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We use numerical simulations to investigate force and stress transmission in cohesive granular media covering a wide class of materials encountered in nature and industrial processing. The cohesion results either from capillary bridges between particles or from the presence of a solid binding matrix filling fully or partially the interstitial space. The liquid bonding is treated by implementing a capillary force law within a debonding distance between particles and simulated by the discrete element method. The solid binding matrix is treated by means of the Lattice Element Method (LEM) based on a lattice-type discretization of the particles and matrix. Our data indicate that the exponential fall-off of strong compressive forces is a generic feature of both cohesive and noncohesive granular media both for liquid and solid bonding. The tensile forces exhibit a similar decreasing exponential distribution, suggesting that this form basically reflects granular disorder. This is consistent with the finding that not only the contact forces but also the stress components in the bulk of the particles and matrix, accessible from LEM simulations in the case of solid bonding, show an exponential fall-off. We also find that the distribution of weak compressive forces is sensitive to packing anisotropy, particle shape and particle size distribution. In the case of wet packings, we analyze the self-equilibrated forces induced by liquid bonds and show that the positive and negative particle pressures form a bi-percolating structure. [ABSTRACT FROM AUTHOR]

Digital Speckle Radiography (DSR), a technique allowing measurement of the full field displacement maps in a plane within an opaque material, is used to follow the temporal progression of a long-rod (100 mm long, 10 mm diameter) penetrating a granular sample at a variety of rates. Quasi-static rates of 1.5 mm/min are achieved using an Instron machine, 5 m/s is achieved using a drop-weight and velocities between 10 and 200 m/s are achieved using a light gas gun. These experiments are carried out using a series of time delayed flash x-ray images analysed with an optimised Digital Image Cross Correlation algorithm (DICC). The subsequent data sheds considerable light on the response of granular materials to penetration at a variety of rates. Particularly, a large difference in the response of the material between quasi-static and dynamic rates is observed. [ABSTRACT FROM AUTHOR]

Many authors claim that to understand the response of a propellant under dynamic loads, it is necessary to know its mesostructural morphology and the mechanical behaviour of each of its components. However, the scale of the mechanical description of the behaviour of a propellant is relative to its heterogeneities and the loading wavelength. The shorter it is, the more important the topological description of the material is. In our problems, involving the safety of energetic materials, the propellant can be submitted to a large spectrum of loadings. In this paper, an analysis is first presented which gives the domain in terms of stress and strain involved. It shows that we have to describe the behaviour of the material from mesoscale to macroscale. Mesoscale response requires the direct description of the morphology of the sample, whereas macroscale response embedding local information can be obtained from homogenisation techniques. Following this analysis, we show how microtomographical data are used to build the mesoscopic description of the sample, and how Christoffersen’s schematisation is used in the homogenisation process. Then we present the frame and the results obtained in the homogenisation processes for hyperelasticity and damage approach. Standard drop-weight tests used to define reaction thresholds are the support of our analysis for dynamic loading. The physical phenomena are described and discussed with the support of numerical simulation. Some remarks and perspective are given in conclusion. [ABSTRACT FROM AUTHOR]

A branching labyrinth of granular material emerges when a confined granular fluid system is slowly drained. From a series of experiments, simulations and theoretical modeling, we explain the mechanism for the pattern formation in terms of the forces involved. We further show that the granular fluid system undergoes a series of transitions whereby new types of patterns develop as the volume fraction of granular material is increased. A balloon pattern is demonstrated, generated by the sudden inflation of air through narrow necks in the densely packed granular front. [ABSTRACT FROM AUTHOR]

Granular materials, such as athermal suspensions, can either be jammed and rigid, or yield and flow. Recent experiments on granular suspensions in a annular shear cell (vibrated and/or sheared) show a hysteretic freezing and melting transition [1, 2]: a crystallised state is found, which can be melted by sufficient shear. The question is open on what are the mechanisms underlying these phenomena and which are the control parameters. Via Molecular Dynamics simulations, we study the rheology of vibrated and sheared granular materials [3]. In particular, we aim to understand the nature of a critical line separating crystallised and melted states and the “jammed” region in the phase diagram, as well as the connections with thermal glass formers and colloidal suspensions. [ABSTRACT FROM AUTHOR]

We study topological and dynamical properties in sheared granular media particularly when avalanches occur, i.e. at the abrupt increase of the total kinetic energy during the shear process. Our model considers irregular polygonal particles compressed between two moving plates. At the occurrence of an avalanche, we monitor the time evolution of the underlying contact force network. When all contact forces are considered, topological and dynamical quantities vary as a reaction to the avalanche, while the decomposition of the contact force network into strong and weak contacts uncovers a topology that is strongly correlated with the occurrence of avalanches. [ABSTRACT FROM AUTHOR]

