The effect of interphase and interfacial cracks on the elastic and elastoplastic behavior of fiber-reinforced composites

In order to provide a better understanding on the mechanical properties of composite materials, a dual homogenization and finite-element study is carried out in this thesis to examine the elastic stiffness and elastoplastic behavior of a fiber-reinforced composite containing either ductile interphase or interfacial cracks. In the case of interphase, circular fibers are examined. In the other case of interfacial cracks, the fibers are taken to be with elliptic cross-section to demonstrate the influence of the fiber shape.; The influence of interphase is studied by the generalized self-consistent model and by finite element method (FEM) with NASTRAN. Both the traction specified boundary and displacement specified boundary conditions are carried out to determine their influence on the elastoplastic behaviors of the fiber-reinforced composite. The propagation of plastic zone in both the interphase and the ductile matrix, and stress distribution in the interphase are also vividly demonstrated.; A fictitious fiber model is adopted to mimic the effect of double debonding on composites. Based on this model, the elastic stiffness of elliptic fiber-reinforced composites is obtained directly by the homogenization method, and its elastoplastic behavior is further studied by use of the concept of secant moduli and an energy-based effective stress which is evaluated by a field fluctuation method. The finite element method is carried out by ANSYS to check the accuracy of the homogenization theory and to find out its associated debonding range in practice. The local stress and deformation behavior of the composite are also given by FEM as the complementary to the homogenization theory. The single debonding is performed also by FEM to compare its influence with that of the double debonding.; A new model to investigate the effect of single debonding on the effective elastic moduli is developed based on the elasticity solution of a circular cylinder with an interface crack. The solution are developed by use of complex variables, and the results of the transverse Young's modulus as well as the plane strain bulk modulus show that this model is capable of providing sufficiently accurate moduli when volume concentration of fibers is not high.