D FE method. Then, the microstructure of 3D woven orthogonal interlock composite is studied in detail experimentally. Based on the observations, a novel model is built by using the Hermit spline functions. The 3D FE analysis are performed on the new model for studying the damage evolution of 3D woven composites under tensile loadings in both warp and weft directions and under shear loadings.The periodic boundary conditions are applied to the two-scale models during the 3D FE analysis in order to ensure that both the displacement and stress are continuous on the boundary surfaces. Once damages occur, the stiffness reduction is considered in particular orientations in both the micro- and macro-scale RUCs, thus, the"element disappear"technique, commonly used in previous researches, is abandoned. The process of Resin Transfer Molding(RTM)is also studied in the thesis. A RTM equipment is designed and manufactured in the laboratory. Specimens of 3D woven composites are manufactured with house-made RTM equipment. Tensile, compressive and shearing tests are performed. The fundamental mechanical properties of plain woven composites and 3D woven composites are obtained. The experimental data agree well with the predicted results, thus, the correctness of the established two-scale models and analysis method have been verified.