Modelling Grain Damage Under Plane Strain Compression Using a Micro-polar Continuum

A novel concept for modelling the influence of grain damage on the reduction of the mean grain diameter and consequently on the compaction behaviour and incremental stiffness is proposed. Herein the constant solid hardness in previous models is replaced by a pressure dependent stiffness factor. The change of grading of the grain sizes caused by grain damage is reflected in a simplified manner by a reduction of the mean grain diameter. In order to distinguish the effect of the loading path on the evolution of grain damage the evolution equations for the solid hardness and the mean grain diameter are decomposed into parts reflecting the influence on isotropic and deviatoric loading. The mean grain diameter is embedded as the internal length in the micro-polar hypoplastic model. Numerical simulations of the influence of grain damage on the compaction behaviour are carried out for biaxial compression tests. The results show that for the case of shear strain localization the reduction of the stiffness factor and the mean grain diameter is more pronounced in the shear band than outside of this zone.