Mechanical Properties Of Granular Materials Research Articles

Modeling the effect of moisture on the flowability of a granular material

The flowability of granular materials is a crucial parameter to consider that impacts many engineering and industrial applications such as slope stability, powder handling and storage. The dependence of mechanical properties of granular materials on their water content is widely encountered but poorly understood. This paper presents a three-dimensional (3D) discrete element method model to simulate the microscale mechanisms of granular materials in terms of interactions between individual particles as well as interactions with surrounding fluid through liquid bridging. Formulation of the contact model is first introduced. Then the numerical model is validated by comparing the simulation results with experimental data reported in literature. Direct shear, repose angle and rotary drum tests are simulated with a synthetic material under various moisture contents. The effects of moisture on the shear strength and flowability are evaluated. The simulation results show that the numerical model presented in this paper is capable of reproducing the dependency of the shear strength on water content. Moreover, it also successfully captures the characteristics of particle flow under wet conditions.

Evolution of meso-structures and mechanical properties of granular materials under triaxial compression state from complex network perspective

The evolution of the network connection of granular materials is investigated by performing a series of numerical simulations in triaxial compression tests with different initial porosities by discrete element method (DEM). Results of evolution characteristics of complex network are reported for both dense and loose assembles. The simulation focuses on the influence of porosity on connectivity evolution, and reveals the correlation between the parameters in macro and mesoscale. Kinds of properties are studied, including degree and its distribution, clustering coefficient, network density and the average shortest path. The results demonstrate the phenomenon of dilataion due to shear deformation are able to be reflected by those mesocope parameters mentioned. Specifically, in the process of the dilatation, the rate of contact disintegration exceeds the rate of contact creation, which means the loss of connectivity, thus the values of some properties decrease, like degree, clustering coefficient and network density, but some increase like the average shortest path. Additionally, the bridging of macro and mesoscope are built regarding the parameters of the Cam-Clay model and complex network. From the results, the parameter M (determined by q = Mp′ at critical state) and the reference parameter $$ {\text{T}} $$ ( $$ T_{j}^{s} = L_{j}^{s} \left( {1 - \log_{{D_{j}^{s} }} t} \right) $$ , calculated according to the average degree $$ D_{j}^{s} $$ and shortest path $$ L_{j}^{s} $$ of the critical state) have a positive correlation. And a linear relationship between the slope of isotropic virgin-consolidation λ and the rate of decline of the average shortest path upon loading is represented as well. These achievements are the first step in an ongoing study of establishing the multi-scale constitutive from complex network perspective.

A 3-D mechanics-based particle shape index for granular materials

Particle shape plays an important role in the mechanical properties of granular materials. This paper studies the effect it has on particle rotations and stress via a mechanics-based 3-D particle shape index, which is based on an earlier 2-D particle shape index. A level set discrete element method (LS-DEM) simulation, with experimental validation, of a specimen of a natural sand with varying particle shapes, subject to triaxial compression, is analyzed. We show that particles whose shapes allow for more moment transmission—as characterized by the particle shape index—experience less deviation in rotation and contribute more to the macroscopic shear stress, and provide possible micromechanical explanations of these trends. The results have implications in the construction of micromechanics-based granular models and higher-order continuum models.

A three-dimensional particle roundness evaluation method

Particle roundness is one of the major shape parameters influencing the micro-and macroscopic mechanical properties of granular materials. Most of the existing particle roundness methods are based on 2D algorithms utilizing particle projections. However, the morphology of a particle projection will be influenced by the subjectively chosen projection angle. Based on the acknowledged Wadell’s definitions of particle corner and particle roundness, an innovative method is proposed to evaluate the 3D roundness of realistic particles in a quantitative and objective manner. This method first reconstructs the particle with a triangular mesh. Then, corner portions are identified by finding the surface vertexes with both a high local curvature and large relative connected area. After corner identification, a sphere-filling method is employed to fill the corner portions with spheres. Finally, the 3D Wadell roundness is computed by comparing the average radius of the filling spheres with the radius of the maximum inscribed sphere. Validation of the proposed 3D roundness evaluation algorithm is conducted by comparing the results of the proposed method with the results from theoretical and other existing approaches. The comparison indicates that the proposed method can be used to accurately and objectively evaluate the roundness of realistic particles.

Influence of particle shape on the microstructure evolution and the mechanical properties of granular materials

In order to study the influence of particle shape on the microstructure evolution and the mechanical properties of granular materials, a two-dimensional DEM analysis of samples with three particle shapes, including circular particles, triangular particles, and elongated particles, is proposed here to simulate the direct shear tests of coarse-grained soils. For the numerical test results, analyses are conducted in terms of particle rotations, fabric evolution, and average path length evolution. A modified Rowe's stress–dilatancy equation is also proposed and successfully fitted onto simulation data.

