# Shear jammed, fragile, and steady states in homogeneously strained granular materials

## TL;DR

This study introduces a novel experimental setup to explore the jamming phase diagram of sheared granular materials, revealing detailed insights into shear-jammed, fragile, and steady states, and identifying critical packing fractions.

## Contribution

The paper presents a new Couette shear setup enabling precise measurements of granular states and phase boundaries, including the first experimental observation of fragile states under shear.

## Key findings

- Identification of the shear-jamming transition at φ_SJ ≈ 0.74.
- Observation of fragile states at φ < φ_SJ under large shear strain.
- Detection of flow deviations and shear band formation at higher packing fractions.

## Abstract

We study the jamming phase diagram of sheared granular material using a novel Couette shear set-up with multi-ring bottom. The set-up uses small basal friction forces to apply a volume-conserving linear shear with no shear band to a granular system composed of frictional photoelastic discs. The set-up can generate arbitrarily large shear strain due to its circular geometry, and the shear direction can be reversed, allowing us to measure a feature that distinguishes shear-jammed from fragile states. We report systematic measurements of the stress, strain and contact network structure at phase boundaries that have been difficult to access by traditional experimental techniques, including the yield stress curve and the jamming curve close to $\phi_{SJ}\approx 0.74$, the smallest packing fraction supporting a shear-jammed state. We observe fragile states created under large shear strain over a range of $\phi < \phi_{SJ}$. We also find a transition in the character of the quasi-static steady flow centered around $\phi_{SJ}$ on the yield curve as a function of packing fraction. Near $\phi_{SJ}$, the average contact number, fabric anisotropy, and non-rattler fraction all show a change of slope. Above $\phi_{F}\approx 0.7$ the steady flow shows measurable deviations from the basal linear shear profile, and above $\phi_c\approx 0.78$ the flow is localized in a shear band.