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A large triaxial apparatus for the study of granular materials under repeated loading used at LNEC

Correia, A. Gomes; Gillett, S.
Fonte: Balkema Publicador: Balkema
Tipo: Conferência ou Objeto de Conferência
Publicado em //1996 ENG
Relevância na Pesquisa
66.31%
The improvement of analytical road pavement and railway infrastructure design methods requires a rational study of the mechanical behaviour of the constituent materials. The repeated load triaxial apparatus is one of the most common and useful tools for determining the parameters used in these design methods. This paper presents a description of a repeated load triaxial apparatus designed to test unbound granular materials of up to 0/60 gradings. The importance of the compacted density in the behaviour of these materials necessitates a laboratory compaction method that reproduces the in-situ conditions. Consequently a method of compaction and control of the density is also presented. By using this apparatus the most common parameters used in analytical pavement design (resilient modulus and Poisson’s ratio) are acquired. This is illustrated by testing a 0/20 graded crushed granite used as base course for road construction. This test apparatus may also be used to investigate the permanent deformation characteristics of base course materials.

Stability of Granular Materials under Vertical Vibrations

Deng, Rensheng; Wang, Chi-Hwa
Fonte: MIT - Massachusetts Institute of Technology Publicador: MIT - Massachusetts Institute of Technology
Tipo: Artigo de Revista Científica Formato: 300763 bytes; application/pdf
EN_US
Relevância na Pesquisa
56.36%
The influence of periodic vibrations on the granular flow of materials is of great interests to scientists and engineers due to both theoretical and practical reasons. In this paper, the stability of a vertically vibrated granular layer is examined by linear stability analysis. This includes two major steps, firstly, the base state at various values of mass holdup (Mt) and energy input (Qt) is calculated and secondly, small perturbations are introduced to verify the stability of the base state by solving the resultant eigenvalue problem derived from the linearized governing equations and corresponding boundary conditions. Results from the base state solution show that, for a given pair of Mt and Qt, solid fraction tends to increase at first along the layer height and then decrease after a certain vertical position while granular temperature decreases rapidly from the bottom plate to the top surface. This may be due to the existence of inelastic collisions between particles that dissipate the energy input from the bottom. It is also found that more energy input results in a lower solid fraction and a higher granular temperature. The stability diagram is constructed by checking the stability property at different points in the Mt-Qt plane. For a fixed Mt...

On the Electrostatics of Pneumatic Conveying of Granular Materials

Zhu, Kewu; Yao, Jun; Wang, Chi-Hwa
Fonte: MIT - Massachusetts Institute of Technology Publicador: MIT - Massachusetts Institute of Technology
Tipo: Artigo de Revista Científica Formato: 297129 bytes; application/pdf
EN_US
Relevância na Pesquisa
66.24%
In this work the electrostatics of the pneumatic conveying of granular materials in a non-conducting (PVC) vertical pipe is studied using Electrical Capacitance Tomography (ECT) system. The non-conducting wall in general attains static charges arising from particle-wall collisions in the initial periods of conveying process and then reaches equilibrium with the surroundings. The polarity of particles and conveying pipe inner wall agrees reasonably well with the contact potential difference measurements. The perturbations in the capacitance signal due to charge accumulation are larger with smaller air superficial velocity. The denser flow regimes give larger wall residual charge. Wall charging process shows similar trend by surface potential and ECT measurements. The addition of small amount (0.5% by weight) of anti-static agent (Larostat-519) in the powder form decreases the electrostatic charge generation by altering the patterns for particle-particle and particle-wall collisions.; Singapore-MIT Alliance (SMA)

Hollow cylinder apparatus for testing unbound granular materials of pavements; Road Materials and Pavement Design