Evolution of the Mesoscopic Parameters and Mechanical Properties of Granular Materials upon Loading

Biaxial tests of granular materials under different lateral pressures were numerically simulated to study both the macromechanical behavior and evolution of mesoscopic parameters, including the coordination number, degree, clustering coefficient, and average shortest path length, which can provide information bridging mesoscale to macroscale of the mechanical properties of granular materials in a different and new way. The analysis results demonstrate that, with the increasing lateral pressure, there is a higher coordination number for denser granular material, and the coordination number of the samples eventually tends to the similar value at critical state with the same lateral pressure for different initial porosities; the distribution of the degree is very similar for samples with different initial porosities at critical state; the evolution of the clustering coefficients with axial strain is almost the same as that of the coordination number. At critical state, the clustering coefficients of all samples are almost identical, which means that the internal structures of samples with various initial porosities are similar; the average shortest path decreases with increasing lateral pressure and tends to be stable in the critical state. Additionally, the diameter of the contact network of granular material hardly changes at different lateral pressures at critical state.

Self-organized magnetic particles to tune the mechanical behavior of a granular system

Above a certain density a granular material jams. This property can be controlled by either tuning a global property, such as the packing fraction or by applying shear strain, or at the micro-scale by tuning grain shape, inter-particle friction or externally controlled organization. Here, we introduce a novel way to change a local granular property by adding a weak anisotropic magnetic interaction between particles. We measure the evolution of the pressure, P, and coordination number, Z, for a packing of 2D photo-elastic disks, subject to uniaxial compression. A fraction Rm of the particles have embedded cuboidal magnets. The strength of the magnetic interactions between particles is too weak to have a strong direct effect on P or Z when the system is jammed. However, the magnetic interactions play an important role in the evolution of latent force networks when systems containing a large enough fraction of the particles with magnets are driven through unjammed to jammed states. In this case, a statistically stable network of magnetic chains self-organizes before jamming and overlaps with force chains once jamming occurs, strengthening the granular medium. This property opens a novel way to control mechanical properties of granular materials.

Моделирование упруговязкопластических свойств земляного полотна. Постановка задачи 1

A brief analysis of the physical and mechanical properties of materials, used in the construction of the rail bed, has shown that in scientific and technical literature there is scattered unsystematic information obtained under different physical conditions for bulk materials (gravel and sand). Unstructured data on the physical and mechanical properties of granular materials does not allow formulating a relatively complete dynamic theory for a wide range of static and dynamic loads, vibration acceleration and humidity conditions in relation to rail transport. In this article the authors formulate the problem of stability of the system “clayey soils in the main area - protective layer of sand cushions - ballast crushed stone prism” under the impact of harmonic oscillations on the basis of physico-chemical mechanics. Loam or clay belongs to isotropic visco-plastic media, sand and gravel - to disperse granular media. Filling the pore space of the gravel with weeds leads to the retention of precipitation and melt liquid. During deflation processes clogging of the gravel layer is increased. Despite the enormous practical value, properties of granular materials, until recently, practically not been studied. Under appropriate conditions this material may behave as a solid and as liquid and as gas. Each phase has unique properties that distinguish granular materials from all other substances. If the vibration field is applied to layer of gravel or sand, the energy of this motion is passed even through individual granules. As a result, with sufficiently high amplitude of jitter a phase transition occurs: the entire mass of granular material gets in motion, begins to flow due to reduction in friction between the particles and behaves as a kind of “granular liquid”. Fluid state of gravel stone occurs at much large values of the amplitude of vibration because of its angular shape. The result is a hydrodynamic instability, then convection starts and the vortices spontaneously appear.

Mechanical Properties of Granular Materials and Their Impact on Load Distribution in Silo: A Review

Abstract Mechanical properties of granular materials and their impact on load distribution in storage silo were discussed with special focus on materials of biological origin. Granular materials classification was briefly outlined. The evolution of constitutive models of granular materials developed in the frame of mechanics of continuum was addressed. Analytical methods, Finite Element Methods (FEM), and Discrete Element Methods (DEM) of estimation of silo pressure were discussed. Special attention was paid to the following issues: dynamic pressure switch in the first moment of silo discharge, asymmetry of loads due to eccentric discharge, and impact of uncontrolled increase of moisture content of grain on silo pressures.

