Structural and Topological Nature of Plasticity in Sheared Granular Materials

TL;DR

This paper investigates the microscopic origins of plasticity in sheared granular materials, revealing that structural defects called coplanar tetrahedra flips are key to understanding deformation and shear band formation.

Contribution

It identifies coplanar tetrahedra flips as the microscopic structural defects responsible for plasticity in disordered granular packings, using advanced tomography techniques.

Findings

Coplanar tetrahedra are key structural defects in granular plasticity.

Plastic events are discrete neighbor-switching flips of tetrahedra.

Flip events align with principal stress directions.

Abstract

Upon mechanical loading, granular materials yield and undergo plastic deformation. The nature of plastic deformation is essential for the development of the macroscopic constitutive models and the understanding of shear band formation. However, we still do not fully understand the microscopic nature of plastic deformation in disordered granular materials. Here we used synchrotron X-ray tomography technique to track the structural evolutions of three-dimensional granular materials under shear. We establish that highly distorted coplanar tetrahedra are the structural defects responsible for microscopic plasticity in disordered granular packings. The elementary plastic events occur through flip events which correspond to a neighbor switching process among these coplanar tetrahedra (or equivalently as the rotation motion of 4-ring disclinations). These events are discrete in space and possess specific orientations with the principal stress direction.