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This talk summarises the numerical and experimental studies of brittle, hollow, cylindrical particles, called shells. It addresses the influence of shell properties both at the particle and assembly scales. Extreme compressibility has been recorded in the oedometer tests. Due to the large internal porosity of the shell, the breakage phenomena lead to highly compressive deformations with a significant stress dissipation. Using the Discrete Element Method (DEM), we have investigated in depth the micro-mechanical phenomena governing this macroscopic response. By quantifying the breakage and separating the double-porosity of the material, the foundations of a future constitutive model have been established throughout a simple prediction model applicable to the engineering practice.

Powders &amp; Grains 2021

High compressibility caused by particle breakage: a DEM investigation

[EPJ Web of Conferences, Volume 249 (2021)](https://www.epj-conferences.org/articles/epjconf/abs/2021/03/contents/contents.html)

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Powders & Grains 2021

Powders & Grains 2021 will be held in Buenos Aires (Argentina, VIRTUAL) July 5, 13, 21, 29 and Aug 6, 2021. The aim of the conference is to give an up-to-date picture of the broad research field of granular media, powders and grains. Contributions from experts around the world will focus on research topics that cover a wide range of open problems in the field.

technical paper

The present contribution tackles the issue of incipient conditions for initiation of erosion by a fluid flow at the surface of cohesive materials. To this end, a typical assessment procedure consists in subjecting a soil sample to progressive hydrodynamic stresses induced by a submerged impinging jet flow whose injection velocity is gradually increased. This paper presents the results of an extensive use of this protocol both in experiments and numerical simulations, the latter being based on a coupled DEM and LBM approach. Here we consider the specific case of weakly cemented soils, either made experimentally of glass beads bonded by solid bridges or modelled numerically by a solid bond rheology with a parabolic yield condition involving the micromechanical traction, shearing and bending of the bonds. The results show that, as expected, the hydrodynamic stress for erosion onset substantially increases with solid cohesion as compared to cohesion-less cases but can, however, be satisfactorily predicted by a simple extension of the usual Shields criterion that only applies for cohesion-less granular sediments. This extension includes a cohesion number, the granular Bond number, with a simple definition based on tensile yield values.

Extending the Shields criterion to erosion of weakly cemented granular soils

We report numerical simulations of surface granular flows confined between two sidewalls. These systems exhibit both very slow and very energetic flows.
Zhu et al. [Zhu et al. granular Matter, 22, 82, 2020] have shown that in energetic confined systems, the Froude number at sidewalls and the sidewall effective friction coefficient are linked through a unique relation.
We show that this relation is also valid for creep flows.
It is independent of the angle of the flow but depends on the sidewall-grain friction coefficient. Our results shed light on boundary conditions that have to be used at sidewalls in continuum theories aiming to capture the behavior of granular systems from creeping to energetic flows.


Sidewall friction in confined surface flows of granular materials

Mixing granular particles of different sizes is a common way of increasing the packing fraction. Recently, a model predicting the packing fraction, taking into account the inhomogeneity of the mixed small and large particles, has been proposed by S. Pillitteri et al. Under certain conditions, this model can be simplified and analytical solutions can be found. We present here these solutions, compared to experimental data, and the physical interpretation they can bring.

Forced segregation in binary granular mixtures

As a follow-up of an earlier work on the numerically exact Coulomb friction in two-dimensional simulations, we present here the relations and implementation for three-dimensional discrete element particles.

Implementation of numerically exact Coulomb friction for discrete element simulations in three dimensions

A discrete element study of a reinforced footing by rigid inclusions, submitted to cyclic loadings is presented. The discrete element method (based on molecular dynamics method) enabled to understand the load transfer mechanisms into the granular layer (just above the inclusions) with cyclic loadings. A multiscale study showed that the soil above the rigid inclusions retrieves the larger forces, illustrating the role of the granular layer as a load transfer layer, while the settlement of both the granular layer and loading slab increase with loading. Efficiency to transfer the load on piles and ability (capacity of the granular material to postpone the overloads to the piles) decreases with cycling, but keeping high values at the end of cycles. The transfer of forces in the granular layer is achieved by two mechanisms, interacting together (inverted pyramid above rigid inclusions and arching), confirming results found in the literature.

