Slow relaxation behavior of cohesive powders

TL;DR

This study investigates the slow stress relaxation behavior of cohesive powders like cocoa and limestone during uniaxial compression, revealing power-law relaxation and proposing a simple model to improve simulations and macroscopic models.

Contribution

It introduces a simple algebraic model capturing stress relaxation in cohesive powders, supported by experimental data from two different testing devices.

Findings

Stress relaxation follows a power law after compression.

The model includes a response timescale and a relaxation parameter.

Results aid in improving granular material simulations.

Abstract

We present findings from uniaxial (oedometric) compression tests on two cohesive industrially relevant granular materials (cocoa and limestone powder). Experimental results are presented for the compressibility, tested with two devices -- the FT4 Powder Rheometer and the custom made lambdameter. We focus on the stress response and the slow relaxation behavior of the cohesive samples tested. After compression ends, at constant volume, the ongoing stress relaxation is found to follow a power law consistently for both cohesive powders and for the different testing equipments. A simple (incremental algebraic evolution) model is proposed for the stress relaxation in cohesive powders, which includes a response timescale along with a second, dimensionless relaxation parameter. The reported observations are useful for both the improvement of discrete element simulations and constitutive macroscopic models for cohesive granular materials.