Characterization of the material response in the granular ratcheting

TL;DR

This paper investigates the steady-state granular ratcheting behavior under cyclic loading, characterizing it with three measurable macroscopic variables and analyzing the influence of micro-mechanical factors through molecular dynamics simulations.

Contribution

It introduces a simple macroscopic characterization of granular ratcheting in terms of three variables and explores the micro-mechanical influences using detailed simulations.

Findings

Permanent strain accumulation becomes independent of cycle number after transient

Contact stiffness and friction significantly affect the response

Hysteresis in stress-strain influences energy dissipation

Abstract

The existence of a very special ratcheting regime has recently been reported in a granular packing subjected to cyclic loading \cite{alonso04}. In this state, the system accumulates a small permanent deformation after each cycle. After a short transient regime, the value of this permanent strain accumulation becomes independent on the number of cycles. We show that a characterization of the material response in this peculiar state is possible in terms of three simple macroscopic variables. They are defined that, they can be easily measured both in the experiments and in the simulations. We have carried out a thorough investigation of the micro- and macro-mechanical factors affecting these variables, by means of Molecular Dynamics simulations of a polydisperse disk packing, as a simple model system for granular material. Biaxial test boundary conditions with a periodically cycling load were implemented. The effect on the plastic response of the confining pressure, the deviatoric stress and the number of cycles has been investigated. The stiffness of the contacts and friction has been shown to play an important role in the overall response of the system. Specially elucidating is the influence of the particular hysteretical behavior in the stress-strain space on the accumulation of permanent strain and the energy dissipation.