The behavior of granular media is challenging to understand and the study has been a focus of research for decades. The complexity of granular media originates from their heterogeneous and discrete nature. The properties of the constituent individual particles intrinsically control the macroscopic response of a granular material. In particular, the shape of particles underpins many facets of the mechanical behavior of the material, including packing, strength, deformation, failure, flow and breakage and others, while the micromechanics of the is still lack of deep understanding due to the limitation on the shape characterization and modelling.

This thesis presents a micromechanical investigation of the influence of complex particle shape on the shear behavior of granular material based...[ Read more ]

The behavior of granular media is challenging to understand and the study has been a focus of research for decades. The complexity of granular media originates from their heterogeneous and discrete nature. The properties of the constituent individual particles intrinsically control the macroscopic response of a granular material. In particular, the shape of particles underpins many facets of the mechanical behavior of the material, including packing, strength, deformation, failure, flow and breakage and others, while the micromechanics of the is still lack of deep understanding due to the limitation on the shape characterization and modelling.

This thesis presents a micromechanical investigation of the influence of complex particle shape on the shear behavior of granular material based on 2D DEM simulations. A series of Fourier shape descriptors were used to quantify the shape of particles, which allows us to construct particles with quantitatively controlled shape properties, including irregularity, elongation and roughness. Packings of granular particles generated with controlled shape properties were subjected to monotonic biaxial shear tests and both their macroscopic and microscopic responses were carefully examined. At the macroscopic scale, the packing density, shear strength and volumetric responses were found to correlate closely with the shape properties in different ways. At the microscopic level, two new methods were introduced to quantify the micromechanical response, one intending to recognize the force chain network from both mechanical and geometrical perspectives and the other considering rolling resistance as an alternative to particle shape. Based on these methods, the anisotropy and force transmission mechanisms were analyzed. We found irregularity, elongation and roughness all enhance the packing strength by increasing the fabric anisotropy and mobilized friction. Irregularity and elongation are found to have significantly different influences on the material behavior due to different contact features. Roughness influences the micromechanics in a similar way with irregularity, but it is less dominant when irregularity is present. We concluded the particle shapes influences the micromechanical behaviors of granular media from at least two aspects: the micromechanics, for which particle shapes induce rolling resistance effect, and the micro-geometries, for which contact features influences the contact distributions and particle arrangement. The quantitative consideration of shape properties in this study enables us to attribute physical explanations to shape-induced rolling resistance which has long been considered in most DEM simulations in a phenomenological manner. It was found irregularity/roughness or elongation dominant particles offer rolling resistance at the grain scale in rather different manners due to different contact features, which cannot be fully explained by conventional rolling resistance models.

The thesis was also devoted to the study of energy flows in granular media for better understanding of their thermomechanics During a shearing process, past studies showed the energy in a granular system is typically stored in the weak contact force networks wherein the interacted forces are concentrated and dissipated and plastic slipping happens. To validate this mode of energy storage for future constitutive behaivor of granular media, we tracked the energy flows in granular media based on 3D DEM simulations. Under quasi-static shear, the energy flows feature a transformation from stored elastic energy to friction dissipation. If the rolling resistance is considered, it will add an additional rolling energy flow which accumulates during the shear process. In particular, we verified an important concept in constitutive modelling of granular materials, the frozen energy, and corroborated it with theoretical predictions. The findings led to improved understanding of the thermomechanical behavior of granular media and offered sounded physical basis for constitutive modelling of these materials in the future.