Internal state of granular assemblies near random close packing

TL;DR

This paper investigates the internal structure of granular assemblies near random close packing, revealing how different assembling procedures influence fabric anisotropy, density, and coordination number, with implications for elastic properties.

Contribution

It demonstrates that density, coordination number, and fabric anisotropy can vary independently depending on the assembly method, challenging previous assumptions about their interdependence.

Findings

Lubrication and vibration procedures produce different packing structures.

Less dense packings can have higher coordination numbers.

Elastic moduli relate to internal state variables and stress states.

Abstract

The structure of random sphere packings in mechanical equilibrium in prescribed stress states, as studied by molecular dynamics simulations, strongly depends on the assembling procedure. Frictionless packings in the limit of low pressure are devoid of dilatancy, and consequently share the same random close packing density, but exhibit fabric anisotropy related to stress anisotropy. Efficient compaction methods can be viewed as routes to circumvent the influence of friction. Simulations designed to resemble two such procedures, lubrication and vibration (or ``tapping'') show that the resulting granular structures differ, the less dense one having, remarkably, the larger coordination number. Density, coordination number and fabric can thus vary independently. Calculations of elastic moduli and comparisons with experimental results suggest that measurable elastic properties provide information on those important internal state variables.