Deformation of Cohesive Granular Materials: Micro influences Macro

Abstract

Granular materials and particulate matter display interesting bulk behaviors from static to dynamic, solid to liquid or gas like states: sand can be compressed and behave like a solid, or flow in a slurry like a liquid or fly in the air as a sand storm. The mystery of bridging the gap between the particulate, microscopic state and the macroscopic, continuum description is one of the challenges of modern research. Powders is a special class of granular materials that contain very fine particles that may flow freely when shaken or tilted, but may stick when left at rest or being compressed. During storage and transportation processes, the material undergoes various modes of deformation and stress conditions, e.g. due to compression or shear. In many applications, it is important to know when powders are yielding, i.e., when they start to flow under shear; in other cases it is necessary to know how much stress is needed to keep them flowing. The flow behaviour changes dramatically from very low to very high stress conditions. The main focus of this thesis is to investigate how the micro-mechanical properties influence the macroscopic bulk responses of granular materials and it is structured as two parts: the former one devoted to laboratory experiments and the latter one to numerical simulations. The focuses of the first part are (i) characterization of granular materials at different length scales, for both dry non-cohesive and cohesive materials, (ii) investigate the flow behaviour in both low and high stress regimes using the same materials, (iii) explore different testing devices to identify the most appropriate techniques on powder flow measurement. While the focus of second part is (iv) the development of the constitutive model to describe granular flows based on micro-mechanical insights from discrete particle simulations.