107 research outputs found
Identification of rolling resistance as a shape parameter in sheared granular media
Using contact dynamics simulations, we compare the effect of rolling
resistance at the contacts in granular systems composed of disks with the
effect of angularity in granular systems composed of regular polygonal
particles. In simple shear conditions, we consider four aspects of the
mechanical behavior of these systems in the steady state: shear strength, solid
fraction, force and fabric anisotropies, and probability distribution of
contact forces. Our main finding is that, based on the energy dissipation
associated with relative rotation between two particles in contact, the effect
of rolling resistance can explicitly be identified with that of the number of
sides in a regular polygonal particle. This finding supports the use of rolling
resistance as a shape parameter accounting for particle angularity and shows
unambiguously that one of the main influencing factors behind the mechanical
behavior of granular systems composed of noncircular particles is the partial
hindrance of rotations as a result of angular particle shape.Comment: Soumis a Physical Review E; Statistical, Nonlinear, and Soft Matter
Physics http://link.aps.org/doi/10.1103/PhysRevE.84.01130
Force chains and contact network topology in packings of elongated particles
By means of contact dynamic simulations, we investigate the contact network
topology and force chains in two-dimensional packings of elongated particles
modeled by rounded-cap rectangles. The morphology of large packings of
elongated particles in quasistatic equilibrium is complex due to the combined
effects of local nematic ordering of the particles and orientations of contacts
between particles. We show that particle elongation affects force distributions
and force/fabric anisotropy via various local structures allowed by steric
exclusions and the requirement of force balance. As a result, the force
distributions become increasingly broader as particles become more elongated.
Interestingly, the weak force network transforms from a passive stabilizing
agent with respect to strong force chains to an active force-transmitting
network for the whole system. The strongest force chains are carried by
side/side contacts oriented along the principal stress direction.Comment: Soumis a Physical Review
Stress-strain behavior and geometrical properties of packings of elongated particles
We present a numerical analysis of the effect of particle elongation on the
quasistatic behavior of sheared granular media by means of the Contact Dynamics
method. The particle shapes are rounded-cap rectangles characterized by their
elongation. The macroscopic and microstructural properties of several packings
subjected to biaxial compression are analyzed as a function of particle
elongation. We find that the shear strength is an increasing linear function of
elongation. Performing an additive decomposition of the stress tensor based on
a harmonic approximation of the angular dependence of branch vectors, contact
normals and forces, we show that the increasing mobilization of friction force
and the associated anisotropy are key effects of particle elongation. These
effects are correlated with partial nematic ordering of the particles which
tend to be oriented perpendicular to the major principal stress direction and
form side-to-side contacts. However, the force transmission is found to be
mainly guided by cap-to-side contacts, which represent the largest fraction of
contacts for the most elongated particles. Another interesting finding is that,
in contrast to shear strength, the solid fraction first increases with particle
elongation, but declines as the particles become more elongated. It is also
remarkable that the coordination number does not follow this trend so that the
packings of more elongated particles are looser but more strongly connected.Comment: Submited to Physical Review
Force transmission in a packing of pentagonal particles
We perform a detailed analysis of the contact force network in a dense
confined packing of pentagonal particles simulated by means of the contact
dynamics method. The effect of particle shape is evidenced by comparing the
data from pentagon packing and from a packing with identical characteristics
except for the circular shape of the particles. A counterintuitive finding of
this work is that, under steady shearing, the pentagon packing develops a lower
structural anisotropy than the disk packing. We show that this weakness is
compensated by a higher force anisotropy, leading to enhanced shear strength of
the pentagon packing. We revisit "strong" and "weak" force networks in the
pentagon packing, but our simulation data provide also evidence for a large
class of "very weak" forces carried mainly by vertex-to-edge contacts. The
strong force chains are mostly composed of edge-to-edge contacts with a marked
zig-zag aspect and a decreasing exponential probability distribution as in a
disk packing
Quasistatic rheology, force transmission and fabric properties of a packing of irregular polyhedral particles
By means of contact dynamics simulations, we investigate a dense packing
composed of polyhedral particles under quasistatic shearing. The effect of
particle shape is analyzed by comparing the polyhedra packing with a packing of
similar characteristics except for the spherical shape of the particles. The
polyhedra packing shows higher shear stress and dilatancy but similar
stress-dilatancy relation compared to the sphere packing. A harmonic
approximation of granular fabric is presented in terms of branch vectors
(connecting particle centers) and contact force components along and
perpendicular to the branch vectors. It is found that the origin of enhanced
shear strength of the polyhedra packing lies in its higher force anisotropy
with respect to the sphere packing which has a higher fabric anisotropy.
Various contact types (face-vertex, face-face, etc) contribute differently to
force transmission and fabric anisotropy. In particular, most face-face
contacts belong to strong force chains along the major principal stress
direction whereas vertex-face contacts are correlated with weak forces and
oriented on average along the minor principal stress direction in steady
shearing
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