Cohesive Granular Materials Research Articles

The effect of cohesion on the discharge of a granular material through the orifice of a silo

We present the results of both experimental and numerical investigations of the silo discharge for a cohesive granular material. In our study, thanks to a cohesion-controlled granular material (CCGM) we propose to investigate the effect of the cohesive length lc, on the discharge of a silo for two different configurations, one axisymmetrical, and one quasi-2D rectangular silo. In both configurations, an adjustable bottom is used to control the size of the orifice. As observed for cohesionless granular material by previous studies, the mass flow rate and the density through an orifice are mostly controlled by the diameter of the orifice D. The experimental results of the quasi-2D silo are compared with continuum numerical simulations.

Quasistatic response of loose cohesive granular materials

DEM-simulated model cohesive assemblies of spherical grains of diameterd, with contact tensile strengthF0, once prepared in loose states, are quasistatically subjected to growing isotropic pressureP, and then to triaxial compression, maintaining lateral stresses σ2= σ3=Pwhile increasing axial stress σ1=P+qand strain є1. Reduced pressureP*=d2P/F0varies from 0.1 (cohesion dominated case, for which systems typically equilibrate with solid fraction Ф ≃ 0.35), to large values for which the cohesionless behavior is retrieved. In triaxial compression, while the moderate strain response (є1~ 0.1) is influenced by initial coordination numbers and mesoscale heterogeneities, the approach to the critical state, as bothq(deviator) and Ф steadily increase, gets slower for smallerP*. Critical ratioq/P strongly increases for decreasingP*, as roughly predicted in an “effective stress” scheme. Anomalously small elastic moduli are observed in the gel-like structures. While extensive geometric rearrangements take place, no shear banding is observed. Loose cohesive granular assemblies are thus capable of large quasistatic stable plastic strains and ductile rupture.

Failure processes of cemented granular materials.

The mechanics of cohesive or cemented granular materials is complex, combining the heterogeneous responses of granular media, like force chains, with clearly defined material properties. Here we use a discrete element model simulation, consisting of an assemblage of elastic particles connected by softer but breakable elastic bonds, to explore how this class of material deforms and fails under uniaxial compression. We are particularly interested in the connection between the microscopic interactions among the grains or particles and the macroscopic material response. To this end, the properties of the particles and the stiffness of the bonds are matched to experimental measurements of a cohesive granular medium with tunable elasticity. The criterion for breaking a bond is also based on an explicit Griffith energy balance, with realistic surface energies. By varying the initial volume fraction of the particle assembles we show that this simple model reproduces a wide range of experimental behaviors, both in the elastic limit and beyond it. These include quantitative details of the distinct failure modes of shear-banding, ductile failure, and compaction banding or anticracks, as well as the transitions between these modes. The present work, therefore, provides a unified framework for understanding the failure of porous materials such as sandstone, marble, powder aggregates, snow, and foam.

On a general correlation for the discharge rate of grains through slots in sidewalls of silos due to gravity

In this work we discuss the general validity of a phenomenological correlation for the mass flow rate of dry non cohesive granular material that outflows, due to the gravity, from slots in very thin vertical sidewalls of bins. The correctness of such a formula is analyzed by comparing it to other correlations published elsewhere which reports cases of very large, similar and very small channel width W relative to its length D.

Insights into the rheology of cohesive granular media.

Characterization and prediction of the "flowability" of powders are of paramount importance in many industries. However, our understanding of the flow of powders like cement or flour is sparse compared to the flow of coarse, granular media like sand. The main difficulty arises because of the presence of adhesive forces between the grains, preventing smooth and continuous flows. Several tests are used in industrial contexts to probe and quantify the "flowability" of powders. However, they remain empirical and would benefit from a detailed study of the physics controlling flow dynamics. Here, we attempt to fill the gap by performing intensive discrete numerical simulations of cohesive grains flowing down an inclined plane. We show that, contrary to what is commonly perceived, the cohesive nature of the flow is not entirely controlled by the interparticle adhesion, but that stiffness and inelasticity of the grains also play a significant role. For the same adhesion, stiffer and less dissipative grains yield a less cohesive flow. This observation is rationalized by introducing the concept of a dynamic, "effective" adhesive force, a single parameter, which combines the effects of adhesion, elasticity, and dissipation. Based on this concept, a rheological description of the flow is proposed for the cohesive grains. Our results elucidate the physics controlling the flow of cohesive granular materials, which may help in designing new approaches to characterize the "flowability" of powders.

