Linking dynamics between anchors and granular materials

Micromechanical modelling of oval particulates subjected to bi-axial compression

\nPore size distribution is an important feature of granular materials in the context of filtration and erosion in soil hydraulic structures. Present study focuses on the evolution characteristics of pore size distribution for numerically simulated granular assemblies while subjected to various compression boundary constrain, namely, conventional drained triaxial compression, one-dimensional or oedometric compression and isotropic compression. We consider the effects initial packing of the granular assembly, loose or dense state. A simplified algorithm based on Delaunay tessellation is used for the estimation of pore size distribution for the deforming granular assemblies at various stress states. The analyses show that, the evolution of pore size is predominantly governed by the current porosity of the granular assembly while the stress path or loading process has minimal influence. Further it has also been observed that pore volume distribution reaches towards a critical distribution at the critical porosity during shear enhanced loading process irrespective of the deformation mechanism either compaction or dilation.\n

The objective of this paper is to resolve different length scales in structure analysis using an interface coupling the discrete element method (DEM) with the finite element method (FEM). This approach distinguishes itself from other methods in so far that no overlapping domains between finite and discrete elements exist. Both domains are separated in the physical space and the numerical simulation domain. The proposed approach is relevant to almost all engineering applications that deal with granular matter such as storage in hoppers, transport on conveyor belts or the displacement of granular material as in mixers or the excavation of the soil. For these applications an engineering device such as mixer blades or cutting tools are in contact with granular matter. Contacts with individual particles generate contact forces that act on both the engineering device and the granular material. The latter experiences a displacement of individual particles whereby the engineering structure responds with deformation and stresses. In order to predict and optimise both the behaviour and motion of granular material and the structures in contact, numerical simulation tools are increasingly employed. Simulations are popular especially because experiments are often expensive, time-consuming and sometimes even dangerous. The continuous increase in computing power is now enabling researchers to implement numerical methods that do not focus on the granular assembly as an entity, but rather deduce its global characteristics from observing the individual behaviour of each grain. An interaction between granular media and a structure relies on a transfer of forces between them. Granular media consists of an ensemble of particles of which a number of particles may be in contact with a surface e.g. walls as surfaces of solid structures. The contact is resolved similar to inter-particle contacts by a representative overlap. It defines the position of impact as well as the force acting on the particle at this position. The same force, however, in the opposite direction defines a mechanical load for the structure. In order to determine the effect of forces on the solid structure, it is discretised using finite elements. The impact of the force is transferred to the nodes

A topological view on microscopic structural evolution for granular material under loading and unloading path

Granular materials reveal their complexity and some of their unique features when subjected to shear deformation. They can dilate, behave like a solid or a fluid, and are known to carry external forces preferentially as force chains. In this dissertation, we employ laboratory experiments to study the complex behavior of granular materials under shear. We introduce a multiscale approach in which the underlying grain-scale mechanics are experimentally measured and homogenized to obtain enriched macroscopic quantities. First, we investigate granular avalanches spontaneously generated by a rotating drum. Measurements of grain kinematics are directly incorporated into a rate-dependent plasticity model that explains and reproduces the life cycle of laboratory avalanches. The results presented here feature dilatancy as the key material parameter governing the triggering of an avalanche. Second, we report a set of experiments performed on a custom-built mechanical device that allows a specimen composed of a two-dimensional analogue granular assembly to be subjected to quasi-static shear conditions. A numerical force inference technique, the Granular Element Method (GEM), provides direct observation and quantitative characterization of force chain structures in assemblies made of realistic grains. Equipped with a complete description of the grain-scale mechanics, we show that shear deformation creates geometrical (fabric) and mechanical (force) anisotropy. Finally, the influence of grain shape on grain-scale processes is studied. We find that grain interlocking is a prominent deformation mechanism for non-circular grains that ultimately promotes a significant increase in macroscopic shear strength. By seamlessly connecting grain-scale information to continuum scale experiments, this dissertation sheds light on the multiscale mechanical behavior of granular assemblies under shear.

A combined experimental and DEM investigation of grain interlocking in sheared granular assemblies

As a critical factor governing the strength and deformation behaviours of granular materials, dilatancy characteristics inherently exhibit intricate complexity arising from stochastic particle arrangements and dynamic motion patterns. This article introduces a stress-dilatancy relationship considering the variation of contact number within granular assemblies from the perspective of micromechanics. At microscopic scale, average contact force and displacement expressions are derived using the true stress tensor. And then the variation of contact number is introduced to further optimise the description of the energy dissipation behaviours of granular materials. Following that, the stress dilatancy relationship for granular materials considering of the evolution of fabric anisotropy are established through macro-micro energy conservation. The stress-dilatancy relationship can well describe the phenomenon that the stress ratio for dense sand at the phase transition point is inconsistent with the critical stress ratio. Compared with the reults of experiments obtained from Ottawa sand, the established stress-dilatancy relationship can better describe macroscopic deformation response than the classical flow law such as the Cam-Clay model and Rowe dilatancy model. Moreover, under true triaxial stress paths and different initial anisotropies, the mechanical responses of the proposed stress-dilatancy relationship are consistent with the results of discrete element method simulation experiments.

