EVOLUTION OF STRESS IN DENSE GRANULAR MATERIALS AT THE PEAK STATE OF LOADING

When dense granular materials are tested in shear or biaxial compression, the deviatoric stress increases with increasing strain until the deviator stress reaches a peak state. At the peak state, the deviatoric stress is stationary, although the material’s volume may be changing. The current study concerns the micro-scale processes that produce a stationary macro-scale stress at the peak state. We consider an unusual decomposition (partition) of the deviatoric stress, in which both changes in the contact forces and in their directions are separately acknowledged. The rate of stress is expressed as a sum of both contributions, and each contribution is further separated into the eects of the tangential and the normal contact forces. We then conduct numerical experiments to quantify each contribution to the average, stationary stress rate at the peak state. The Discrete Element Method is employed in simulations of a large dense assembly of two-dimensional circular disks. The results show that, at the peak state, the average stress is stationary, but that normal contact forces are, on average, increasing. This average increase in the normal forces is oset, in part, by concurrent changes in the tangential forces. The increase in normal force is also oset by changes in the directions of the normal contact forces, since the orientations of the contact normals change when neighboring particle pairs rotate around each other. This unusual result provides further understanding of how stress can evolve in a micro-scale view even as the macro-scale stress is stationary