Caicedo Hormaza, Bernardo; Ocampo Terreros, Manuel Santiago; Vallejo Rosero, Luis Eduardo; Monroy, Faindry Julieth
Fonte: Pontifícia Universidade Javeriana Publicador: Pontifícia Universidade Javeriana
Formato: 455 - 479
Relevância na Pesquisa
66.57%
13 - 3; Unbound granular materials used in pavement structures are subjected to a complex stress path which includes rotational stresses. Hollow cylinder apparatuses (HCAs) are suitable laboratory devices for reproducing stress paths found in the field. However, as the size of the HCA depends on the size of the granular particles, their use for testing unbound granular materials for pavements has not been reported in the literature. This paper presents the development of a new large size hollow cylinder apparatus designed to study the response of unbound granular materials subjected to stress rotation. This large HCA has hydraulic actuators for vertical movement and torsion while shear and vertical stresses are servo-controlled to reproduce stress paths produced by a heavy vehicle moving on a pavement structure. Confining stress is applied through the use of rings with controllable stiffness. This paper highlights the role of ring stiffness on stress paths. The results show the capabilities of the large HCA for studying the behaviour of unbound granular materials used in pavement structures.; Unbound granular materials used in pavement structures are subjected to a complex stress path which includes rotational stresses. Hollow cylinder apparatuses (HCAs) are suitable laboratory devices for reproducing stress paths found in the field. However...

Modelling crushing of granular materials as a poly-disperse mixture; Proceedings of the 18th International Conference on Soil Mechanics and Geotechnical Engineering

Caicedo Hormaza, Bernardo; Ocampo Terreros, Manuel Santiago; vallejo Rosero, Luis Eduardo
Fonte: Presses des Ponts Publicador: Presses des Ponts
Formato: 1003 - 1006
Relevância na Pesquisa
66.33%
This paper presents a new model to assess the evolution of the grain size distribution of granular materials during loading. This model is based on the theory of poly-disperse mixtures proposed by De Larrard, 2000. Using this model it is possible to evaluate the compacity of the mixture depending on the grain size distribution, the shape of the particles and the compaction energy. Markov processes are used to assess the evolution of the grain size distribution, Markovian transition probabilities for each grain size are evaluated experimentally using gyratory compaction. Finally the experimental results are compared with the results of the model showing a very good agreement.; This paper presents a new model to assess the evolution of the grain size distribution of granular materials during loading. This model is based on the theory of poly-disperse mixtures proposed by De Larrard, 2000. Using this model it is possible to evaluate the compacity of the mixture depending on the grain size distribution, the shape of the particles and the compaction energy. Markov processes are used to assess the evolution of the grain size distribution, Markovian transition probabilities for each grain size are evaluated experimentally using gyratory compaction. Finally the experimental results are compared with the results of the model showing a very good agreement.; Paris

Modelling crushing of granular materials using Markovian processes; Proceedings of the TC105 ISSMGE International Symposium on Geomechanics from micro to macro, Cambridge, UK, 1-3 September 2014

Caicedo Hormaza, Bernardo; Ocampo Terreros, Manuel Santiago; Vallejo Rosero, Luis Eduardo
Fonte: CRC Press Publicador: CRC Press
Formato: 989 - 994
Relevância na Pesquisa
66.33%
This paper proposes an analytical model to assess the evolution of the grain size distribution due to crushing of granular materials during cyclic loading. The model, that requires very low computational cost, is based on the theory of polydisperse mixtures proposed by De Larrard, 2000 and the theory of Markovian processes. The evaluation of the performance of the model was carried out experimentally using a gyratory compactor and numerically using a discrete element model that includes crushing. The comparison of the results obtained experimentally with those obtained using the models, numerical and analytical, shows very good agreement. For this reason, the analytical model proposed here is a useful alternative to be included in a model based in continumm mechanics.; This paper proposes an analytical model to assess the evolution of the grain size distribution due to crushing of granular materials during cyclic loading. The model, that requires very low computational cost, is based on the theory of polydisperse mixtures proposed by De Larrard, 2000 and the theory of Markovian processes. The evaluation of the performance of the model was carried out experimentally using a gyratory compactor and numerically using a discrete element model that includes crushing. The comparison of the results obtained experimentally with those obtained using the models...

