arXiv subjects

G. Combe

Publications and source records attributed to G. Combe.

7 recordsLinked to original sources

arXiv2021

High compression of granular assemblies of brittle hollow tubular particles

This paper is devoted to the micro-mechanical origins of the high compressibility of brittle tubular particle assemblies. The material is extremely porous due to the presence of a large hole within the tube-shaped particle. The release of the inner void, protected by a fragile shell, gives the material a very strong ability to compress. The compressive response is investigated by means of the Discrete Element Method, DEM, using crushable-elements. To address the complexity of the model, a step-by-step break-down is developed. The paper comprises the comparison of the numerical results with both results obtained by the authors and existing experiments. With the insights provided by the DEM, we have sought to better understand the phenomena that originate at the grain scale, and that govern macroscopic behaviour. Grain breakage was proven to control the compressive behaviour, and thus, the importance of internal pores dominates the inter-particle voids. Then, a novel concept of compressibility analysis has been proposed using the separation of the double porosity and the quantification of the pore collapse through primary grain breakage. Finally, a general, geometrical development of a semi-analytical model has been proposed aiming the prediction of the evolution of double porosity vs axial strain.

cond-mat.mtrl-sci↗

arXiv2014

Mechanical properties of inclined frictional granular layers

We investigate the mechanical properties of inclined frictional granular layers prepared with different protocols by means of DEM numerical simulations. We perform an orthotropic elastic analysis of the stress response to a localized overload at the layer surface for several substrate tilt angles. The distance to the unjamming transition is controlled by the tilt angle $α$ with respect to the critical angle $α_c$. We find that the shear modulus of the system decreases with $α$, but tends to a finite value as $α\to α_c$. We also study the behaviour of various microscopic quantities with $α$, and show in particular the evolution of the contact orientation with respect to the orthotropic axes and that of the distribution of the friction mobilisation at contact.

cond-mat.soft↗

arXiv2013

Mechanical response of an inclined frictional granular layer approaching unjamming

We present an orthotropic elastic analysis of frictional granular layers under gravity by studying their stress response to a localized overload at the layer surface for several substrate tilt angles. The distance to the unjamming transition is controlled by the tilt angle α with respect to the critical angle α_c. We find that the shear modulus of the system decreases with α, but reaches a finite value as αtends to α_c. We also analyze the vibration modes of the system and show that the soft modes play an increasing, though not crucial, role approaching the transition.

cond-mat.soft↗

arXiv2012

Polygons vs. clumps of discs: a numerical study of the influence of grain shape on the mechanical behaviour of granular materials

We performed a series of numerical vertical compression tests on assemblies of 2D granular material using a Discrete Element code and studied the results with regard to the grain shape. The samples consist of 5,000 grains made from either 3 overlapping discs (clumps - grains with concavities) or six-edged polygons (convex grains). These two grain type have similar external envelopes, which is a function of a geometrical parameter $α$. In this paper, the numerical procedure applied is briefly presented followed by the description of the granular model used. Observations and mechanical analysis of dense and loose granular assemblies under isotropic loading are made. The mechanical response of our numerical granular samples is studied in the framework of the classical vertical compression test with constant lateral stress (biaxial test). The comparison of macroscopic responses of dense and loose samples with various grain shapes shows that when $α$ is considered a concavity parameter, it is therefore a relevant variable for increasing mechanical performances of dense samples. When $α$ is considered an envelope deviation from perfect sphericity, it can control mechanical performances for large strains. Finally, we present some remarks concerning the kinematics of the deformed samples: while some polygon samples subjected to a vertical compression present large damage zones (any polygon shape), dense samples made of clumps always exhibit thin reflecting shear bands. This paper was written as part of a CEGEO research project www.granuloscience.com

cond-mat.soft↗

arXiv2012

Particle shape dependence in 2D granular media

Particle shape is a key to the space-filling and strength properties of granular matter. We consider a shape parameter $\eta$ describing the degree of distortion from a perfectly spherical shape. Encompassing most specific shape characteristics such as elongation, angularity and nonconvexity, $\eta$ is a low-order but generic parameter that we used in a numerical benchmark test for a systematic investigation of shape-dependence in sheared granular packings composed of particles of different shapes. We find that the shear strength is an increasing function of $\eta$ with nearly the same trend for all shapes, the differences appearing thus to be of second order compared to $\eta$. We also observe a nontrivial behavior of packing fraction which, for all our simulated shapes, increases with $\eta$ from the random close packing fraction for disks, reaches a peak considerably higher than that for disks, and subsequently declines as $\eta$ is further increased. These findings suggest that a low-order description of particle shape accounts for the principal trends of packing fraction and shear strength. Hence, the effect of second-order shape parameters may be investigated by considering different shapes at the same level of $\eta$.

cond-mat.soft↗

arXiv2006

Scale separation in granular packings: stress plateaus and fluctuations

It is demonstrated, by numerical simulations of a 2D assembly of polydisperse disks, that there exists a range (plateau) of coarse graining scales for which the stress tensor field in a granular solid is nearly resolution independent, thereby enabling an `objective' definition of this field. Expectedly, it is not the mere size of the the system but the (related) magnitudes of the gradients that determine the widths of the plateaus. Ensemble averaging (even over `small' ensembles) extends the widths of the plateaus to sub-particle scales. The fluctuations within the ensemble are studied as well. Both the response to homogeneous forcing and to an external compressive localized load (and gravity) are studied. Implications to small solid systems and constitutive relations are briefly discussed.

cond-mat.stat-mech↗

arXiv2005

Sensitivity of the stress response function to packing preparation

A granular assembly composed of a collection of identical grains may pack under different microscopic configurations with microscopic features that are sensitive to the preparation history. A given configuration may also change in response to external actions such as compression, shearing etc. We show using a mechanical response function method developed experimentally and numerically, that the macroscopic stress profiles are strongly dependent on these preparation procedures. These results were obtained for both two and three dimensions. The method reveals that, under a given preparation history, the macroscopic symmetries of the granular material is affected and in most cases significant departures from isotropy should be observed. This suggests a new path toward a non-intrusive test of granular material constitutive properties.

cond-mat.soft↗