Title: Microscopic Insights into the Mechanics of Granular Materials

Abstract: The widespread occurrence of granular materials in nature (like soil and beach sand) and industries (such as cement and pharmaceuticals) has spurred extensive research into their mechanical behavior. Key phenomena, such as stress saturation with depth, rate-independent shear stress, and large stress fluctuations, are well documented. Our study, based on a combination of particle dynamics simulations and experiments, investigates the emergence of these features from the characteristics of grain structure. We adopted tools from network analysis to quantify the particle arrangement and its evolution. While previous studies often treat particles as rigid, our analysis reveals that the stress within the material is fundamentally related to the elastic deformation of the particles. To investigate stress transmission in static columns filled under gravity, we construct a coarse-grained representation of the force network, called “force lines”. These lines demonstrate lateral transmission of the load within the material, caused by anisotropy in the contact network, and resulting in the saturation of the stress with depth. 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. A cascade failure mechanism is proposed that unveils a system-spanning loss of stability in the network during the slip phases. This microscopic description of flow explains the rate independence of stress and establishes stress fluctuations as a hallmark of the slow granular flow regime. 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. These findings emphasize the relationships between the microstructure and the mechanical response of grain assemblies.