We study granular materials characterized by the possibility of interlocking between the particles. The Interlocking is modeled as the combined effect of sliding and rolling resistance at the contacts, and it involves two contact parameters: a coefficient of sliding friction and a coefficient of rolling friction. This model is introduced within the framework of the Contact Dynamics method, and it is applied to investigate the effect of the two contact friction coefficients on the behavior of large polydisperse granular samples sheared in a numerical simple shear device. The macroscopic behavior of the samples is investigated in terms of the steady-state shear strength and the fabric properties of the packing. We find that two regimes can be distinguished in which the steady-state shear strength is controlled by either sliding friction or rolling friction. Moreover, our results show that the introduction of rolling friction strongly affects the microstructure of the packing. In particular, with high values of the sliding and rolling friction coefficients, the force-carrying backbone takes an increasingly columnar aspect involving a weak proportion of particles and affecting the mechanical role played by the weak network. [ABSTRACT FROM AUTHOR]

We investigate the influence of particle shape nonconvexity on the shear strength and texture of granular materials. We consider regular trimers (clumps composed of three circular particles) described by a nonconvexity parameter depending on the relative positions of the particles. The contact dynamics method is used to shear large samples of trimers with different levels of nonconvexity. The solid fraction is a nonmonotonous function of nonconvexity. The shear strength increases with nonconvexity and saturates. We show that the increase of shear strength results from the presence of multiple contacts between trimers leading to enhanced frictional interlocking. The double and triple contacts are less in proportion but carry the largest forces in the packing. The saturation is related to the fact that the contact orientation anisotropy declines and compensates the increase of the normal force anisotropy, so that the shear strength remains constant. [ABSTRACT FROM AUTHOR]

Applying the principle of least dissipation rate to the biaxial shear geometry leads to the prediction that there are preferred shear band patterns according to the aspect ratio of the shear cell. By means of the Mohr-Coulomb angle these patterns can be classified in terms of two integer indices. They are expected to have a certain life time which is particularly long for low index configurations. This explains the temporal fluctuations of the stress ratio in “steady state flow,” which so far were believed to be random. [ABSTRACT FROM AUTHOR]

We study the macroscopic behaviour of granular material, as a consequence of the interactions of individual grains at the micro scale. A two-scale approach of computational homogenization is considered. On the micro-level, we consider granular structures modelled using the Discrete Element Method (DEM). Grain interactions are modelled by normal and tangential contact laws with friction (Coulomb’s criterion). On the macro-level, we use a Finite Element Formulation (FEM). The upscaling technique consists in using the response of the DEM model at each Gauss point of the FEM discretisation to derive numerically the constitutive response. In this process, a tangent operator is generated together with the stress increment corresponding to the strain increment in the Gauss point. In order to get more insight on the consistency of the resulting constitutive response, we compute the determinant of the acoustic tensor associated with the tangent operator. This quantity is known to be an indicator of a possible loss of uniqueness locally, at the macro scale, by strain localisation in shear band. Different numerical studies have been performed, as listed hereafter. We have considered different number of grains in the REV cell. Periodic boundary conditions have been compared with the ordinary wall conditions. [ABSTRACT FROM AUTHOR]

The planar shearing of dense systems of bidisperse as well as polydisperse, non-cohesive, hard, round, dissipative and frictional particles is simulated in two and three dimensions using Contact Dynamics. The system is sheared by smooth, frictional walls at constant shear velocities, subject to a constant normal force. Depending on the shear velocity, the system behavior belongs to one of the three different regimes (from high to low shear velocities): a) a fluid like state with almost homogeneous shearing, b) a block like state with symmetric velocity profiles and fluidization only at the walls, c) a state of broken symmetry, where a block moves essentially with one wall while possessing a shear zone at the other. The two shear velocities separating theses regimes do not depend on the distance between the walls. [ABSTRACT FROM AUTHOR]

Sinusoidal horizontal vibrations have been applied to a parallelepiped containing rounded sand grains. To model the granular medium behavior, a commercial software based on Molecular Dynamics has been used. The influence on the rheologic behavior of many process parameters, such as the dimensionless acceleration and the frequency has been studied. The velocity, density, pressure and mass flow fields have been computed, both in dynamic and static modes and compared with experimental results. The correlation between these different parameters has been also examined. [ABSTRACT FROM AUTHOR]

