51 research outputs found
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
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
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
Shear strength properties of wet granular materials
We investigate shear strength properties of wet granular materials in the
pendular state (i.e. the state where the liquid phase is discontinuous) as a
function of water content. Sand and glass beads were wetted and tested in a
direct shear cell and under various confining pressures. In parallel, we
carried out three-dimensional molecular dynamics simulations by using an
explicit equation expressing capillary force as a function of interparticle
distance, water bridge volume and surface tension. We show that, due to the
peculiar features of capillary interactions, the major influence of water
content over the shear strength stems from the distribution of liquid bonds.
This property results in shear strength saturation as a function of water
content. We arrive at the same conclusion by a microscopic analysis of the
shear strength. We propose a model that accounts for the capillary force, the
granular texture and particle size polydispersity. We find fairly good
agreement of the theoretical estimate of the shear strength with both
experimental data and simulations. From numerical data, we analyze the
connectivity and anisotropy of different classes of liquid bonds according to
the sign and level of the normal force as well as the bond direction. We find
that weak compressive bonds are almost isotropically distributed whereas strong
compressive and tensile bonds have a pronounced anisotropy. The probability
distribution function of normal forces is exponentially decreasing for strong
compressive bonds, a decreasing power-law function over nearly one decade for
weak compressive bonds and an increasing linear function in the range of
tensile bonds. These features suggest that different bond classes do not play
the same role with respect to the shear strength.Comment: 12 page
Anisotropic nonlinear elasticity in a spherical bead pack: influence of the fabric anisotropy
Stress-strain measurements and ultrasound propagation experiments in glass
bead packs have been simultaneously conducted to characterize the
stress-induced anisotropy under uniaxial loading. These measurements, realized
respectively with finite and incremental deformations of the granular assembly,
are analyzed within the framework of the effective medium theory based on the
Hertz-Mindlin contact theory. Our work shows that both compressional and shear
wave velocities and consequently the incremental elastic moduli agree fairly
well with the effective medium model by Johnson et al. [J. Appl. Mech. 65, 380
(1998)], but the anisotropic stress ratio resulting from finite deformation
does not at all. As indicated by numerical simulations, the discrepancy may
arise from the fact that the model doesn't properly allow the grains to relax
from the affine motion approximation. Here we find that the interaction nature
at the grain contact could also play a crucial role for the relevant prediction
by the model; indeed, such discrepancy can be significantly reduced if the
frictional resistance between grains is removed. Another main experimental
finding is the influence of the inherent anisotropy of granular packs, realized
by different protocols of the sample preparation. Our results reveal that
compressional waves are more sensitive to the stress-induced anisotropy,
whereas the shear waves are more sensitive to the fabric anisotropy, not being
accounted in analytical effective medium models.Comment: 9 pages, 8 figure
Short-time dynamics of a packing of polyhedral grains under horizontal vibrations
We analyze the dynamics of a 3D granular packing composed of particles of
irregular polyhedral shape confined inside a rectangular box with a retaining
wall sub jected to horizontal harmonic forcing. The simulations are performed
by means of the contact dynamics method for a broad set of loading parameters.
We explore the vibrational dynamics of the packing, the evolution of solid
fraction and the scaling of dy- namics with the loading parameters. We show
that the motion of the retaining wall is strongly anharmonic as a result of
jamming and grain rearrangements. It is found that the mean particle
displacement scales with inverse square of frequency, the inverse of the force
amplitude and the square of gravity. The short- time compaction rate grows in
proportion to frequency up to a characteristic frequency, corresponding to
collective particle rearrangements between equilibrium states, and then it
declines in inverse proportion to frequency
- …