Views on Microstructures in Granular Materials (original) (raw)
Views on Microstructures in Granular Materials: the Preface
2020
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Some statistics about research on granular materials during the last decade
EPJ Web of Conferences
In this work we present several statistical analyses about research on the subject of granular materials during the last ten years. These statistics are based on the set of articles published in the Granular Materials section of Physical Review E during this period, comprising roughly 1000 documents (including rapid communications). We estimate the degree of contribution of countries, academic institutions, authors, and articles, in terms of the frequency of appearance on the articles' affiliations and the number of citations. On the first hand, these analyses serve as a recognition to researchers and research communities that have notably contributed to the development of knowledge on granular materials. On the second hand, the presented statistics allow for identifying countries and institutions where research in granular materials is the most developed, and also places where it can be developed in the future. Finally, these analyses allow for highlighting which cooperations amongst countries have been the most active during the last decade, which in turn allows for identifying potential links to be created and developed across this research network.
Micro structures of granular materials with various grain size distributions
Powder Technology, 2012
The effect of the grain size distribution on the micro structure of granular materials has not been fully understood. Especially, few works is currently available on the coordination number of the well-graded granular mixtures due to the difficulty and uncertainness of experimental measurement. This research aims at studying systematically the effects of grain size distribution on the coordination number and constructing a model to predict the frequency distribution of the coordination number of granular materials on the basis of their grain size distribution. Using a two-dimensional Discrete Element Method, nine types of disk assemblies were packed densely or loosely and their coordination numbers and void ratios were measured. Simulation results indicated that the coordination number and the void ratio generally decreased with increasing grain size dispersion. The authors found a general relationship between the number of contact points and the particle size which can be applied to granular materials having various grain size distributions. In the case of densely packed assemblies, this relationship is applicable to predict the frequency distribution of coordination number only from their grain size distribution.
Microstructural characteristics of planar granular solids
2013
The microstructure of granular materials is the main factor determining their macroscopic behaviour. We study systematically the statistical characteristics of volume elements (called quadrons) of microstructures of mono-and polydisperse planar disc packs granular and report a number of new results. The packs analysed were of different intergranular friction coef¿cients ,μ, contained about 20,000 discs each and were brought to mechanical equilibrium under identical isotropic compression stresses from three different initial states: loose, intermediate and dense. Our ¿ndings are the following. (i) The rattlers volume fraction φ r is not affected by the disc size distribution (DSD). (ii) Excluding the rattlers, we ¿nd that the relation between the packing fraction φ and the mean coordination numberz is independent of the initial state. Together with result (i), this allows us to separate the effects of the DSD and the initial state on the microstructure. (iii) We relate analyticallȳ z, φ and the (normalised) mean quadron volumev. (iv) Combining (iii) and a relation betweenz and the mean cell order,ē, derived from Euler's topological relation, we show that (ii) is a result of the geometrical relation betweenv andē. (v) The probability density function of the quadron volumes, normalised byv , is universal for all the studied systems and it can be ¿t reasonably well by a Γ distribution.
Acta Mechanica, 2008
The formation of strain localization influences the stability and stiffness of the soil mass or geosystem. The thickness of shear bands provides insight into overall strength and stiffness inside the granular body, and the shear band angle gives information about the failure surface in a given soil or soil mass. Thus, it is important to be able to predict when a shear band forms and how this zone of intense deformation is located and oriented within the granular medium. A rational finite element analysis for capturing the formation and development of shear bands has been formulated by using a micropolar continuum in finite element analysis. Hardening parameters including nonsymmetric stress and length scale are implemented for a realistic stressstrain analysis. Implementation and simulations of the model will be discussed in the Part II of this research. The micro-macro structure relation for finite element analysis based on this formulation paper will be simulated in the corresponding paper.
Structural changes in granular materials: The case of irregular polygonal particles
International Journal of Solids and Structures, 2005
This paper presents a series of numerical simulations of biaxial tests performed on assemblies of two-dimensional irregular polygonal particles. Each sample is prepared with a technique similar to dry pluviation. Different aspect ratios (1-3) are considered and the behavior of granular samples is analyzed from both a global and a local point of view. More precisely, the influence of the particle aspect ratio on both inherent (initial) and induced anisotropy is investigated. New internal variables which are related to the orientation of particles are proposed. They give new insight into the specific mechanisms that control the behavior of irregular polygonal materials. Associated to global variables, they demonstrate the existence of a critical state irrespective of the investigated aspect ratios. However, for materials with higher aspect ratios (2 and 3), their inherent anisotropy prevents any extensive reorganization, this means that, within the range of usual strains considered in civil engineering, the particle reorientation remains in progress and considerable deformations are required to reach the critical state.
