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In DEM simulations contact mechanics between particles is considered with simple models. A majority of the DEM studies includes a spring and dashpot or springs with two different rigidities during impact and rebound, respectively. These models specify the required contact forces in the normal and tangential directions. The contact forces also produce corresponding rotational resistance with respect to the particle's center. Most models do not consider additional contact couples. Iwashita and Oda were the first to include the locally generated couples at the contact surface, i.e. rolling resistance. They found that without such consideration, the high void ratio in the shear band observed in tri-axial tests of granular materials could not be reproduced. In a later study it was found that without rolling resistance, the simulated shear forces could not match the experimental tests in an annular shear cell. Paradoxically, rolling resistance appears to have negligible effect in two-sphere collisions. In this study, we examine the reason for the dependence of the global shear stress on the rolling resistance. It is found that rolling resistance promotes internal structure by increasing the contact duration. This effect is negligible under low solid fraction, consistent with the pairwise collision case, but its effect increases rapidly when solid fraction reaches a threshold. [ABSTRACT FROM AUTHOR]

DEM simulations of direct shear test have been performed to investigate the frictional behavior of granular media. The size dependence of bulk friction during the pre- and post- failure conditions is studied. Results show that the ratio of particle diameter to box size affects the bulk friction significantly. The particles’ translation, rotation, percentage of slipping contact, mean stress and best fit strain distributions are analyzed to reveal the relationship between the evolution of micromechanics parameters and the bulk friction. [ABSTRACT FROM AUTHOR]