Generalized Shields criterion for weakly cohesive granular materials

The erosion of natural sediments by a superficial fluid flow is a generic situation in many usual geological or industrial contexts. However, there is still a lack of fundamental knowledge about erosional processes, especially concerning the role of internal cohesion and adhesive stresses on issues such as the critical flow conditions for the erosion onset or the kinetics of soil mass loss. This contribution investigates the influence of cohesion on the surface erosion by an impinging jet flow based on laboratory tests with artificially bonded granular materials. The model samples are made of spherical glass beads bonded either by solid bridges made of resin or by liquid bridges made of a highly viscous oil. To quantify the intergranular cohesion, the capillary forces of the liquid bridges are here estimated by measuring their main geometrical parameters with image-processing techniques and using well-known analytical expressions. For the solid bonds, the adhesive strength of the materials is estimated by direct measurement of the yield tensile forces and stresses at the particle and sample scales, respectively, with specific traction tests developed for this purpose. The proper erosion tests are then carried out in an optically adapted device that permits a direct visualization of the scouring process at the jet apex by means of the refractive index matching technique. On this basis, the article examines qualitatively the kinetics of the scour crater excavation for both scenarios, namely, for an intergranular cohesion induced by either liquid or solid bonds. From a quantitative perspective, the critical condition for the erosion onset is discussed specifically for the case of the solid bond cohesion. In this respect, we propose here a generalized form of the Shields criterion based on a common definition of a cohesion number from yield tensile values, derived at both micro- and macroscales. The article finally shows that the proposed form manages to reconcile the experimental data for cohesive and cohesionless materials, the latter in the form of the so-called Shields curve along with some previous results of the authors which have been appropriately revisited.

Mathematical modeling of axisymmetric flow of granular materials

A mathematical model of the axisymmetric flow of cohesive granular materials is considered. A plastically compressible material is considered as a mathematical model. This material has an additional characteristic - its own rotation of particles. The existence of this characteristic leads to the asymmetry of the stress tensor and the presence of torque stresses. Asymmetric components of the stress tensor are balanced by normal pressure due to the presence of rolling friction. It is shown that the distinctive characteristic of the axisymmetric state is the layering of its flow. The condition for the flow of cohesive bulk material is obtained.

Steady state rheology of homogeneous and inhomogeneous cohesive granular materials

This paper aims to understand the effect of different particle/contact properties like friction, softness and cohesion on the compression/dilation of sheared granular materials. We focus on the local volume fraction in steady state of various non-cohesive, dry cohesive and moderate to strong wet cohesive, frictionless-to-frictional soft granular materials. The results from (1) an inhomogeneous, slowly sheared split-bottom ring shear cell and (2) a homogeneous, stress-controlled simple shear box with periodic boundaries are compared. The steady state volume fractions agree between the two geometries for a wide range of particle properties. While increasing inter-particle friction systematically leads to decreasing volume fractions, the inter-particle cohesion causes two opposing effects. With increasing strength of cohesion, we report an enhancement of the effect of contact friction i.e. even smaller volume fraction. However, for soft granular materials, strong cohesion causes an increase in volume fraction due to significant attractive forces causing larger deformations, not visible for stiff particles. This behaviour is condensed into a particle friction—Bond number phase diagram, which can be used to predict non-monotonic relative sample dilation/compression.

Qualitative and quantitative DEM analysis of cohesive granular material behaviour in FT4 shear tester

The macroscopic behaviour of cohesive granular material in the FT4 shear tester is studied using the discrete element method (DEM). The shear test is simulated faithfully to the experimental procedure (filling, compaction, pre-shearing and shearing). The angle of internal friction and the apparent bulk cohesion are the macroscopic properties analysed as a result of the variation of the microscopic parameters: the sliding friction coefficient and the adhesive surface energy. The simplified JKR model was used to account for the cohesive contact between spheres. The results of the shear test show that the adhesive forces influence the dilatancy of the granular bed and the incipient failure point. In general, the shear stress increases with the adhesive energy. The sliding friction coefficient and the adhesive energy affect the yield locus and therefore the angle of internal friction and the apparent cohesion. Two correlations were established between the angle of internal friction and sliding friction coefficient and between cohesion and adhesive energy. The effect of the initial consolidation on the shear test results is also discussed.