In this study, we investigate with the Discrete Element Method (DEM) the mechanical behavior of a cohesionless granular material under undrained true triaxial conditions, considering both monotonic and cyclic loading. We link the microstructure evolution within the granular assembly to its macroscopic cyclic response. To capture the mechanical response of our reference material (Karlsruhe fine sand), a rolling resistance linear contact model along with spherical particles is calibrated through a trial-and-error process, adjusting the model parameters to capture the experimentally observed behavior as close as possible. A series of cyclic undrained triaxial tests were simulated to investigate the micromechanical processes underlying liquefaction of sand under cyclic shearing. We analyzed the evolution of various fabric indices, including the redundancy index, contact normal orientations, and fabric anisotropy in relation to the pre- and post-liquefaction responses. The results reveal that a redundancy index below unity provides a unified criterion for the loss of the isostatic condition within the granular assembly, which triggers the onset of liquefaction. Throughout the cyclic loading process, sliding-dominant contact-yielding mechanisms remain prevalent. Additionally, significant changes in contact normal orientation and increasing fabric anisotropy dependent on the induced axial strain occur as the sample undergoes post-liquefaction deformation.

The physical nature of contact moments within granular assemblies is reviewed and a new approach is developed for the homogenisation of stress within these materials. This approach revolves around capturing the effects of contact moments through the concept of contact eccentricity. By this method it is possible to calculate an expression for bulk stress that is both symmetric for material in equilibrium and fully consistent with the usual definition of bulk stress as an ensemble average of material stress over a representative volume element. The technique is demonstrated in a simple two dimensional example, as well as a larger scale discrete element modelling simulation of a steady state direct shear experiment.

One significant factor influencing geothermal energy exploitation is the variation of the mechanical properties of rock in high temperature environments. Since rock is typically a heterogeneous granular material, thermal fracturing frequently occurs in the rock when the ambient temperature changes, which can greatly influence the geothermal energy exploitation. A numerical method based on the numerical manifold method (NMM) is developed in this study to simulate the thermo-elastic fracturing of rocklike granular materials. The Voronoi tessellation is incorporated into the pre-processor of NMM to represent the grain structure. A contact-based heat transfer model is developed to reflect heat interaction among grains. Based on the model, the transient thermal conduction algorithm for granular materials is established. To simulate the cohesion effects among grains and the fracturing process between grains, a damage-based contact fracture model is developed to improve the contact algorithm of NMM. In the developed numerical method, the heat interaction among grains as well as the heat transfer inside each solid grain are both simulated. Additionally, as damage evolution and fracturing at grain interfaces are also considered, the developed numerical method is applicable to simulate the geothermal-related thermal fracturing process.

Granular lubrication is a promising extreme environment lubrication candidate. Lubricated by granular media, the startup dynamic process of a plain journal is significant to the mechanical performance, which is not well investigated yet. A research model focusing on the startup process of a granular flow lubrication bearing is constructed by the discrete element method. The granules filled in slim gaps extrude and collide with each other, which transfers the force to the shaft and causes instability to the shaft movement. During the movement from rest to stable running position, repeated position mutations of the shaft were found to occur. The variations in axis resistance torque and average unbalanced force are indistinctive, whereas those in average contact force, coordination number, and slip ratio are apparent. In this process, the force interacts within the inner granular assembly, which results in the formation of a force chain network. The breakage and reconstruction of this network can be used to analyze the movement and force variation characteristics. The movement and load tend to stabilize after a certain running period. The results provide a deeper understanding of the unique behavior of the granular-lubricated bearing induced by granular media.

The time evolution of silo discharge is investigated for different granular materials made of spherical or elongated grains in laboratory experiments and with discrete element model (DEM) calculations. For spherical grains, we confirm the widely known typical behavior with constant discharge rate (except for initial and final transients). For elongated particles with aspect ratios between 2 ⩽ L/d ⩽ 6.1, we find a peculiar flow rate increase for larger orifices before the end of the discharge process. While the flow field is practically homogeneous for spherical grains, it has strong gradients for elongated particles with a fast-flowing region in the middle of the silo surrounded by a stagnant zone. For large enough orifice sizes, the flow rate increase is connected with a suppression of the stagnant zone, resulting in an increase in both the packing fraction and flow velocity near the silo outlet within a certain parameter range.

Experimental evaluation of mechanically stabilized earth walls with recycled crumb rubbers

An assessment of discrete element approaches to infer intergranular forces from experiments on 2D granular media

When the granular matter is being pushed a certain point can be found, before arriving at which the force increases linearly, and after that it increases exponentially. The granular matter is a large assemblage of solid grains, which are fundamentally different from any other type of matters, such as solid blue and liquids; they have different physical properties. The meso-scale of force chain bridges single particles and granular matter, and leads to unique properties and behaviours of granular matters, thus acts as the key issue in the study of granular matter. In this work, we first find the variation of different forces by simulation. We then propose a discrete element model based on rigorous Hertzian contact law and Mindlin-Deresiewicz contact theory for normal and tangential contact forces, respectively, and then point out that the change of force chain is the key to the granular dynamics. The force distribution and the force chain length distribution are calculated and analyzed to show the change rule and the origin of the drag force.