Impact of Shear History on Powder Flow Characterization Using a Ring Shear Tester

Abstract

In this study, we have investigated the impact of repeated shear displacement on powder flow properties. We show that when multiple yield loci are obtained using the same bulk solid specimen by stepping through different stress levels (i.e., stress walk [SW]), the shear deformation of the powder in a rotational shear cell, that is, Schulze Ring Shear Tester, is maximized, reducing the powder shear strength. This approach is material and time sparing; however, it imprecisely predicts better powder flowability. The magnitude of the change in the unconfined yield strength, σ, due to this prolonged shear displacement appears to be material-dependent, being less impactful for free-flowing powders. Using the SW and the individual yield loci-generated flow properties, we have demonstrated that in hopper design, the shear displacement effect impacts the computed critical arching diameter more than the critical mass flow angle. This knowledge of powder flow properties highlights limitations associated with the SW. An exponential function was found to describe the relationship between the change in σ at the highest major principal stress and the density weighted flowability, ff, with an R of 0.98. Such a model could be a valuable tool for correcting shear strength results obtained from SW, saving time, and material.

From Quasi-static to Intermediate Regimes in Shear Cell Devices: Theory and Characterisation

“…And yet, different dimensions, sample volume and pre-shearing procedures lead to disparities in the measurements from different testers. The impact the history of a powder has in its response to stress is well documented (Swize et al, 2019), but the sample preparation procedures are still specific to each device. The work of Berry et al (2014) and Salehi et al (2017a) compares the measurements of Brookfield, Schulze and Jenike testers, and Koynov et al (2015) reports on a comparative study of shear cells evaluating the flow of coarse (59 mm) and fine (4 mm) alumina powder.…”

“…The discrepancies observed across different devices (Koynov et al, 2015) are a reflection of an overly simplified description of the system and inconsistencies in the experimental procedures. The effect of the stress history of a sample is well-known (Swize et al, 2019), but without a common way to condition the powder, it is challenging to draw any direct comparison between different tests and samples. The measurements are also not universal.…”

Transport of Compressed Woody Biomass: Correlating Rheology and Microcompounder Measurements

“…The granular flow of bulk solids, such as coal and powders in hoppers and bins, has been studied both theoretically and experimentally, − starting with the pioneering work of Jenike to predict the minimum outlet opening size for the proper discharge of cohesive powders from hoppers. , Today, shear cells are used to measure parameters that affect the flow of granular materials such as cohesion, unconfined yield strength, bulk density, and friction factors at corresponding consolidation stress. , “Flowability” of bulk solids in granular rheology studies is classified by dividing the consolidation stress by its corresponding unconfined yield strength …”

Demonstration of the Feasibility of Predicting the Flow of Pharmaceutically Relevant Powders from Particle and Bulk Physical Properties

“…Nevertheless a guide for potential performance of powders in LIW feeders suggests that a Flow Function Coefficient (FFC), measured on a shear cell or equivalent [13][14][15][16] , of greater than 3 is a good predictor of acceptable (i.e. predictable and reproducible gravimetric feeding) performance in a feeder 7 .…”