Internal states of model isotropic granular packings. II. Compression and pressure cycles

TL;DR

This study investigates how isotropic compression and pressure cycles affect the geometric and mechanical properties of spherical bead packings, revealing reversible volume changes but irreversible contact network alterations due to frictional hysteresis.

Contribution

It provides detailed numerical analysis of how pressure cycles influence contact networks and structural properties in granular packings, highlighting irreversible changes caused by frictional effects.

Findings

Solid fraction varies nearly reversibly with pressure

Contact networks change irreversibly due to frictional hysteresis

Initial configurations with high coordination lose contacts after compression

Abstract

This is the second paper of a series of three investigating, by numerical means, the geometric and mechanical properties of spherical bead packings under isotropic stresses. We study the effects of varying the applied pressure P (from 1 or 10 kPa up to 100 MPa in the case of glass beads) on several types of configurations assembled by different procedures, as reported in the preceding paper. As functions of P, we monitor changes in solid fraction \Phi, coordination number z, proportion of rattlers (grains carrying no force) x0, the distribution of normal forces, the level of friction mobilization, and the distribution of near neighbor distances. Assuming the contact law does not involve material plasticity or damage, \Phi is found to vary very nearly reversibly with P in an isotropic compression cycle, but all other quantities, due to the frictional hysteresis of contact forces, change irreversibly. In particular, initial low P states with high coordination numbers lose many contacts in a compression cycle, and end up with values of z and x0 close to those of the most poorly coordinated initial configurations. Proportional load variations which do not entail notable configuration changes can therefore nevertheless significantly affect contact networks of granular packings in quasistatic conditions.