Coulomb-Mohr Granular Materials: Quasi-static Flows and the Highly Frictional Limit

Cox, G.; Thamwattana, N.; McCue, S.; Hill, J.
Fonte: ASME International Publicador: ASME International
Tipo: Artigo de Revista Científica
Publicado em //2008 EN
Relevância na Pesquisa
66.47%
One approach to modeling fully developed shear flow of frictional granular materials is to use a yield condition and a flow rule, in an analogous way to that commonly employed in the fields of metal plasticity and soil mechanics. Typically, the yield condition of choice for granular materials is the Coulomb–Mohr criterion, as this constraint is relatively simple to apply but at the same time is also known to predict stresses that are in good agreement with experimental observations. On the other hand, there is no strong agreement within the engineering and applied mechanics community as to which flow rule is most appropriate, and this subject is still very much open to debate. This paper provides a review of the governing equations used to describe the flow of granular materials subject to the Coulomb–Mohr yield condition, concentrating on the coaxial and doubleshearing flow rules in both plane strain and axially symmetric geometries. Emphasis is given to highly frictional materials, which are defined as those granular materials that possess angles of internal friction whose trigonometric sine is close in value to unity. Furthermore, a discussion is provided on the practical problems of determining the stress and velocity distributions in a gravity flow hopper...

New stress and velocity fields for highly frictional granular materials

McCue, S.; Johnpillai, I.; Hill, J.
Fonte: Oxford Univ Press Publicador: Oxford Univ Press
Tipo: Artigo de Revista Científica
Publicado em //2005 EN
Relevância na Pesquisa
66.36%
The idealized theory for the quasi-static flow of granular materials which satisfy the Coulomb–Mohr hypothesis is considered. This theory arises in the limit as the angle of internal friction approaches π/2, and accordingly these materials may be referred to as being ‘highly frictional’. In this limit, the stress field for both two-dimensional and axially symmetric flows may be formulated in terms of a single nonlinear second-order partial differential equation for the stress angle. To obtain an accompanying velocity field, a flow rule must be employed. Assuming the non-dilatant double-shearing flow rule, a further partial differential equation may be derived in each case, this time for the streamfunction. Using Lie symmetry methods, a complete set of group-invariant solutions is derived for both systems, and through this process new exact solutions are constructed. Only a limited number of exact solutions for gravity-driven granular flows are known, so these results are potentially important in many practical applications. The problem of mass flow through a two-dimensional wedge hopper is examined as an illustration.; Scott W. Mccue, I. Kenneth Johnpillai and James M. Hill

Perturbation solutions for highly frictional granular media

Thamwattana, N.; Hill, J.
Fonte: Royal Soc London Publicador: Royal Soc London
Tipo: Artigo de Revista Científica
Publicado em //2005 EN
Relevância na Pesquisa
56.24%
In this paper, we deal with the materials possessing angles of internal friction ϕ for which 1 − sinϕ is close to zero, and we use the solution for sinϕ = 1 as the leading term in a regular perturbation series, where the correction terms are of order 1 − sinϕ. In this way we obtain approximate analytical solutions which can be used to describe the behaviour of real granular materials. The solution procedure is illustrated with reference to quasi–static flow through wedge–shaped and conical hoppers. For these two problems, the obtained perturbation solutions are shown to be graphically indistinguishable from the numerical solutions for high angles of internal friction, and for moderately high angles of internal friction the perturbation solutions still provide excellent approximations.; Ngamta Thamwattana and James M. Hill

Symmetry analysis for uniaxial compression of a hypoplastic granular material

McCue, S.; Johnpillai, I.; Hill, J.
Fonte: Birkhauser Verlag Ag Publicador: Birkhauser Verlag Ag
Tipo: Artigo de Revista Científica
Publicado em //2005 EN
Relevância na Pesquisa
56.24%
A variety of modelling approaches currently exist to describe and predict the diverse behaviours of granular materials. One of the more sophisticated theories is hypoplasticity, which is a stress-rate theory of rational continuum mechanics with a constitutive law expressed in a single tensorial equation. In this paper, a particular version of hypoplasticity, due to Wu [2], is employed to describe a class of one-dimensional granular deformations. By combining the constitutive law with the conservation laws of continuum mechanics, a system of four nonlinear partial differential equations is derived for the axial and lateral stress, the velocity and the void ratio. Under certain restrictions, three of the governing equations may be combined to yield ordinary differential equations, whose solutions can be calculated exactly. Several new analytical results are obtained which are applicable to oedometer testing. In general this approach is not possible, and analytic progress is sought via Lie symmetry analysis. A complete set or “optimal system” of group-invariant solutions is identified using the Olver method, which involves the adjoint representation of the symmetry group on its Lie algebra. Each element in the optimal system is governed by a system of nonlinear ordinary differential equations which in general must be solved numerically. Solutions previously considered in the literature are noted...

