120 research outputs found
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
Liquid clustering and capillary pressure in granular media
International audienceBy means of extensive lattice Boltzmann simulations, we investigate the process of growth and coalescence of liquid clusters in a granular material as the amount of liquid increases. A homogeneous grain-liquid mixture is obtained by means of capillary condensation, thus providing meaningful statistics on the liquid distribution inside the granular material. The tensile stress carried by the grains as a function of the amount of condensed liquid reveals four distinct states, with a peak stress occurring at the transition from a primary coalescence process, where the cohesive strength is carried mostly by the grains, to a secondary process governed by the increase of the liquid cluster volumes. We show that the evolution of capillary states is correctly captured by a simple model accounting for the competing effects of the Laplace pressure and grain-liquid interface
The Alestle - Vol. 57 No. 30 - 01/11/2005
Vol. 57 No. 3
Experimental characterization of a cohesive zone model using Digital Image Correlation
International audienceIn recent years, cohesive-zone models have been formulated and used to numerically simulate the fracture of solid materials. Cohesive-zone models presented in the literature involve a 'jump' in the displacement field describing crack onset within a predefined interface network corresponding to interfaces between elements of the finite element (FE) mesh. The introduction of a virtual displacement jump is convenient to numerically manage micro-crack or void initiation, growth and coalescence. Until now, the forms of interface laws were mainly chosen in connection with the overall responses of specimens when subjected to standard loadings. In this study, a cohesive-zone model identification method is proposed based on the local material behaviour derived from kinematical measurements obtained by digital image correlation (DIC). A series of tensile loadings were performed for several damageable elastic-plastic materials on standard tensile specimens. Kinematical data analysis enabled early detection and tracking of the zone where the crack occurs. The results of this study highlight the potential of DIC to quantify damage and show how damage assessments can be inserted in cohesive-zone model identification
Analyse des autocontraintes dans un matériau granulaire humide
Nous présentons une analyse de la transmission des contraintes dans un matériau granulaire humide. L'étude repose sur des simulations numériques tridimensionnelles par éléments discrets. On montre que l'action de traction des ponts capillaires induit un réseau de particules autocontraintes qui s'organise sous forme de deux "phases" percolantes de pressions positives et négatives définies au niveau des particules. Des descripteurs statistiques de la microstructure et du réseau des forces sont utilisés pour caractériser la répartition des pressions. Le comportement biphasique des autocontraintes semble persister lorsque l'assemblage humide est soumis à une pression de confinement non nulle. Le nombre de liens en tension diminue et la phase négative se scinde en agrégats et en sites isolés
Volume, Peak discharges and Froude Number of Debris-Flow Surges: 10 Years of Monitoring on the Réal Torrent (France)
This work presents a summary of data on debris-flow monitoring stations focusing on the surge scale rather than full-scale debris-flow event (several fronts and surges with intermediate diluted flows). Surge-scale debris-flow data are not easily accessible for modellers but would be very beneficial for the community. A summary of the data processing protocol is offered, and its application to the monitoring station of the R?al Torrent is described (drainage area: 2 km2, SE France). Investigated bulk surge features are volume, front height, peak discharge, and Froude number. This investigation leads to statistical distributions of these parameters on 34 surges gathered from 2011 to 2020. Their volumes are typically a few thousand cubic metres, their peak flow height is 1 to 2 m, their peak discharge is a few dozens of cubic metres per second and their Froude number is near critical. Results drawn from this work will be a great asset for modellers to better feed their numerical experiments with realistic, field-driven features
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
A benchmark for particle shape dependence
International audienceParticle shape is a major parameter for the space-filling and strength properties of granular materials. For a systematic investigation of shape effect, a numerical benchmark test was set up within a collaborative group using different numerical methods and particles of various shape characteristics such as elongation, angularity and nonconvexity. Extensive 2D shear simulations were performed in this framework and the shear strength and packing fraction were compared for different shapes.We show that the results may be analyzed in terms of a low-order shape parameter h describing the degree of distortion from a perfectly circular shape. In particular, the shear strength is an increasing function of h with nearly the same trend for all shapes, the differences being of second order compared to h. We also observe a nontrivial behavior of packing fraction which, for all our simulated shapes, increases with h from the random close packing fraction for disks, reaches a peak considerably higher than that for disks, and subsequently declines as h is further increased. Finally, the analysis of contact forces for the same value of h leads to very similar statistics regardless of our specific particle shapes
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