Strain-stiffening in random packings of entangled granular chains

TL;DR

This study uses granular chain packings as a model to explore how entanglements contribute to strain-stiffening behavior, revealing that longer chains form entangled networks that significantly increase material stiffness under shear.

Contribution

It demonstrates that entanglements in granular chains induce strain-stiffening, providing a new model to understand polymer-like behavior without thermal motion.

Findings

Long chains form entangled networks that resist shear.

Strain-stiffening occurs only in long-chain packings.

Entangled clusters span the entire system, increasing stiffness.

Abstract

Random packings of granular chains are presented as a model polymer system to investigate the contribution of entanglements to strain-stiffening in the absence of Brownian motion. The chain packings are sheared in triaxial compression experiments. For short chain lengths, these packings yield when the shear stress exceeds a the scale of the confining pressure, similar to packings of spherical particles. In contrast, packings of chains which are long enough to form loops exhibit strain-stiffening, in which the effective stiffness of the material increases with strain, similar to many polymer materials. The latter packings can sustain stresses orders-of-magnitude greater than the confining pressure, and do not yield until the chain links break. X-ray tomography measurements reveal that the strain-stiffening packings contain system-spanning clusters of entangled chains.