Discrete element simulations of load transfer mechanisms for a reinforced granular embankment submitted to loading cycles

The dissipation of energy during the compaction of granular materials was studied by performing
confined drop tower experiments on Ottawa sand. Energy dissipated due to breakage was quantified by evaluating the creation of new surfaces at varying drop heights. Post-compaction particle size distributions (PSD)
were measured and the amount of breakage was quantified by the position of the current PSD relative to the
pre-compaction and ultimate PSD. Our observations revealed that the percentage of input energy dissipated
due to breakage accounted for less than 0.5% of the total energy budget and was a constant proportion regardless of the total energy applied to the system. We also evaluated the effects of die wall friction by measuring
post-compaction PSD in various positions within the sample.

Energy dissipation due to breakage during confined compaction of granular materials

Wet granulation processes can be driven from low to high water content. In this study, we consider the model situation of the growth of a single wet aggregate rolling in a dry granular flow inside a rotating drum. We measure the time evolution of its diameter for dierent grains and liquids, as well as various rotation rates of the drum. Using X-ray tomography, we are able to characterize the internal structure of the granular aggregate at dierent times during the process. We show that the growth rate of the aggregate can be related to the transport of the liquid inside the granule and the capture of grains. We propose a model to rationalize the maximum size of the aggregate and its growth rate.

Wet rolling stones: growth of a granular aggregate under flow

Several granular materials are hydro-sensitive, i.e., contact with water severely affects their morphology and mechanical behaviour. To this broad class belong a series of materials of great relevance for the pharmaceutical and food industry. Former studies have been conducted on the effect that moisture or humidity have on granular flow or on the individual grain mechanical response, but the processes occurring at the micro-level and their influence on the overall granular packing behaviour is yet to be fully understood. This study presents an experimental investigation of the response of a water-sensitive material (couscous) exposed to high relative humidity (97%). 4D (3D plus time) x-ray tomographies were acquired in operando. A data treatment approach based on Dicrete Digital Image Correlation (dDIC) is developed and detailed here. This allows for following each individual grain throughout the 4+ days of the humidification test. This, in turn, allows the study of the inter- and intra-granular strain, which is particularly pronounced in water sensitive materials. The expansion of the individual grains also severely affects the contact network, whose evolving properties are here analysed and correlated to the macroscopic (sample-scale) deformation.

X-ray tomographies of a water-sensitive granular material (couscous) exposed to high relative humidity: an experimental study

To understand the underlying thermal and physical phenomena in selective laser melting (SLM) process, discrete element method (DEM) is applied to generate the randomly packed powder, then computational fluid dynamics (CFD) is adopted to simulate the laser-powder interaction. The penetration and multiple reflection of laser rays is traced. The physics of melting and solidification is captured. The temperature profile indicates the laser travel path and the adsorption and transmission of laser rays with the powder. The wetting behaviour of the melt pool driven by the capillary forces leads to the formation of pores at the connection zone. It has been demonstrated that the developed model can capture the laser-powder interaction for further understanding of SLM process.

Investigation of laser-powder interaction in laser powder bed fusion process in additive manufacturing

In this work, we study bidispese mixtures of self-propelled magnetic particles of different shapes via discrete element method simulations. We show how this particles self-assemble into clusters and how these clusters depends on the ratio of the mixture, the magnetic interaction, and the shape of the grains. It is found that the mix ratio of the system controls the cluster size. Besides, the intensity of the magnetic dipoles and the shape of the grains in the mixture rule the average number of neighbors in contact and the shape of the clusters. By varying the intensity of the interactions, globular, linear and branched clusters were obtained.

High compressibility caused by particle breakage: a DEM investigation

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Extending the Shields criterion to erosion of weakly cemented granular soils

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Highly strained mixtures of bidimensional soft and rigid grains: an experimental approach from the local scale

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High compressibility caused by particle breakage: a DEM investigation

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Next from Powders & Grains 2021

Extending the Shields criterion to erosion of weakly cemented granular soils

Similar lecture

Highly strained mixtures of bidimensional soft and rigid grains: an experimental approach from the local scale

Stay up to date with the latest Underline news!

PRESENTATIONS

CONFERENCES

COMPANY

RESOURCES

Downloads