Real grain shape analysis: characterization and generation of representative virtual grains. application to railway ballast

Grain shape significantly influences the mechanical properties of granular media. In order to explore this effect and to simulate realistic material morphology, we designed a method which well characterizes real grains shape. Starting from a representation of the particle surfaces as a points cloud, this paper presents a method to generate a set of virtual grains that are morphologically representative of real ballast grains. The model relies on a statistical modelling of the ballast grain morphology based on a dimensionality reduction approach (Proper Orthogonal Decomposition) leading to an optimal and nearly exhaustive shape characterization by extracting a hierarchy of shape functions that fully describe the grain sample. We will show the efficiency of the both characterizing and generating methods and describe their advantages, as well as a future outloo

Influence de la polydispersité sur la texture des milieux granulaires

La texture des matériaux granulaires est fortement influencée par le degré de polydispersité des tailles de particules. Dans cet article on présente un modèle de distribution granulométrique associé à un protocole de dépôt géométrique. Ce modèle est suffisamment efficace sur le plan numérique pour permettre une étude systématique des propriétés d'arrangement du milieu (compacité, connectivité, anisotropie) en fonction de la forme et de l'étalement de la courbe granulométrique. Un résultat remarquable de cette étude est la variation non linéaire de la compacité avec l'étalement. On observe notamment un palier où la compacité n'évolue pas avec l'étalement. Une transition a lieu vers les compacités plus élevées lorsque l'étalement permet l'insertion des plus petites particules dans les pores formés par les plus grosses. Cette transition se retrouve au niveau de l'arrangement granulaire qui est relativement régulier sur le pallier et désordonné ensuite

Evaluation de la dégradation du ballast ferroviaire à partir des simulations par éléments discrets des conditions de voie et de l'essai Micro-Deval

En raison de la circulation des trains et des opérations de maintenance (bourrage), les grains de ballast ferroviaire s'usent. Cela entraine une perte graduelle de performance qui nécessite finalement le renouvellement de l'ensemble du ballast. Afin de prédire l'évolution de cette dégradation, une étude multi-échelle est proposée. Des simulations par éléments discrets de l'essai d'attrition Micro-Deval permettent relier le chargement microscopique à la production de particules fines observée expérimentalement. La même approche numérique est utilisée avec des simulations du trafic des trains et de l'opération de bourrage pour déterminer les chargements microscopiques caractéristiques avec les conditions de voie

Texture et comportement des matériaux granulaires fortement polydisperses

Dans cet article, on présente une étude numérique de l’influence de la polydispersité, i.e. distribution étendue des tailles de particules, sur la texture et sur le comportement mécanique des matériaux granulaires. Les simulations en éléments discrets, basées sur la méthode de Dynamique des Contacts, mettent en évidence le rôle dominant des plus grosses particules. De façon inattendue, on montre alors que la résistance au cisaillement de tels matériaux est indépendante de la polydispersité

From liquid to solid bonding in cohesive granular media

We study the transition of a granular packing from liquid to solid bonding in the course of drying. The particles are initially wetted by a liquid brine and the cohesion of the packing is ensured by capillary forces, but the crystallization of the solute transforms the liquid bonds into partially cemented bonds. This transition is evidenced experimentally by measuring the compressive strength of the samples at regular intervals of times. Our experimental data reveal three regimes: 1) Up to a critical degree of saturation, no solid bonds are formed and the cohesion remains practically constant; 2) The onset of cementation occurs at the surface and a front spreads towards the center of the sample with a nonlinear increase of the cohesion; 3) All bonds are partially cemented when the cementation front reaches the center of the sample, but the cohesion increases rapidly due to the consolidation of cemented bonds. We introduce a model based on a parametric cohesion law at the bonds and a bond crystallization parameter. This model predicts correctly the phase transition and the relation between microscopic and macroscopic cohesion.Comment: 20

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

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 $\eta$.Comment: 5 pages, 8 figure

Image-based investigation into the primary fabric of stress transmitting particles in sand

This paper uses three-dimensional images of a natural silica sand to analyse the mechanisms of stress transmission under triaxial compression. As discussed in Fonseca, J., O’Sullivan, C., Coop, M., Lee, P.D., (2012), the irregular morphology and locked fabric that can be found in natural sands lead to the formation of contacts with extended surface areas. However, most of our current understanding of stress-transmission phenomena comes from DEM simulations and photo-elastic experiments using idealised grain shapes and contact topologies. The direct measurement of stress transmission in assemblies of real soil grains is a challenging task. The present study postulates that important insight can be obtained by following the evolution of intergranular contacts as the grains rearrange and by considering how these rearrangements enhance the stability of the material. The methodology consists of measuring the geometrical data of the individual grains and their associated contacts obtained at successive load stages in the post-peak regime (after shear band formation). A statistical analysis of the vectors normal to the contacts reveals a realignment of these vectors in the direction of the major principal stress; this is a clear indication of the formation of force chains. A subsequent analysis shows that these columnar structures of stress-transmitting grains are associated with larger contact surfaces and have distinct patterns in the regions affected by the formation of a shear band. An algorithm based on stability and load-transmission criteria is developed to contribute new insight into the characterisation of load-bearing sand particles

Particle-scale mechanics of sand crushing in compression and shearing using DEM

AbstractIn this paper, the discrete element method is used to explore why differing amounts of breakage, quantified using Hardin's relative breakage parameter (Br), are associated with the critical state line (CSL) and the normal compression line (NCL) at similar stress levels. Virtual samples, initially containing more than 20,000 spherical particles, were isotropically compressed to a range of confining pressures up to 56MPa and subjected to triaxial compression, both considering and disregarding particle crushing. A particle crushing model was developed for these simulations which is both computationally tractable and gives macro-scale results qualitatively in agreement with laboratory tests. The CSLs are both linear in q–p' space. A curved peak envelope, corresponding to a curved Mohr–Coulomb envelope, is obtained for the crushing simulations which is absent when crushing is disabled. Consideration of particle crushing reduces the peak stress, and the volumetric response is much more contractive with crushing at high p'. These simulations capture the behaviour in Br–p' space expected from published laboratory tests. The difference in behaviour along the NCL and CSL is explained by the larger fluctuations in contact force during triaxial shearing than during isotropic compression which was quantified using a newly-defined measure, the contact number ratio. Particle crushing continues after the critical state is attained, contributing to counteract the dilation induced by particle rearrangement

Simulating regoliths in microgravity

Despite their very low surface gravities, the surfaces of asteroids and comets are covered by granular materials – regolith – that can range from a fine dust to a gravel-like structure of varying depths. Understanding the dynamics of granular materials is, therefore, vital for the interpretation of the surface geology of these small bodies and is also critical for the design and/or operations of any device planned to interact with their surfaces. We present the first measurements of transient weakening of granular material after shear reversal in microgravity as well as a summary of experimental results recently published in other journals, which may have important implications for small-body surfaces. Our results suggest that the force contact network within a granular material may be weaker in microgravity, although the influence of any change in the contact network is felt by the granular material over much larger distances. This could mean that small-body surfaces are even more unstable than previously imagined. However, our results also indicate that the consequences of, e.g., a meteorite impact or a spacecraft landing, may be very different depending on the impact angle and location, and depending on the prior history of the small-body surface