Mixing of Bidisperse Cohesive Granular Materials in Food Processes

Mixing of Bidisperse Cohesive Granular Materials in Food Processes

Rheology of Cohesive Granular Particles under Constant Pressure

The rheology of cohesive granular materials, under a constant pressure condition, is studied using molecular dynamics simulations. Depending on the shear rate, pressure, and interparticle cohesiveness, the system exhibits four distinctive phases: uniform shear, oscillation, shear-banding, and clustering. The friction coefficient is found to increase with the inertial number, irrespective of the cohesiveness. The friction coefficient becomes larger for strong cohesion. This trend is explained by the anisotropies of the coordination number and angular distribution of the interparticle forces. In particular, we demonstrate that the second-nearest neighbors play a role in the rheology of cohesive systems.

Effects of confining pressure and loading path on deformation and strength of cohesive granular materials: a three-dimensional DEM analysis

This paper is devoted to numerical analysis of strength and deformation of cohesive granular materials. The emphasis is put on the study of effects of confining pressure and loading path. To this end, the three-dimensional discrete element method is used. A nonlinear failure criterion for inter-granular interface bonding is proposed, and it is able to account for both tensile and shear failure for a large range of normal stress. This criterion is implemented in the particles flow code. The proposed failure model is calibrated from triaxial compression tests performed on representative sandstone. Numerical results are in good agreement with experimental data. In particular, the effect of confining pressure on compressive strength and failure pattern is well described by the proposed model. Furthermore, numerical predictions are studied, respectively, for compression and extension tests with a constant mean stress. It is shown that the failure strength and deformation process are clearly affected by loading path. Finally, a series of numerical simulations are performed on cubic samples with three independent principal stresses. It is found that the strength and failure mode are strongly influenced by the intermediate principal stress.

Nonlinear Model of Soils Under Complex Stress Paths

The direction of stress has an obvious influence on the mechanical properties of soils under the static and dynamic conditions. This is mainly because that geomaterials are anisotropic in general. Mechanical analysis shows that the increment of stress can be divided into the coaxial component and the rotational component of which the coupling change of shear stress and normal stress in the physical space can be used to measure the direction of the principal stress. In light of the shear-normal stress ratio, a simple nonlinear model which can simulate the deformation behavior of soils under complex stress path is proposed. The stress–strain relationship of the model in the form of component is established considering the initial shear modulus and the peak strength of soil which are closely related to the anisotropy of soil and the direction of stress. Comparisons of predicted and measured results of tests under complex stress paths show that the newly build model can capture deformation characteristics of cohesive soil and granular materials reasonable wells.

Patterning of a cohesionless granular layer under pure shear.

The response of a thin layer of granular material to an external pure shear imposed at its base is investigated. The experiments show that, even for noncohesive materials, the resulting deformation of the material is inhomogeneous. Indeed, a novel smooth pattern, consisting of a periodic modulation of the shear deformation of the free surface, is revealed by an image-correlation technique. These observations are in contrast with the previous observation of the fracture pattern in cohesive granular materials subjected to stretching. For cohesive materials, the instability is due to the weakening of the material which results from the rupture of capillary bridges that bond the grains to one another. For noncohesive materials, the rupture of the capillary bridges cannot be invoked anymore. We show that the instability results from the decrease of friction on shearing. PACS: 89.75.Kd: Pattern formation in complex systems; 83.60.Uv: Rheology: fracture; 45.70.Qj: Pattern formation in granular matter.

Scaling laws for the mechanics of loose and cohesive granular materials based on Baxter's sticky hard spheres.

We have conducted discrete element simulations (pfc3d) of very loose, cohesive, granular assemblies with initial configurations which are drawn from Baxter's sticky hard sphere (SHS) ensemble. The SHS model is employed as a promising auxiliary means to independently control the coordination number z_{c} of cohesive contacts and particle volume fraction ϕ of the initial states. We focus on discerning the role of z_{c} and ϕ for the elastic modulus, failure strength, and the plastic consolidation line under quasistatic, uniaxial compression. We find scaling behavior of the modulus and the strength, which both scale with the cohesive contact density ν_{c}=z_{c}ϕ of the initial state according to a power law. In contrast, the behavior of the plastic consolidation curve is shown to be independent of the initial conditions. Our results show the primary control of the initial contact density on the mechanics of cohesive granular materials for small deformations, which can be conveniently, but not exclusively explored within the SHS-based assembling procedure.

Force Transmission Modes of Non-Cohesive and Cohesive Materials at the Critical State.

