# The intertwined roles of particle shape and surface roughness in controlling the shear strength of a granular material

## TL;DR

This study investigates how particle shape and surface roughness influence the shear strength of granular materials, revealing that surface wear significantly reduces strength in lens-shaped particles but not in spheres or tetrahedra.

## Contribution

It demonstrates the systematic impact of surface wear on shear strength in specific particle shapes, providing insights for designing durable granular materials.

## Key findings

- Surface wear causes significant strength loss in lens-shaped particles.
- Spheres and tetrahedra show minor strength degradation under load cycling.
- Surface smoothing correlates with decreased shear strength.

## Abstract

Both the shape of individual particles and their surface properties contribute to the strength of a granular material under shear. Here we show the degree to which these two aspects can be intertwined. In experiments on assemblies of 3D printed, convex lens-shaped particles, we measure the stress-strain response under repeated compressive loading and find that the aggregate's shear strength falls rapidly when compared to other particle shapes. We probe the granular material at mm-scales with X-ray computed tomography and $\mu$m-scales with high-resolution surface metrology to look for the cause of the degradation. We find that wear due to accumulated deformation smooths out the lens surfaces in a controlled and systematic manner that correlates with a significant loss of shear strength observed for the assembly as a whole. The sensitivity of lenses to changes in surface properties contrasts with results for assemblies of 3D printed tetrahedra and spheres, which under the same load cycling are found to exhibit only minor degradation in strength. This case study provides insight into the relationship between particle shape, surface wear, and the overall material response, and suggests new strategies when designing a granular material with desired evolution of properties under repeated deformation.