Instability of Particulate Assemblies under Constant Shear Drained Stress Path: DEM Approach

The discrete element method (DEM) is applied to investigate instability of frictional particulate assemblies sheared under the triaxial constant shear drained (CSD) condition. The instability in loose samples occurs in the contractive regime of the behavior prior to the complete mobilization of the critical state friction angle. In contrast, instability of dense assemblies occurs once the peak stress ratio is attained. For the loose assemblies, certain states for the onset of instability may be found by means of the intersection of the effective stress path with the instability line obtained from constant volume tests, the second-order work, nullification of volumetric strain rate, and the abrupt rise in the axial strain rate. However, only the second-order work and the sudden rise in the axial strain rate criteria can be applied to examine the instability of dense granular samples. Using the DEM data, evolutions of coordination number and contact fabric tensor in both loose and dense assemblies are investigated. Also, as a complement, a modified plasticity model based on the anisotropic critical state theory is presented that can effectively simulate the instability of loose and dense samples sheared under CSD stress paths using a single set of the model parameters.