Mechanical and Structural Contributions to Anisotropy in Granular Materials

TL;DR

This paper introduces a method to distinguish and quantify mechanical and structural anisotropy in granular materials using laboratory data, revealing how these contributions vary with stress conditions.

Contribution

It develops a first-order linearisation approach that isolates mechanical from structural anisotropy, enabling direct measurement from macroscopic tests.

Findings

Both anisotropy components increase with deviatoric stress.

Mechanical anisotropy is generally stronger than structural anisotropy.

The dominance of mechanical anisotropy decreases as deviatoric stress increases.

Abstract

Anisotropy in granular materials arises from both the internal fabric and the directionality of the stress state, yet separating these effects experimentally remains challenging. This study develops a first-order linearisation of the incremental stress-strain response that isolates mechanical anisotropy from structural anisotropy using two independent orientation measures. The formulation enables both contributions to be quantified directly from macroscopic laboratory data. The method is applied to hollow-cylinder tests with systematically varied loading directions. Results show that both anisotropy components intensify as the stress state becomes more deviatoric; mechanical anisotropy is consistently stronger; and its relative dominance decreases with increasing deviatoric stress. Comparison with an isotropic hypoplastic model confirms that mechanically induced directional effects are captured even without fabric anisotropy. The framework offers a practical and physically transparent means for quantifying and comparing anisotropy mechanisms in granular materials.