Abstract: The interlink between particle-scale properties and macroscopic behavior of three-dimensional granular media subjected to mechanical loading is studied intensively by scientists and engineers, but not yet well understood. Here we study the role of key particle-scale properties, such as interparticle friction and particle elastic modulus, in the functioning of dual contact force networks, viz., strong and weak contacts, in mobilizing shear strength in dense granular media subjected to quasistatic shearing. The study is based on three-dimensional discrete element method in which particle-scale constitutive relations are based on well-established nonlinear theories of contact mechanics. The underlying distinctive contributions of these force networks to the macroscopic stress tensor of sheared granular media are examined here in detail to find out how particle-scale friction and particle-scale elasticity (or particle-scale stiffness) affect the mechanism of mobilization of macroscopic shear strength and other related properties. We reveal that interparticle friction mobilizes shear strength through bimodal contribution, i.e., through both major and minor principal stresses. However, against expectation, the contribution of particle-scale elasticity is mostly unimodal, i.e., through the minor principal stress component, but hardly by the major principal stress. The packing fraction and the geometric stability of the assemblies (expressed by the mechanical coordination number) increase for decrease in interparticle friction and elasticity of particles. Although peak shear strength increases with interparticle friction, the deviator strain level at which granular systems attain peak shear strength is mostly independent of interparticle friction. Granular assemblies attain peak shear strength (and maximum fabric anisotropy of strong contacts) when a critical value of the mechanical coordination number is attained. Irrespective of the interparticle friction and elasticity of the particles, the packing fraction and volumetric strain are constant during steady state. Volumetric strain in sheared granular media increases with interparticle friction and elasticity of the particles. We show that the elasticity of the particles does not enhance dilation in frictionless granular media. The results presented here provide additional understanding of the role of particle-scale properties on the collective behavior of three-dimensional granular media subjected to shearing.

Abstract: We study the consequences of the interplay between electrostatic forces, mechanical contact forces, and frictional properties of grains upon the bulk frictional properties of charged granular media subjected to quasistatic shearing. We show that, the variations in short-range electrostatic forces between the grains (which are often ignored in the existing studies) dominantly affect the bulk friction. Charging enhances the fabric anisotropy of heavily loaded contacts--this enhances the bulk friction, more significantly, in the case of low frictional granular systems.

Abstract: To describe the heterogeneous nature of stress transmission in granular materials, the concept of the "strong" network consisting of contacts with large normal forces has been proposed by Radjaï [Phys. Rev. Lett. 80, 61 (1998)]. The shear stress is mainly determined by this strong network. The dual viewpoint is adopted here, by not only considering the forces at contacts, but also the deformation. It is shown that the strain increments are determined by the tangential component of the relative displacements at the contacts. A "mobile" network consisting of contacts with large tangential relative displacements is defined that primarily accounts for the strain increments. The investigation of the relation between the strong and the mobile networks shows that these networks are largely unrelated. An alternative network is defined that consists of contacts at which the contribution to the work input is large. It is found that this work input occurs primarily through the tangential forces and tangential relative displacements.

Abstract: The prevalence of particulate materials in modern industrial processes and products provides a significant motivation to achieve fundamental understanding of the bulk behaviour of particulate media. The rapid progress being made with atomic force microscopy and related particle characterization techniques pushes the limits of micro- and nanotechnologies such that interparticle interactions can be engineered to fabricate particulate assemblies to deliver specific functionalities. In this paper, primarily based on discrete element method simulations that we performed over the past 10 years, we summarize the key findings on the role of force transmission networks in dense particulate systems subjected to shearing. In general, the macroscopic strength characteristics in particulate systems is dictated by the distribution of heavily loaded contacts, also referred to as 'strong' force chains. Surprisingly, they constitute only a limited proportion of all contacts in particulate systems. They act like a 'granular brain' (memory networks) at particle scale. We show that the structural arrangement of the force chains and their evolution during loading depends on the single-particle properties and the initial packing condition in particulate assemblies. Further, the 'nature' of force chains in sheared granular media induces larger 'solid' grains to behave like 'fluid' particles, retarding their breakage. Later, we probe for ways by which we can control the signature of memory networks in packed beds, for example by applying an external electrical field in a densely packed particulate bed subjected to shearing (combined electromechanical loading). Though further research is required to account for more realistic conditions and preferably to allow particles to self-organize to strength specifications, understanding the hidden memory networks in particulate materials could be exploited to optimize their collective strength.

Abstract: Based on the discrete element method, the nature of normal contact force distribution and the effect of microstructure (contact fabric) on stresses in granular media sheared under constant mean stress condition is analyzed. The particles are tested in a periodic cell, having a nearly monodispersed system of spherical particles ("hard" and "soft"). The granular systems were initially isotropically compressed to have different solid fractions in order to obtain "dense" and "loose" samples. To study the nature of the force distribution, the granular medium was considered as both (i) noncohesive and (ii) with low values of interface energy. For the granular systems considered here, the nature of force distribution is shown to be dependent on shear history. The amount of interface energy introduced in the granular system does not seem to change the nature of normal force distribution significantly. However, it improves the postpeak stability in agreement with previous research [C. Thornton, Geotechnique 50, 43 (2000)]. The simulation of systems subjected to quasistatic shearing, in general, reveals that in a hard system (both dense and loose), the normal contact force distribution (i) at "peak" shear strength is purely an exponential decay throughout the entire range of force scale that is used, and (ii) at "isotropic" and "steady" states, the contact normal force distribution is bimodal with forces greater than average decaying exponentially at both the states, while the forces less than average tend to be half-Gaussian at the "isotropic" state and a second-order polynomial function at the "steady" state. For the soft (dense) system, the normal contact force distribution at "peak" shear strength is bimodal with forces greater than average decaying exponentially while the forces less than average tend to be a second-order polynomial function. However, for the soft system at both "isotropic" and "steady" states, the contact normal force distribution is half-Gaussian throughout the entire range of force scale that is used. It has been pointed out that in a granular system undergoing slow shearing, the shear strength of the system seems to depend on the ability of the material to form strong fabric anisotropy of contacts carrying strong (greater than average) force.