Extracting Contact Forces in Cohesive Granular Ensembles

TL;DR

This paper introduces an extension to the PeGS software that enables the measurement of both tensile and compressive contact forces in cohesive granular materials using photoelasticity, advancing micro-mechanical analysis.

Contribution

The authors develop a novel method to distinguish and measure tensile and compressive forces in cohesive granular systems using an extended open-source photoelastic analysis tool.

Findings

Effective in dilute systems with up to 25% bonded dimers

Allows for the separate evaluation of tensile and compressive forces

Provides a first step towards experimental micro-mechanical studies of cohesion

Abstract

Interparticle cohesion is prevalent in stored powders, geological formations, and infrastructure engineering, yet a comprehensive understanding of the effects of its micro-mechanics on bulk properties has not been established experimentally. One challenge has been that while photoelasticy has been widely and successfully used to measure the vector contact forces within dry granular systems, where the particle-particle interactions are solely frictional and compressive in nature, it has seen little development in systems where tensile forces are present. The key difficulty has been the inability to distinguish between compressive and tensile forces, which appear identically within the photoelastic response. Here, we present a novel approach which solves this problem, by an extension to the open-source PeGS (Photoelastic Grain Solver) software available at https://github.com/photoelasticity. Our new implementation divides the procedure of finding vector contact forces into two steps: first evaluating the vector contact forces on the non-cohesive particles present in the ensemble, followed by using an equilibrium constraint to solve for the forces in the bonded particles. We find that in the dilute limit, for up to 25% bonded dimers, we can solve for all forces since each particle has only one force bearing contact that can potentially transmit tensile forces. While the case of dimers is an idealised version of cohesive granular ensemble, it provides an important first step towards experimentally studying the micro-mechanics of cohesive granular materials.