B. Ebrahimian - School of Civil Engineering, Faculty of Engineering, University of Tehran, Tehran, Iran
A. Noorzad - School of Civil Engineering, Faculty of Engineering, University of Tehran, Tehran, Iran

Shear localization phenomenon is not simulated properly within the framework of classical continuum due to the lack of material characteristic length. Consequently, the numerical solutions of rate boundary value problems exhibit mesh sensitivity and become mathematically ill-posed. It is widely accepted that post-bifurcation behavior can be well captured by micropolar (Cosserat) continuum to eliminate the limitations of classical continuum in problems involving shear localization. In this paper, an elasto-plastic micro-polar (Cosserat) model is proposed to capture the formation of shear band in biaxial compression tests. The model takes into account Cosserat rotations and couple stresses via the mean grain diameter as characteristic length. Applications of the model in simulations of biaxial compression tests with finite element method in Updated Lagrangian (UL) formulation are presented. Particular attention is paid to the effects of mesh size, mean grain diameter and boundary condition on the evolution of shear localization. It is found that prescribed boundary condition have strong influence on the pattern of shear banding in biaxial compression test. The numerical results are compared with those of experiments. Qualitative agreement is observed between numerical predictions and experimental measurements.

shear band, biaxial compression test, micro-polar (Cosserat) continuum,characteristic length, granular material