Quasi-static incremental behavior of granular materials: elastic-plastic coupling and micro-scale dissipation

TL;DR

This study demonstrates that granular materials exhibit elastic-plastic coupling at micro- and macro-scales, challenging traditional assumptions, with detailed DEM simulations revealing complex irreversible behaviors during loading and reversal.

Contribution

The paper provides the first detailed DEM analysis showing elastic-plastic coupling in granular materials, emphasizing micro-scale contact behavior and its impact on macro-scale modeling.

Findings

Granular materials are coupled at micro- and macro-scales.

Contact sliding persists during load reversal, indicating irreversible micro-scale behavior.

The elastic domain in granular materials is smaller than the simulated strain increments.

Abstract

The paper addresses a common assumption of elastoplastic modeling: that the recoverable, elastic strain increment is unaffected by alterations of the elastic moduli that accompany loading. This assumption is found to be false for a granular material, and discrete element (DEM) simulations demonstrate that granular materials are coupled materials at both micro- and macro-scales. Elasto-plastic coupling at the macro-scale is placed in the context of thermomechanics framework of Tomasz Hueckel and Hans Ziegler, in which the elastic moduli are altered by irreversible processes during loading. This complex behavior is explored for multi-directional loading probes that follow an initial monotonic loading. An advanced DEM model is used in the study, with non-convex non-spherical particles and two different contact models: a conventional linear-frictional model and an exact implementation of the Hertz-like Cattaneo-Mindlin model. Rectilinear true-triaxial probes were used in the study (i.e., no direct shear strain), with tiny strain increments of $2\times 10^{-6}$. At the micro-scale, contact movements were monitored during small increments of loading and load-reversal, and results show that these movements are not reversed by a reversal of strain direction, and some contacts that were sliding during a loading increment continue to slide during reversal. The probes show that the coupled part of a strain increment, the difference between the recoverable (elastic) increment and its reversible part, must be considered when partitioning strain increments into elastic and plastic parts. Small increments of irreversible (and plastic) strain and contact slipping and frictional dissipation occur for all directions of loading, and an elastic domain, if it exists at all, is smaller than the strain increment used in the simulations.