Micromechanics approach to stiffness characterization of woven composites

Abstract: Thesis consider the stiffness of woven composites, methods of its calculation and stiffness reduction due to damage accumulation during the service. First, reliable methods for stiffness calculation of undamaged woven composites are considered. There are a large amount of simple approximate methods available in literature. They are very convenient to use but the error introduced by used assumptions may be considerable. In the first part of thesis a detailed investigation of this question is performed comparing approximate results with reiterated homogenization method. The second part of thesis deals with carbon fiber reinforced ceramic matrix composites and with the stiffness reduction due to damage accumulation. The morphology of the considered porous plain weave composite is complicated by stitches (fiber bundles) in the out-of-plane direction. Experimental data are presented showing dramatic changes in all elastic properties due to loading. FEM modeling is performed in order to analyze the initial stiffness of this material and its reduction versus the determined number of microcracks. An analytical model convenient for use is derived that gives results rather close to those obtained by finite element method. The Fracture Mechanics approach is used to estimate the critical energy release rate G_IC for several damage modes.