PhD Thesis Defence: Ravi Gautam

Micromechanical understanding of the macroscopic features of dense granular materials

The widespread occurrence of granular materials in nature (soil, beach sand, rubble pile asteroids) and in several industries (cement, food grains, pharmaceutical powders) has motivated numerous studies on the mechanics of granular media. Various features of static and sheared media have been known for long, such as the saturation of stress with depth in static columns, the rate-independence of the shear stress, large stress fluctuations, and dilation in slowly sheared packings. However, how these features emerge from grain interactions remain unexplained. In this study we investigate stress transmission in grain networks and their rearrangement during shear to provide a micromechanical explanation of these features, using a combination of particle dynamics simulations and experiments.

While many previous studies have treated particles as rigid entities, our analysis demonstrates that the stress within the material is fundamentally related to the elastic deformation of the particles. The stiffness of the particles and inter-particle friction greatly influence the contact network and, therefore, the macroscopic mechanical response. Our simulations of simple shear show that the material exhibits stick-slip dynamics: the contact network is stable in the stick phases, and the network rearranges in short-lived slip phases. We propose a cascade failure mechanism that reveals a system-spanning loss of stability in the contact network during the slip phases. This microscopic description of flow relates stick-slip dynamics to the stress fluctuations, and establishes the origin of the rate independence of the stress. Further, by treating the granular medium as an elastic continuum in the stick phase, we show that dilatancy results from anisotropy in the grain network.

To investigate stress transmission in static columns filled under gravity, we construct a coarse-grained representation of the force network, called “force lines” (Krishnaraj, 2020). The force lines show that the weight is preferentially transmitted to the side walls, resulting in the saturation of the stress with depth. Our experiments on a two-dimensional column filled with photoelastic disks show how the anisotropic microstructure in the grain contact network results in lateral transmission of load to the side walls, and thereby saturation of the stress with depth. These findings emphasize the relationships between the microstructure and the mechanical response of grain assemblies.

Reference:
Krishnaraj, K. P. “Flow and structure of dense granular materials”, PhD thesis, Indian Institute of Science (2020).