An investigation on the mechanical properties of granular materials in interface with asphaltic concrete

Asphaltic concrete has been used as waterproofing core in embankment dams, since 1948. In this application, granular materials are used around the asphaltic concrete as filter, which makes a complicated behaviour in the interface and needs to be researched by experiments and modelling. This paper describes experimental work and the results of investigating the mechanical behaviour of the interface between aggregates and asphaltic concrete. Small scale shear strength test has been used in this study, in which the shear surface is considered as the interface. The effect of bitumen penetration grade, moisture, density and angularity of aggregates on the shear strength parameters at different levels of normal stresses and a constant shear rate have been studied. The results show that the shear strength parameters increases with increasing the density and normal stress level. It is also shown that the shear parameters decrease with increasing moisture content in the interface. The penetration grade of bitumen does not have a significant effect on the parameters. It is also demonstrated that the ratio of interaction between the aggregate and asphalt concrete decreases in a certain range of variation.

DEM simulations of stiff and soft materials with crushable particles: an application of expanded perlite as a soft granular material

The mechanical behavior of granular materials is largely affected by particle breakage. Physical and mechanical properties of granular materials, such as grain size distribution, deviatoric and volumetric behavior, compressibility and mobilized friction angle are affected by particle crushing. This paper focuses on the evolution of the above mentioned characteristics using the Discrete Element Method (DEM). Behaviors of stiff and soft materials are studied using well established crushing criteria. Results from simulations indicate that stiff materials, have a typical fractal distribution of particle size, which is dominant when confining pressure increases. The fractal characteristic parameter of grain size effect is discussed. Evolution of shear stresses and volumetric strains during shearing are also predicted and analyzed. Expanded perlite, selected as a soft material, is investigated in terms of shear and volumetric behavior. For perlite, triaxial compression tests and corresponding DEM simulations are also performed. Results show good agreement between experiments and simulations and support the fact that the DEM can be considered as a useful tool to predict the behavior of crushable granular materials.

Thermodynamic analysis of simple shear granular flows

Granular flow is the collective moment of a large number of solid particles. A so-called Granular Solid Hydrodynamics (GSH) was recently proposed and expected to provide unifying descriptions of mechanical properties of granular materials. In this work, the simple shear flows with constant volume are firstly simulated with the discrete element method (DEM) where the volume fraction of solids ranges from 0.2 to 0.68. Meanwhile, the conventional kinetic theory is used for dilute systems analysis (i.e. volume fraction less than 0.6 in this study). We found the kinetic theory results about the normal stress with different volume fractions and shear rates agree well with DEM results. After improve the expression of granular temperature, GSH theory is employed to study the granular flows and the obtained results coincide with DEM and kinetic results. It is concluded that the improved GSH theory could coherently describe mechanical behavior of granular flows from quasi-static deformation until rapid flows.

Influence of the Fractal Dimension on the Mechanical Properties of Granular Materials

To determine the size of a grain, we associated its form to that of a equivalent sphere. The grains size is then measured by an equivalent diameter, which is not enough to describe the behavior materials with irregular grains shape. To understand these effects, a new technique was developed by Mandelbrot (1979) which is based on the fractal geometry. To clarify this notion, the grains shape is characterized using the fractal dimension (Df), which is a number measuring the degree of irregularity or the fragmentation of a grain. Mechanical tests were performed. The fractal dimension was calculated for different grains constituting the samples before and after each test while studying its evolution after crushing. The results confirm that the fractal dimension affects the measurement of mechanical properties of the granular materials.

The static contact statuses between granular materials and flat-bottomed steel silos

Abstract In this paper, finite element models have been developed to simulate the static contact statuses between granular materials and flat-bottomed steel silos in three forms: Near Contact, Sliding Contact and the Sticking Contact. The finite element models employ the Drucker–Prager yield criterion, which includes the volumetric dilation effects of granular materials. The static contact statuses are investigated with flat bottom steel silos containing different granular materials. The silo geometries vary at height/diameter ratios (H/D) of 0.75, 1.5 and 3.0. The granular materials are selected as coal, wheat, rounded gravel and sunflower seeds. Results show that the mechanical properties of granular materials (include bulk density, elastic modulus, Poisson's ratio, dilation angle, internal friction angle, cohesion) and silo designs (especially height/diameter ratio) have significant effects on the contact statuses at the interface between the granular materials and the silos. Since larger sticking/sliding contact area inevitably cause unloading difficulties or friction damage, contact statuses between the granular materials and the flat-bottomed steel silos should be optimized in the silo design in order to boost storing efficiency and economic benefit.

