Element test experiments and simulations: from dry towards cohesive powders

Findings from experiments and particle simulations for dry and cohesive granular materials are presented with the goal to reach quantitative agreement between simulations and experiments. Results for the compressibility, tested with the FT4 Powder Rheometer are presented. The first simulation results involve the strain controlled uniaxial compression of frictionless polydisperse spheres in a biaxial box using a linear visco-elastic contact model.\ud \ud As main result, the evolution of pressure as a function of volume fraction is reported. Our anisotropic, uniaxial findings compare astonishingly well with results for purely isotropic compression. Concerning the second stress response, namely anisotropy, we present the evolution of the deviatoric stress as a function of the volume fraction, which cannot be measured with the FT4 experiment, but requires a bi-axial experiment

Hydrostatic and shear behavior of frictionless granular assemblies under different deformation conditions

Stress- and structure-anisotropy (bulk) responses to various deformation modes are studied for dense packings of linearly elastic, frictionless, polydisperse spheres in the (periodic) triaxial box element test configuration. The major goal is to formulate a guideline for the procedure of how to calibrate a theoretical model with discrete particle simulations of selected element tests and then to predict another element test with the calibrated model (parameters).\ud 　Only the simplest possible particulate model material is chosen as the basic reference example for all future studies that aim at the quantitative modeling of more realistic frictional, cohesive powders. Seemingly unrealistic materials are used to exclude effects that are due to contact non-linearity, friction, and/or non-sphericity. This allows us to unravel the peculiar interplay of stress, strain, and microstructure, i.e. fabric.\ud 　Different elementary modes of deformation are isotropic, deviatoric (volume-conserving), and their superposition, e.g. a uniaxial compression test. Other ring-shear or stress-controlled (e.g. isobaric) element tests are referred to, but are not studied here. The deformation modes used in this study are especially suited for the bi- and triaxial box element test set-up and provide the foundations for understanding and predicting powder flow in many other experimental devices. The qualitative phenomenology presented here is expected to be valid, even clearer and magnified, in the presence of non-linear contact models, friction, non-spherical particles and, possibly, even for strong attractive/ adhesive forces.\ud 　The scalar (volumetric, isotropic) bulk properties, the coordination number and the hydrostatic pressure scale qualitatively differently with isotropic strain. Otherwise, they behave in a very similar fashion irrespective of the deformation path applied. The deviatoric stress response (i.e. stressanisotropy), besides its proportionality to the deviatoric strain, is cross-coupled to the isotropic mode of deformation via the structural anisotropy; likewise, the evolution of pressure is coupled via the structural anisotropy to the deviatoric strain, leading to dilatancy/compactancy. Isotropic/uniaxial over-compression or pure shear respectively slightly increase or reduce the jamming volume fraction below which the packing loses mechanical stability. This observation suggests a necessary generalization of the concept of the jamming volume fraction from a single value to a “wide range” of values as a consequence of the deformation history of the granular material, as “stored/memorized” in the structural anisotropy.\ud 　The constitutive model with incremental evolution equations for stress and structural anisotropy takes this into account. Its material parameters are extracted from discrete element method (DEM) simulations of isotropic and deviatoric (pure shear) modes as volume fraction dependent quantities. Based on this calibration, the theory is able to predict qualitatively (and to some extent also quantitatively) both the stress and fabric evolution in another test, namely the uniaxial, mixed mode during compression. This work is in the spirit of the PARDEM project funded by the European Unio

Characterization of cohesive powders for bulk handling and DEM modelling

The flow behaviour of granular materials is relevant for many industrial applications including the pharmaceutical, chemical, consumer goods and food industries. A key issue is the accurate characterisation of these powders under different loading conditions and flow regimes, for example in mixers, pneumatic conveyors and silo filling and discharge. This paper explores the experimental aspects of cohesive powder handling at different compaction levels and flow regimes, namely inertial and quasi-static regimes. So far, laboratory element test set-ups capable of defining the full stress states at very low compaction levels have not been fully explored in literature. In contrast the mechanical behaviour of cohesive powders under relatively high consolidation stress (several kPa upward) can be carefully measured using element tests such as biaxial test, true triaxial and hollow cylinder tests. However in practice these tests are expensive and slow to conduct and are almost never performed for many industrial applications requiring material characterisation. Here we investigate simpler techniques that could be used for filling this important gap with the focus of providing test data for model calibration and simulation validation in line with the spirit of the European Commission funded PARDEM Marie Curie ITN Project. We perform particle and bulk characterisation on limestone powder with 4.7μm and 31.3 μm mean particle size, detergent powder with differences in formulation, cocoa powder with low and high fat content - relevant for different industrial applications. Of particular significance is the 4.7μm limestone powder which is the PARDEM reference powder that have been created and extensively used in a collaborative European PARDEM Project (www.pardem.eu). In the inertial, low consolidation stress regimes - more relevant for powder transport and conveying applications - we present experimental findings on the flowability and avalanching behaviour of the reference material in a rotating drum. On the other hand, in the quasi-static, higher consolidation regime, we perform shear tests with the Edinburgh Powder Tester (EPT), an extended uniaxial tester and the commercially available Freeman FT4 Powder Rheometer. For macroscopic quantities, we report the unconfined yield strength as a function of applied stress. These material characteristics provide important scientific insights for developing innovative solutions for cohesive powder handling problems. From these experiments and for best practice guideline, we highlight subtle issues associated with the experimental setup and measurements. The experiments lead to a rich quantitative description of the flow behaviour and failure properties of the materials which provide the material data for DEM model calibration and validation

