A COMPARATIVE STUDY OF MICRO-MODELS AND MICRO-MECHANICS BASED BIFURCATION ANALYSIS

Present paper discusses a simplified square cell model (SSCM) derived from elementary mechanics and a finite clement- based 3-phase cylindrical fiber model (FECM) in comparison. The latter is developed with primary object of determining stress variation at the microlevel and follow the evolution of damage. This model is seen to be very effective because of uncoupling of harmonics in the solution process and promises to be an effective tool for nonlinear analysis of composites. It is seen that the determination of the elastic constants depends on the average stresses and strains, and both SSCM and FECM models are satisfactory for this purpose. However, for an accurate determination of initial yielding under combined loading, a micro-model which gives a good description of the variation of stresses across the model is required. Thus square cell models fail to predict accurately the first yield in the context of stress states which are nearly hydrostatic. The nonlinear response of a lamina or a laminate undergoing shear-softening is investigated using the micro-models. The values of the parameters characterizing the nonlinear response of the matrix are determined from the shear response of the lamina with respect to the principal axes. It is found that these values are strongly dependent on the model employed. However, once these parameters are determined, both the models are capable of giving satisfactory predictions, at least in the cases studied here. Accurate correlation between in-situ properties of matrix and its bulk properties is required to get correct nonlinear parameters and to be able to judge correctness of a micro-model.