We investigate the water distribution and the link between suction and water content in granular media. Firstly, we examine the effect of suction on the shape and the volume of the liquid bridge for different parameters (grain radius, inter-particle distance, contact angle, surface tension). This local behaviour is then used in a discrete element study of a sample composed of several thousands of grains. We focus our study on the pendular state. The existence of a liquid film around the grains which involves the continuity of the liquid phase is assumed. The water distribution and the water content associated with a given suction are calculated. Then retention curves of the granular media are built. Four different methods are used. The first is based on the local expression of the capillary force coupled with the “gorge method,” the second is based on the Laplace equation, and the third and the fourth are based on the integration of the differential equation that defines the liquid bridge shape. A parametric study is made to bring to light the effect of macroscopic parameters (grain-size distribution, density) and physical parameters (liquid/air surface tension, contact angle) on the water retention curve. Finally, numerical data are compared to experimental results. [ABSTRACT FROM AUTHOR]

This controlled study examined the feasibility of a simple percussive approach to drilling through unconsolidated regolith deposits on Mars. The experiments showed that the approach is feasible at the low power levels and low confining pressures used, and that the rate of impact is more important to the penetration rate than is the mass of the impactor (hammer). More massive impactors tend to lower energy efficiency, as they do in terrestrial pile-driving. Unexpectedly, penetration plotted against applied energy tends to cluster into parallel linear trends. Within a given cluster, penetration is very sensitive to applied energy, while between clusters, the same penetration requires different energy levels. The clusters are separated by gaps whose widths may be related to the average grain size of the material being penetrated. The layered nature of natural sedimentary deposits is reflected in the cumulative energy-penetration plots, which could thus serve to record bedding thickness and frequency during Mars exploration. This study has shown that percussive drilling using a down-the-hole hammer design may be feasible in unconsolidated fine regolith near the ground surface. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

Experimental results of ultrasonic wave propagation through unconsolidated granular materials are reported. Influence of the applied static pressure and the propagation distance on the linear and nonlinear acoustic properties is studied. The aim of this work is to understand better the linear and nonlinear acoustic propagation through granular assemblages when the wavelength is of the order of the bead diameter. In particular, the role of the so-called force chains is currently not well understood, as well as the influence of the contact distribution of the medium. In particular, the self-demodulation of shear wave packets with variation of the excitation amplitude is documented for different configurations of granular media (propagation distance, static pressure etc ...). Modeling is currently in progress and should allow together with these results to understand better the acoustic propagation through granular assemblages. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

Results of experimental research of nonlinear properties of the granular medium with fluid in the field of acoustics oscillations are considered. It is found that the acoustical respond of some separate granule of the medium is drastically changes in time under the pure tone excitation. Granular oscillation occurs essentially nonlinear rather under moderate amplitude elastic wave propagation when the granular acceleration is essentially less then gravity acceleration. Sound intensity fluctuations for the harmonic and subharmonic components are revealed in the records of up to 62 hours long. Viscous fluid suppresses fluctuations and makes the granular medium more linear for acoustical excitation. Data of the experiments are compared with the results of a simulation of sound wave propagation in granular medium with nonlinear contacts between the granules. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

Explosions near the seafloor result in shock waves entering a much more complicated medium than water or air. Nonlinearities may be increased by two processes inherent to granular media: (1) a poroelastic nonlinearity comparable to the addition of bubbles to water, and (2) the Hertz force resulting from elastic deformation of grains, proportional to the Youngs modulus of the grains times the strain rate to the power 3/2. These two types of nonlinearity for shock propagation into the seafloor are investigated using a variant of the NPE model. The traditional Taylor series expansion of the equation of state (pressure as a function of density) is not appropriate to the Hertz force in the limit of small strain. We present a simple nonlinear wave equation model for compressional waves in marine sediments that retains the Hertz force explicitly with overdensity to the power 3/2. Numerical results for shock propagation are compared with similarity solutions for quadratic nonlinearity and for the fractional nonlinearity of the Hertz force. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

Experiments have shown that the thickness of shear bands depends upon the mean particle size. These observations suggest that a proper continuum representation of granular behavior must be augmented in a manner that introduces a characteristic length scale. Such generalized continua include micro-polar continua (which involve couple-stress or higher-order stress) and strain-gradient material models. The paper presents the results of DEM simulations that test whether such micro-polar or strain-gradient effects are present in granular materials. The 2D simulation experiments were conducted as “flexure tests” of roughly square assemblies of circular particles. The platens along two opposite sides were rotated to produce a fiexural curvature of the assembly between these sides. Particles along the rotating sides were constrained to translate and rotate in conformity with the sides; whereas, interior particles were free to adjust their movements so that their local equilibrium was maintained. The constraining movements of the platen particles were produced by applying external forces and moments. By measuring incremental forces and moments, we compute the incremental bending moment on the particle assembly. This incremental moment is compared with that of a simple classical continuum. After being compressed to near the peak strength, the bending stiffness is significantly greater than would be computed from classical stiffness moduli. That is, at large strains, granular stiffness depends, in part, on the gradient of strain, not just the strain alone. The results are consistent with micro-polar media or with a strain gradient-dependent material. [ABSTRACT FROM AUTHOR]