Contact forces in anisotropic frictional granular material

The probability density functions (PDFs) of contact forces in anisotropic, cohesionless and frictional granular materials are studied numerically and theoretically. Using discrete element simulations of biaxial deformation of a large two-dimensional assembly consisting of 200,000 disks, it is observed that the PDFs for the normal and tangential components of the contact forces depend significantly on contact orientation. The PDFs exhibit exponential decay and the PDF for the tangential component of the contact forces is not always symmetrical with respect to zero tangential force. The shape of the PDF for the normal component of the contact forces changes with shear strain. A qualitative explanation for this change is given that is related to the biaxial deformation mechanism in which the disrupted contacts are predominantly oriented in the direction of the minor principle stress. A maximum entropy method is employed to study these PDFs theoretically, using a prescribed stress tensor as constraint. It is found that the theoretical results correspond qualitatively to many of the results obtained from the discrete element simulations. Discrepancies between theory and simulations are attributed to the fact that the kinematics have not been taken into account in the theory.