Micromechanical simulations not only enables to visualize the physical phenomena involved during failure, such as matrix failure, fiber/matrix debonding or ply delamination, but also can be used as very powerful tools for performing various virtual mechanical tests. Using this methodology different loading scenarios difficult and expensive to apply through experiments, can be simulated by means of numerical simulation of a representative volume element (RVE) of the microstructure. In this thesis, computational micromechanics is employed to examine the performance of the Tsai-Wu and Hashin criteria in prediction of matrix failure in composits in several multi-axial stress states. Micromechanical simulation of composite material failure requires a pressure-dependent failure model for the polymeric matrix. Available pressure-dependent damage formulations assume a certain shape of the stress-strain law under uniaxial loading. However, upon close iection none of the available formulations is able to reproduce the assumed shape. In this thesis, firstly, a new methodology for developing consistent pressure-dependent damage models for polymeric materials is presented. Secondly, macromodels are calibrated with the results from RVEs including 25 elastic fibers and a pressure-dependent damaging matrix in five basic virtual mechanical tests. Various stress combinations are applied to the RVE using PBCs and the failure envelopes from micromechanics are compared with those of Tsai-Wu and Hashin. Considering the failure envelopes obtained for longitudinal shear/longitudinal tension and also transverse tension/longitudinal tension, it is found that the stress in fiber direction has a significant effect on the predicted failure load. Although this effect is taken into account in the Tsai-Wu criterion, it is left out of consideration in the Hashin criterion. The big difference between the predictions of the two macroscopic failure criteria and micromodel response in combined loadings including transverse tension and longitudinal shear indicates that the interaction between these two stress components is very different depending on whether they are in the same plane or not. Afterwards, the performance of the model is enhanced by formulating an elastic-plastic-damage model for the matrix material and employing cohesive elements between fibers and matrix. There is a good agreement between the micromechanical simulations and experimental results. Keywords: Pressure-dependent damage model, Micromechanicas, Faiure envelope, Polymer composite, Combined loading.
