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|Title: ||Investigation of micro- and macro-phenomena in densely packed granular media using the discrete element method|
|Authors: ||Zhou, Chong|
|Supervisor(s): ||Ooi, Jin|
|Issue Date: ||2011|
|Publisher: ||The University of Edinburgh|
|Abstract: ||Granular materials are in abundance in nature and are estimated to
constitute over 75% of all raw materials passing through the industry.
Granular or particulate solids are thus of considerable interest to many
industrial sectors and research communities, where many unsolved
challenges still remain.
This thesis investigates the micro- and macro-phenomena in densely
packed particulate systems by means of the Discrete Element Method
(DEM), which is a numerical tool for analysing the internal complexities
of granular material as the mechanical interactions are considered
at the grain scale. It presents an alternative approach to
phenomenological continuum approaches when studying localisation
problems and finite deformation problems in granular materials.
In order to develop a comprehensive theoretical understanding of particulate
matter and to form a sound base to improve industrial processes,
it is desirable to study the mechanical behaviour of granular
solids subject to a variety of loading conditions. In this thesis, three
loading actions were explored in detail, which are biaxial compression,
rigid object penetration and progressive formation of granular piles.
The roles of particle shape and contact friction in each of these loading
scenarios were investigated. The resulting packing structures were
compared and studied to provide a micromechanical insight into the
development of contact force network which governs the collective response.
The interparticle contact forces and displacements were then
used to evaluate the equivalent continuum stress and strain components
thus providing the link between micro- and macroscopic descriptions.
The information collected from the evolution of strong contact
network illustrates the underlying mechanism of force transmission
DEM simulations presented in this thesis demonstrate strong capability
in predicting the bulk behaviour as well as capturing local
phenomenon occurring in the system. The research first simulates
a testing environment of biaxial compression in DEM, in which the
phenomenon of strain localisation was investigated, with special attention
given to the interpretation of underlying failure mechanism.
Several key micromechanical quantities of interest were extracted to
understand the bifurcation instability, such as force chains, contact
orientation, particle rotation and void ratio. In the simulation of progressive
formation of granular piles, a counterintuitive pressure profile
with a significant pressure dip under the apex was predicted for three
models under certain conditions. Both particle shape and preparation
history were shown to be important in the resulting pressure distribution.
During the rigid body penetration into a granular sample, the
contact forces were used to evaluate the equivalent continuum stress
components. Significant stress concentration was developed around
the punch base which further led to successive collapse and reformation
of force chains. Taking the advantage of micromechanical analysis
at particle scale, two distinct bearing failure mechanisms were
identified as the penetration proceeded.
To further quantify the nature of strain mobilisation leading to failure,
Particle Image Velocimetry (PIV) was employed to measure the
deformation over small strain interval in association with shear band
propagation in the biaxial test and deformation pattern in the footing
test. The captured images from DEM simulation and laboratory
experiments were evaluated through PIV correlation. This optical
measuring technique is able to yield a significant improvement in the
accuracy and spatial resolution of the displacement field over highly
strained and localised regions. Finally, a series of equivalent DEM simulations
were also conducted and compared with the physical footing
experiments, with the objective of evaluating the capability of DEM
in producing satisfactory predictions.|
|Sponsor(s): ||Dorothy Hodgkin Postgraduate Award (DHPA)|
British Engineering and Physical Sciences Research Council (EPSRC) and Shell UK Ltd.
|Keywords: ||discrete element method|
|Appears in Collections:||Engineering thesis and dissertation collection|
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