100 research outputs found
How granular materials deform in quasistatic conditions
Based on numerical simulations of quasistatic deformation of model granular
materials, two rheological regimes are distinguished, according to whether
macroscopic strains merely reflect microscopic material strains within the
grains in their contact regions (type I strains), or result from instabilities
and contact network rearrangements at the microscopic level (type II strains).
We discuss the occurrence of regimes I and II in simulations of model materials
made of disks (2D) or spheres (3D). The transition from regime I to regime II
in monotonic tests such as triaxial compression is different from both the
elastic limit and from the yield threshold. The distinction between both types
of response is shown to be crucial for the sensitivity to contact-level
mechanics, the relevant variables and scales to be considered in
micromechanical approaches, the energy balance and the possible occurrence of
macroscopic instabilitie
Discrete numerical simulation, quasistatic deformation and the origins of strain in granular materials.
International audienceSystematic numerical simulations of model dense granular materials in monotonous, quasistatic deformation reveal the existence of two different régimes. In the first one, the macroscopic strains stem from the deformation of contacts. The motion can be calculated by purely static means, without inertia, stress controlled or strain rate controlled simulations yield identical smooth rheological curves for a same sample. In the second régime, strains are essentially due to instabilities of the contact network, the approach to the limits of large samples and of small strain rates is considerably slower and the material is more sensitive to perturbations. These results are discussed and related to experiments : measurements of elastic moduli with very small strain increments, and slow deformation (creep) under constant stress
Quasistatic rheology and the origins of strain
Published in special issue of "Comptes-Rendus Physique" on granular materialsInternational audienceFeatures of rheological laws applied to solid-like granular materials are recalled and confronted to microscopic approaches via discrete numerical simulations. We give examples of model systems with very similar equilibrium stress transport properties -- the much-studied force chains and force distribution -- but qualitatively different strain responses to stress increments. Results on the stability of elastoplastic contact networks lead to the definition of two different rheological regimes, according to whether a macroscopic fragility property (propensity to rearrange under arbitrary small stress increments in the thermodynamic limit) applies. Possible consequences are discussed
Discrete numerical simulation, quasistatic deformation and the origins of strain in granular materials
Systematic numerical simulations of model dense granular materials in
monotonous, quasistatic deformation reveal the existence of two different
r\'egimes. In the first one, the macroscopic strains stem from the deformation
of contacts. The motion can be calculated by purely static means, without
inertia, stress controlled or strain rate controlled simulations yield
identical smooth rheological curves for a same sample. In the second r\'egime,
strains are essentially due to instabilities of the contact network, the
approach to the limits of large samples and of small strain rates is
considerably slower and the material is more sensitive to perturbations. These
results are discussed and related to experiments : measurements of elastic
moduli with very small strain increments, and slow deformation (creep) under
constant stress.Comment: Third international symposium on deformation characteristics of
geomaterials, Lyon : France (2003
Experimental validation of nonextensive scaling law in confined granular media
In this letter, we address the relationship between the statistical
fluctuations of grain displacements for a full quasistatic plane shear
experiment, and the corresponding anomalous diffusion exponent, . We
experimentally validate a particular case of the so-called Tsallis-Bukman
scaling law, , where is obtained by fitting the
probability density function (PDF) of the measured fluctuations with a
-Gaussian distribution, and the diffusion exponent is measured independently
during the experiment. Applying an original technique, we are able to evince a
transition from an anomalous diffusion regime to a Brownian behavior as a
function of the length of the strain-window used to calculate the displacements
of grains in experiments. The outstanding conformity of fitting curves to a
massive amount of experimental data shows a clear broadening of the fluctuation
PDFs as the length of the strain-window decreases, and an increment in the
value of the diffusion exponent - anomalous diffusion. Regardless of the size
of the strain-window considered in the measurements, we show that the
Tsallis-Bukman scaling law remains valid, which is the first experimental
verification of this relationship for a classical system at different diffusion
regimes. We also note that the spatial correlations show marked similarities to
the turbulence in fluids, a promising indication that this type of analysis can
be used to explore the origins of the macroscopic friction in confined granular
materials.Comment: 8 pages 4 figure
Jamming transition in a two-dimensional open granular pile with rolling resistance
We present a molecular dynamics study of the jamming/unjamming transition in
two-dimensional granular piles with open boundaries. The grains are modeled by
viscoelastic forces, Coulomb friction and resistance to rolling. Two models for
the rolling resistance interaction were assessed: one considers a constant
rolling friction coefficient, and the other one a strain dependent coefficient.
The piles are grown on a finite size substrate and subsequently discharged
through an orifice opened at the center of the substrate. Varying the orifice
width and taking the final height of the pile after the discharge as the order
parameter, one can devise a transition from a jammed regime (when the grain
flux is always clogged by an arch) to a catastrophic regime, in which the pile
is completely destroyed by an avalanche as large as the system size. A finite
size analysis shows that there is a finite orifice width associated with the
threshold for the unjamming transition, no matter the model used for the
microscopic interactions. As expected, the value of this threshold width
increases when rolling resistance is considered, and it depends on the model
used for the rolling friction.Comment: 9 pages, 6 figure
FEMxDEM multi-scale modelling with second gradient regularization
The multi-scale FEMxDEM approach is an innovative numerical method for geotechnical problems,
using at the same time the Finite Element Method (FEM) at the engineering macro-scale and the
Discrete Element Method (DEM) at the scale of the microstructure of the material. The link between
scales is made via computational homogenization. In this way, the continuum numerical constitutive
law and the corresponding tangent matrix are obtained directly from the discrete response of the
microstructure [1,2,3].
In the proposed paper, a variety of operators, rather than the tangent consistent for the Newton-
Raphson method, is tested in a challenging attempt to improve the poor convergence performance.
The independence of the DEM computations between the different elements is exploited to develop a
parallelized code using an OpenMP paradigm. At the macro level, a second gradient constitutive
relation is implemented in order to enrich the first gradient Cauchy relation bringing meshindependency
to the model. The second gradient regularization, together with the speedup provided
by the parallelization allows by first time to the FEMxDEM model to be applied to real scale
problems with the desired mesh refinement.
Some results are given exhibiting the above findings with emphasis on aspects related to strain
localisation
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