Non-coaxial plastic flow of granular materials through stress probing analysis

The current study investigates the non-coaxial plastic flow in granular media where the principal directions of incremental plastic strain differ from those of the current stress (and of the current fabric tensor). The stress probing technique is used with Discrete Element Method simulations to capture the plastic response of a 2D granular assembly to loading increments along different directions in stress space, including non-coaxial ones. A general form for the plastic flow is suggested, based on multi-mechanism plasticity theory which is demonstrated to be capable of capturing the essential characteristics of the non-coaxial plastic flow. Deviations between DEM results and model predictions are associated with the incrementally nonlinear nature of the response. The analysis of the results indicate that, at a given stress state, the plastic yielding direction is not coaxial with the current stress. The significance of the non-coaxial plastic flow is shown to be around 20% for lower deviatoric stresses, but this value diminishes as the peak stress is achieved, hinting to the dominance of a single mechanism at failure. The study concludes by comparing the developed model with two well-known non-coaxial models in the literature, where the performance of the developed model is found to be close to the double-shearing theory. (C) 2021 Elsevier Ltd. All rights reserved.