Study of temporal and spatial evolution of deformation and breakage of dry granular materials using x-ray computed tomography and the discrete element method
Particles exist in great abundance in nature, such as in sands and clays, and they
also constitute 75% of the materials used in industry (e.g., mineral ores, formulated
pharmaceuticals, dyes, detergent powders). When a load is applied to a bulk assembly of
soil particles, the response of a geomaterial at the bulk (macro) scale, originates from the
changes that take place at the particle scale. If particle breakage occurs, the shape and
size of the particles comprising the bulk are changed; this induces changes in the contact
network through which applied loads are transmitted. As a result, changes at the micro-scale
can significantly affect the mechanical behaviour of a geomaterial at a macro-scale.
It is therefore unsurprising that the mechanisms leading to particle breakage are a subject
of intense research interest in several fields, including geomechanics.
In this thesis, particle breakage of two dry granular materials is studied, both
experimentally and numerically. The response of the materials is investigated under
different stress paths and in all the tests grain breakage occurs. High resolution x-ray
computed micro-tomography (XCT) is used to obtain 3D images of entire specimens
during high confinement triaxial compression tests and strain controlled oedometric
compression tests. The acquired images are processed and measurements are made of
the temporal and spatial evolution of breakage, local variations of porosity, volumetric
and shear strain and grading. The evolution and spatial distribution of quantified
breakage including the resulting particle size distribution for the whole specimen and
for specific areas, are presented and further related to the localised shear and volumetric
strains that developed in the specimens.
In addition, the discrete element method (DEM) was used to provide further micro-mechanical
insight of the underlying mechanisms leading to particle breakage. Classical
DEM simulations, using a Hertz-Mindlin contact model and non-breakable spheres,
was first deployed to study the initiation and likelihood of particle breakage under
oedometric compression. Moreover, a bonded DEM model was used to create clumps
that represent each particle and simulate breakage of particles under single particle
compression. The DEM model parameters were obtained from results of single particle
compression test and the models were validated against the quantitative 3D information
of the micro-scale, acquired from the XCT analysis