deformation of cohesive granular materials: micro influences macro

Hao Shi is a PhD student in the research group Multiscale Mechanics (MSM). His supervisor is prof.dr. S. Luding from the Faculty of Engineering Technology (ET).

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.

In the first part of the study, we perform a wide and systematic experimental investigation to assess the  influences of particle size and inter-particle cohesion on powder flows at various stress regimes. We choose limestone powders as a reference material because of its insensitivity to the environmental change through the whole study. Initially, we investigate the effect of particle size on limestone powder yielding in low to moderate stress regimes and we found an interesting non-monotonic trend of bulk friction and cohesive strength due to the interplay between inter-particle cohesion and geometrical interlocking. We also propose a simple empirical model based on van der Waals interaction to describe the behaviour of cohesive strength.

Next, we further enter the high stress regime by compacting our powders at high loads, and investigate the effect of particle size on the powder compaction and the tensile strength of the final tablet. The geometrical influence which dominates at low stress regime are found to be irrelevant at high pressure regime.

Finally, we try to bridge the limit of different dynamic and quasi-static flow tests at low towards zero confining pressure and found a good agreement between these two types of test. This novel approach gives access to a stress regime normally forbidden to conventional shear cell experiments.

In the second part of this study, instead of simulating each single case as presented in the first part, we aim on finding a good generalized rheological model with the help of discrete particle simulations (DPM) to describe different types of granular flow under various conditions. We first give an overview of recent progress and some new insights about the collective mechanical behavior of granular, deformable particles from diluted to jammed states. Then we systematically investigate the interplay between inter-particle friction and cohesion on sheared homogeneous and inhomogeneous granular media at steady state and therefore extend our rheological model towards a more generalized description.