Characterisation of cohesive powders for bulk handling and DEM modelling

The flow behaviour of granular materials is relevant for many industrial applications including the pharmaceutical, chemical, consumer goods and food industries. A key issue is the accurate characterisation of these powders under different loading conditions and flow regimes, for example in mixers, pneumatic conveyors and silo filling and discharge. This paper explores the experimental aspects of cohesive powder handling at different compaction levels and flow regimes, namely inertial and quasi-static regimes. So far, laboratory element test set-ups capable of defining the full stress states at very low compaction levels have not been fully explored in literature. In contrast the mechanical behaviour of cohesive powders under relatively high consolidation stress (several kPa upward) can be carefully measured using element tests such as biaxial test, true triaxial and hollow cylinder tests. However in practice these tests are expensive and slow to conduct and are almost never performed for many industrial applications requiring material characterisation. Here we investigate simpler techniques that could be used for filling this important gap with the focus of providing test data for model calibration and simulation validation in line with the spirit of the European Commission funded PARDEM Marie Curie ITN Project. We perform particle and bulk characterisation on limestone powder with 4.7µm and 31.3 µm mean particle size, detergent powder with differences in formulation, cocoa powder with low and high fat content - relevant for different industrial applications. Of particular significance is the 4.7µm limestone powder which is the PARDEM reference powder that have been created and extensively used in a collaborative European PARDEM Project (www.pardem.eu). In the inertial, low consolidation stress regimes - more relevant for powder transport and conveying applications - we present experimental findings on the flowability and avalanching behaviour of the reference material in a rotating drum. On the other hand, in the quasi-static, higher consolidation regime, we perform shear tests with the Edinburgh Powder Tester (EPT), an extended uniaxial tester and the commercially available Freeman FT4 Powder Rheometer. For macroscopic quantities, we report the unconfined yield strength as a function of applied stress. These material characteristics provide important scientific insights for developing innovative solutions for cohesive powder handling problems. From these experiments and for best practice guideline, we highlight subtle issues associated with the experimental setup and measurements. The experiments lead to a rich quantitative description of the flow behaviour and failure properties of the materials which provide the material data for DEM model calibration and validation

Force correlations, anisotropy, and friction mobilization in granular assemblies under uniaxial deformation

We study dense, frictional, polydisperse 3D granular assemblies under uniaxial deformation with Discrete Element Method (DEM) simulations. The overall goal – beyond the scope of the present study – is to link microscopic parameters and observations with the macroscopic behavior, for different elementary deformation modes. At present, we focus on the behavior of the force/contact network during uniaxial deformation, for different coefficients of friction. We discuss the stress and structural anisotropy and the relationship between force intensity weighted by contact state (sticking or sliding, at the Coulomb limit) or force strength. Furthermore, we study the orientational distribution of contacts and forces and the contribution of friction to structural anisotropy. We find that initial isotropic states are irrecoverable, since the structural anisotropy is independent of the deviatoric stress behavior both with and without friction. Contacts display an interesting anisotropy of order four in the presence of friction

Evolution of the effective moduli for anisotropic granuar materials during pure shear

We analyze the behavior of a frictionless dense granular packing sheared at constant volume. Goal is to predict the evolution of the effective moduli along the loading path. Because of the structural anisotropy that develops in the system, volumetric and deviatoric stresses and strains are cross coupled via four distinct quantities, the classical bulk and shear moduli and two anisotropy moduli. Here, by means of numerical simulation, we apply small perturbations to various equilibrium states that previously experienced different pure shear strains and investigate the effect of the microstructure (2 nd rank fabric tensor) on the elastic bulk response. Besides the expected dependence of the bulk modulus on the isotropic fabric, we find that both the isotropic density of contacts and the (deviatoric) orientational anisotropy affect the anisotropy moduli. Interestingly, the shear modulus of the material depends also on the actual stress state, along with the (isotropic and anisotropic) contact configuration

Experiments on the avalanching of cohesive powders in a rotating drum

Based on image processing, a novel method is presented for characterizing the flowability of cohesive powders undergoing the dynamic avalanching regime in a rotating drum. To benchmark the results, we compare them with the well-established flow of a non-cohesive sample and highlight quantitative differences between the two cases. As main result, we present the probability distribution and fluctuations of the average surface angle and the angle of stability. Cohesive powders display repeatable and reproducible data with considerably higher mean and average - different from the sample with glass beads

Force correlations, anisotropy, and friction mobilization in granular assemblies under uniaxial deformation

We study dense, frictional, polydisperse 3D granular assemblies under uniaxial deformation with Discrete Element Method (DEM) simulations. The overall goal – beyond the scope of the present study – is to link microscopic parameters and observations with the macroscopic behavior, for different elementary deformation modes. At present, we focus on the behavior of the force/contact network during uniaxial deformation, for different coefficients of friction. We discuss the stress and structural anisotropy and the relationship between force intensity weighted by contact state (sticking or sliding, at the Coulomb limit) or force strength. Furthermore, we study the orientational distribution of contacts and forces and the contribution of friction to structural anisotropy. We find that initial isotropic states are irrecoverable, since the structural anisotropy is independent of the deviatoric stress behavior both with and without friction. Contacts display an interesting anisotropy of order four in the presence of friction