This paper investigates the force transmission modes, mainly described by probability density distributions, in non-cohesive dry and cohesive wet granular materials by discrete element modeling. The critical state force transmission patterns are focused on with the contact model effect being analyzed. By shearing relatively dense and loose dry specimens to the critical state in the conventional triaxial loading path, it is observed that there is a unique critical state force transmission mode. There is a universe critical state force distribution pattern for both the normal contact forces and tangential contact forces. Furthermore, it is found that using either the linear Hooke or the non-linear Hertz model does not affect the universe force transmission mode, and it is only related to the grain size distribution. Wet granular materials are also simulated by incorporating a water bridge model. Dense and loose wet granular materials are tested, and the critical state behavior for the wet material is also observed. The critical state strength and void ratio of wet granular materials are higher than those of a non-cohesive material. The critical state inter-particle distribution is altered from that of a non-cohesive material with higher probability in relatively weak forces. Grains in non-cohesive materials are under compressive stresses, and their principal directions are mainly in the axial loading direction. However, for cohesive wet granular materials, some particles are in tension, and the tensile stresses are in the horizontal direction on which the confinement is applied. The additional confinement by the tensile stress explains the macro strength and dilatancy increase in wet samples.

Compaction of noncohesive and cohesive granular materials under vibrations: Experiments and stochastic model.

We study the time evolution of the compaction of a noncohesive or cohesive granular material submitted to shaking through experiments and a stochastic model. Beyond well-known empirical expressions, we show that the characteristic time scales depend on the number of objects in the assembly. For a noncohesive granular material, the compaction time scale is governed by the number of individual grains in the system. In the case of a cohesive granular material, a two-scale model (individual particles and clusters) allows one to mimic the time evolution of the compaction of an actual cohesive powder driven by horizontal vibrations. In this case, the two time scales are associated with the numbers of clusters and grains, respectively.

Thinning sea ice weakens buttressing force of iceberg m\xe9lange and promotes calving

At many marine-terminating glaciers, the breakup of mélange, a floating aggregation of sea ice and icebergs, has been accompanied by an increase in iceberg calving and ice mass loss. Previous studies have argued that mélange may suppress calving by exerting a buttressing force directly on the glacier terminus. In this study, I adapt a discrete element model to explicitly simulate mélange as a cohesive granular material. Simulations show that mélange laden with thick landfast sea ice produces enough resistance to shut down calving at the terminus. When sea ice within mélange thins, the buttressing force on the terminus is reduced and calving is more likely to occur. When a calving event does occur, it initiates a propagating jamming wave within mélange, which causes local compression and then slow mélange expansion. The jamming wave can also initiate widespread fracture of sea ice and further increase the likelihood of subsequent calving events.

Influence of dry cohesion on the micro- and macro-mechanical properties of dense polydisperse powders & grains

Understanding how cohesive granular materials behave is of interest for many industrial applications, such as pharmaceutical or food and civil engineering. Models of the behaviour of granular materials on the microscopic scale can be used to obtain macroscopic continuum relations by a micro-macro transition approach. The Discrete Element Method (DEM) is used to inspect the influence of cohesion on the micro and macro behaviour of granular assemblies by using an elasto-plastic cohesive contact model. Interestingly, we observe that frictional samples prepared with different cohesion values show a significant difference in pressure and coordination number in the jammed regime; the differences become more pronounced when packings are closer to the jamming density, i.e. the lowest density where the system is mechanically stable. Furthermore, we observe that cohesion has an influence on the jamming density for frictional samples, but there is no influence on the jamming density for frictionless samples.

Experimental investigation of impinging jet erosion on model cohesive granular materials

Erosion of soils affects both natural landscapes and engineering constructions as embankment dams or levees. Improving the safety of such earthen structures requires in particular finding out more about the elementary mechanisms involved in soil erosion. Towards this end, an experimental work was undertaken in three steps. First, several model materials were developed, made of grains (mostly glass beads) with solid bridges at particle contacts whose mechanical yield strength can be continuously varied. Furthermore, for most of them, we succeeded in obtaining a translucent system for the purpose of direct visualization. Second, these materials were tested against surface erosion by an impinging jet to determine a critical shear stress and a kinetic coefficient [2, 3]. Note that an adapted device based on optical techniques (combination of Refractive Index Matching and Planar Laser Induced Fluorescence [3]) was used specifically for the transparent media. Third, some specifically developed mechanical tests, and particularly traction tests, were implemented to estimate the mechanical strength of the solid bridges both at micro-scale (single contact) and at macro-scale (sample) and to investigate a supposed relationship with soil resistance to erosion.