Face stability analysis of shallow shield tunnels in dry sandy ground using the discrete element method

Controlling the face stability of shallow shield tunnels is difficult due to the inadequate understanding of face failure mechanism. The failure mechanism and the limit support pressure of a tunnel face in dry sandy ground were investigated by using discrete element method (DEM), which has particular advantages for revealing mechanical properties of granular materials. The contact parameters of the dry sand particles were obtained by calibrating the results of laboratory direct shear tests. A series of three-dimensional DEM models for different ratios of the cover depth to the diameter of the tunnel ( C/ D = 0.5, 1, and 2; i.e., relative depth) were then built to simulated the process of tunnel face failure. The limit support pressure, failure zone and soil arching were discussed and compared with other methods. The results of DEM simulations show that the process of tunnel face failure can be divided into two stages. With the increase of the horizontal displacement of the tunnel face, the support pressure decreases to the limit support pressure and then increases to the residual support pressure. The limit support pressure increases with the rise of relative depth and then tends to be constant. In the process of tunnel face failure, the failure zone is gradually enlarged in size and expands to the ground surface. The numerical results also demonstrate that soil arching occurs in the upper part of the failure zone and the soil becomes loosened in the failure zone. Consequently, the comprehensive analysis of tunnel face failure may help to guarantee safe construction during tunneling.

Fibre-reinforcement of granular materials: DEM visualisation and analysis

Mechanical properties of granular materials can be significantly improved by the inclusion of small amounts of short synthetic fibres. This phenomenon has been experimentally studied before by many researchers who have found that the shear strength of a granular material can be significantly improved. This study presents a visualisation of the phenomenon using discrete element method (DEM) simulations of direct shear tests conducted on mixtures of an idealised granular material and randomly oriented fibres. Snapshots showing the deformation of the samples, the velocity vectors of the particles and the force chains produced inside the samples are presented at different stages of the tests. Changes in shear strength and porosity are also illustrated. It was found that the reinforcement produced depends mostly on the rigidity of the fibres.

Mechanical properties of granular materials: A variational approach to grain‐scale simulations

AbstractThe mechanical properties of cohesionless granular materials are evaluated from grain‐scale simulations. A three‐dimensional pack of spherical grains is loaded by incremental displacements of its boundaries. The deformation is described as a sequence of equilibrium configurations. Each configuration is characterized by a minimum of the total potential energy. This minimum is computed using a modification of the conjugate gradient algorithm.Our simulations capture the nonlinear, path‐dependent behavior of granular materials observed in experiments. Micromechanical analysis provides valuable insight into phenomena such as hysteresis, strain hardening and stress‐induced anisotropy. Estimates of the effective bulk modulus, obtained with no adjustment of material parameters, are in agreement with published experimental data. The model is applied to evaluate the effects of hydrate dissociation in marine sediments. Weakening of the sediment is quantified as a reduction in the effective elastic moduli. Copyright © 2008 John Wiley & Sons, Ltd.

A practical approach to grain shape quantification

Grain shape is a key factor affecting the mechanical properties of granular materials. However, grain shape quantification techniques to distinguish one granular material from another have not reached a stage of development for inclusion in modeling the behavior of granular materials. Part of the problem is the equipment of choice for grain shape measurement is the scanning electron microscopes. This is a relatively expensive and complex device. In this paper, we investigate a practical approach using light microscopy to quantify grain shape and to identify the key shape parameters that can distinguish grains. A light microscope was found to produce grain images with sufficient quality for the purpose of observing the grain shape profile. Several grain shape parameters were determined for eight different sands. We found Circularity, Roundness, Compactness, Sphericity, Aspect Ratio and ModRatio to be the key shape parameters that differentiate these sand grains.

Mechanical behaviour of granular materials used in analogue modelling: insights from grain characterisation, ring-shear tests and analogue experiments

The mechanical behaviour of several dry granular materials is investigated through ring-shear tests, grain characterisation, and simple analogue experiments analysed by X-ray computed tomography. An improved knowledge of granular materials is essential to determine their suitability as analogues for upper crustal rocks in experimental models and to compare analogue and numerical experiments. The ring-shear tests show that the granular materials have an elastic/frictional plastic behaviour with strain-hardening preceding failure at peak strength, followed by strain softening until a dynamic-stable value is reached. This is similar to the behaviour exhibited by experimentally deformed rocks. The physical characteristics of the grains determine the amount of diffuse deformation before failure, the percentage of strain softening and act on the thickness of the shear zones before broadening. Initial shear zone width in extensional and contractional experiments is between 11 and 16 times the mean grain size. The angle of internal friction defining one of the mechanical properties of granular materials and thus fault dip is not only related to physical characteristics of the grains and to the handling technique used (e.g. sieving or pouring), but also to the overburden and to the experimental setup used.

Mechanical properties of wet granular materials

We elaborate on the impact of liquids upon the mechanical properties of granularmaterials. We find that most of the experimental and simulation results may beaccounted for by a simple model assuming frictionless, spherical grains, with ahysteretic attractive interaction between neighbouring grains due to capillary forces.