Predicting granule behaviour through micro-mechanistic investigations
International Journal of Mineral Processing, 2003
A novel micro-manipulator device has been developed to observe and measure, directly, the behaviour of binder liquid bridges between pairs of solid particles. The objective has been to develop the fundamental understanding of the role of the liquid and solid properties in the growth and consolidation of granules, from the initial contact between the liquid and particles to the resultant multi-particle bodies. On the particle level, it is the liquid bridges that are responsible for the strength of ''wet'' agglomerates, since they hold the particles together. In this paper, results of experiments will be reported that identify the role of liquid surface tension, bridge Laplace pressure, liquid viscosity and, hence, wetting behaviour, in the axial strength of the bridges. In particular, the differences in bridge shape when particles of different surface properties come together (i.e. in mineral mixtures) provides a crucial insight into whether granules will grow successfully or not. A parabolic approach to describing the shapes adopted by the liquid bridges, from which parameters such as resistance to deformation can be calculated, will be shown. From the theoretical behaviour of individual bridges, determined through the direct experimental observations, a simple model has been constructed, which relates granule porosity, liquid content and the physicochemical properties of the materials to the agglomerate hardness. Some experimental measurements using spherical particles and powders commonly granulated in the pharmaceutical industry, such as lactose, will be compared to the model predictions and the role of interparticle friction and liquid surface tension and viscosity will be shown quantitatively.
Journal of Structural Geology, 2003
Laboratory tests have been carried out to determine the physical properties of high sphericity and low density, fine grained dry granular materials. In particular, we tested hollow aluminium microspheres and hollow siliceous microspheres. Shear tests have been carried out with a Casagrande apparatus for different values of normal stress (from 12.27 to 491.8 kPa) in order to determine the frictional properties of the microspheres. Diagrams of the shear stress against displacement have been used to describe the strength of the investigated materials at different confining pressures. The shear tests show that hollow aluminium and siliceous microspheres successfully replicate the mechanical behaviour of weaker layers within natural sedimentary successions undergoing deformation. The use of these artificial granular materials in specifically designed experiments indicated that they can be successfully used for constructing more "natural" multilayers in the laboratory.
Systematic description of the effect of particle shape on the strength properties of granular media
EPJ Web of Conferences
In this paper, we explore numerically the effect of particle shape on the mechanical behavior of sheared granular packings. In the framework of the Contact Dynamic (CD) Method, we model angular shape as irregular polyhedral particles, non-convex shape as regular aggregates of four overlapping spheres, elongated shape as rounded cap rectangles and platy shape as square-plates. Binary granular mixture consisting of disks and elongated particles are also considered. For each above situations, the number of face of polyhedral particles, the overlap of spheres, the aspect ratio of elongated and platy particles, are systematically varied from spheres to very angular, non-convex, elongated and platy shapes. The level of homogeneity of binary mixture varies from homogenous packing to fully segregated packings. Our numerical results suggest that the effects of shape parameters are nonlinear and counterintuitive. We show that the shear strength increases as shape deviate from spherical shape. But, for angular shapes it first increases up to a maximum value and then saturates to a constant value as the particles become more angular. For mixture of two shapes, the strength increases with respect of the increase of the proportion of elongated particles, but surprisingly it is independent with the level of homogeneity of the mixture. A detailed analysis of the contact network topology, evidence that various contact types contribute differently to stress transmission at the micro-scale.
Microscopic study on stress-strain relation of granular materials
Chinese Science Bulletin, 2009
A biaxial shearing test on granular materials is numerically simulated by distinct element method (DEM). The evolution of the microstructures of granular materials during isotropic compression and shearing is investigated, on which a yield function is derived. The new yield function has a similar form as the one used in the modified Cam-clay model and explains the yield characteristics of granular materials under the isotropic compression and shear process through the change of the contact distribution N(θ) defining the contacts at particle contact angle θ.