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F Radjai

Publications and source records attributed to F Radjai.

9 recordsLinked to original sources
PubMed2005

Multi-scale analysis of the stress state in a granular slope in transition to failure.

By means of contact dynamics simulations, we analyze the stress state in a granular bed slowly tilted toward its angle of repose. An increasingly large number of grains are overloaded in the sense that they are found to carry a stress ratio above the Coulomb yield threshold of the whole packing. Using this property, we introduce a coarse-graining length scale at which all stress ratios are below the packing yield threshold. We show that this length increases with the slope angle and jumps to a length comparable to the depth of the granular bed at an angle below the angle of repose. This transition coincides with the onset of dilation in the packing. We map this transition into a percolation transition of the overloaded grains, and discuss it in terms of long-range correlations and granular slope metastability.

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PubMed2005

Self-stresses and crack formation by particle swelling in cohesive granular media.

We present a molecular-dynamics study of force patterns, tensile strength, and crack formation in a cohesive granular model where the particles are subjected to swelling or shrinkage gradients. Nonuniform particle size change generates self-equilibrated forces that lead to crack initiation as soon as the strongest tensile contacts begin to fail. We find that the tensile strength is well below the theoretical strength as a result of inhomogeneous force transmission in granular media. The cracks propagate either inward from the edge upon shrinkage or outward from the center upon swelling. We show that the coarse-grained stresses are correctly predicted by an elastic model that incorporates particle size change as metric evolution.

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PubMed2005

Intermittent flow of a collection of rigid frictional disks in a vertical pipe.

We study the quasistatic flow of a collection of rigid frictional disks pushed upward (against the gravity) inside a narrow vertical pipe by a compliant mechanism. The contact dynamics method was used for the numerical simulations in combination with a friction law at disk-disk and wall-disk contacts characterized by discontinuous velocity weakening from a static threshold to a dynamic coefficient of friction. The material is sheared by the rolling of particles at the walls inducing a convective motion in the bulk. We observe a transition from constant flow to an intermittent flow when the driving velocity is reduced below a characteristic velocity that scales as k(-1/2) with the stiffness k of the pushing mechanism. The intermittent flow is composed of alternating phases of creep motion, where the pressure at the bottom of the granular column rises nonlinearly with time, and sudden slip, corresponding to a fast pressure drop. We show that the mean static pressure is correctly predicted by the Janssen model. The interplay between friction mobilization at the walls and structural changes in the bulk gives rise to a broad distribution of slip amplitudes characterized by a power law with an exponent approximately -1.7 that appears to be robust with respect to our system parameters.

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PubMed2002

Model for granular texture with steric exclusion.

We propose a method to characterize the geometrical texture of a granular packing at the particle scale including the steric hindrance effect. This method is based on the assumption of a maximum directional disorder (statistical entropy) compatible with both the strain-induced anisotropy of the contact network and steric exclusions. We show that the predicted statistics for the local configurations are in fairly good agreement with our numerical data.

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PubMed2002

Dynamic rearrangements and packing regimes in randomly deposited two-dimensional granular beds.

We study the structural properties of two-dimensional granular packings prepared by random deposition from a source line. We consider a class of random ballistic deposition models based on single-particle relaxation rules controlled by a critical angle, and we show that these local rules can be formulated as rolling friction in the framework of dynamic methods for the simulation of granular materials. We find that a packing prepared by random deposition models is generically unstable, and undergoes dynamic rearrangements. As a result, the dynamic method leads systematically to a higher solid fraction than the geometrical model for the same critical angle. We characterize the structure of the packings generated by both methods in terms of solid fraction, contact connectivity, and anisotropy. Our analysis provides evidence for four packing regimes as a function of solid fraction, the mechanisms of packing growth being different in each regime.

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