An anisotropic yield criterion for composite

The intrinsic structure of a wide range of structural, geological and industrial materials, such as composites, masonry, wood, textiles and several types of rocks and clays, is the major cause of their anisotropic behavior. The material response to the same stress state is strongly linked to the sampling orientation with respect to the principal stress axes. The position and the orientation of microfibers, for example, define a sort of material geometry on which the stress geometry depends. Some of the research efforts made in recent years to achieve a better understanding of constitutive behaviour of quasi-brittle materials have been developed in the framework of plasticity. The theoretical and numerical problems about the variational formulation of non-associated plastic flow rule has been developed although the complexity of the problem welcomes more studies in the future. Most contributions towards establishing a rational failure theory of quasi brittle materials exhibiting a strong anisotropic behaviour are phenomenological: experimental results are interpreted on the basis of a criterion which is assumed to be suitable for the observed mode of failure. The yield condition, based on the generalization of the Mohr-Coulomb criterion, involves six material parameters. A geometrical representation of the failure domain is provided 3 in the case of plane stress. Experimental tests arranged for the identification of the criterion and performed on composite samples are considered and discussed in this paper. It is shown that the agreement between the theory and experiments is reasonably good.