Model methodology for studying crushing of unbound granular materials during compaction; Proceedings of the TC105 ISSMGE International Symposium on Geomechanics from micro to macro, Cambridge, UK, 1-3 September 2014

Ocampo Terreros, Manuel Santiago; Caicedo Hormaza, Bernardo; vallejo Rosero, Luis Eduardo
Fonte: CRC Press Publicador: CRC Press
Formato: 297 - 302
Relevância na Pesquisa
66.51%
A fracture and abrasion model is used in order to understand and visualize the evolution of crushing resulting from compression and shear in unbound granular materials. Modelling was done using numerical simulation with the discrete element method in two dimensions in which each particle in the granular material is represented by a disc. The particle fragmentation criterion is defined on the bases of grain fracturing under diametric compression, and it is estimated as a function of material properties using an adaptation ofWeibull’s statistical theory for fracturing of brittle materials. Stresses in discs are measured using numerical modelling in order to evaluate particle crushing in soils subjected to loads, defining the failure criterion on an individual particle subjected to an arbitrary set of contact forces. This procedure approximates the stress levels inside the grain through a simple analysis which take into account the magnitude, location, and number of contact forces on the particle. This model was validated with gyratory compaction tests in the laboratory with granular materials extracted from three locations near Bogotá, Colombia. Good agreement between the results of the model and laboratory tests was achieved.; A fracture and abrasion model is used in order to understand and visualize the evolution of crushing resulting from compression and shear in unbound granular materials. Modelling was done using numerical simulation with the discrete element method in two dimensions in which each particle in the granular material is represented by a disc. The particle fragmentation criterion is defined on the bases of grain fracturing under diametric compression...

On the fabric tensor of polydisperse granular materials in two dimension

Madadi, Mahyar; Tsoungui, Olivier; Lätzel , Marc; Luding, S
Fonte: Pergamon Press Publicador: Pergamon Press
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
66.37%
The trace of the fabric tensor in static, isotropic, two-dimensional, frictionless, polydisperse granular materials is examined theoretically and numerically. In the monodisperse case, the trace of the fabric tensor equals the product of volume fraction a

Shear strength properties of wet granular materials

Richefeu, Vincent; Youssoufi, Moulay Saïd El; Radjaï, Farhang
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 14/04/2006
Relevância na Pesquisa
56.24%
We investigate shear strength properties of wet granular materials in the pendular state (i.e. the state where the liquid phase is discontinuous) as a function of water content. Sand and glass beads were wetted and tested in a direct shear cell and under various confining pressures. In parallel, we carried out three-dimensional molecular dynamics simulations by using an explicit equation expressing capillary force as a function of interparticle distance, water bridge volume and surface tension. We show that, due to the peculiar features of capillary interactions, the major influence of water content over the shear strength stems from the distribution of liquid bonds. This property results in shear strength saturation as a function of water content. We arrive at the same conclusion by a microscopic analysis of the shear strength. We propose a model that accounts for the capillary force, the granular texture and particle size polydispersity. We find fairly good agreement of the theoretical estimate of the shear strength with both experimental data and simulations. From numerical data, we analyze the connectivity and anisotropy of different classes of liquid bonds according to the sign and level of the normal force as well as the bond direction. We find that weak compressive bonds are almost isotropically distributed whereas strong compressive and tensile bonds have a pronounced anisotropy. The probability distribution function of normal forces is exponentially decreasing for strong compressive bonds...

Isostatic phase transition and instability in stiff granular materials

Moukarzel, Cristian F.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
56.3%
In this letter, structural rigidity concepts are used to understand the origin of instabilities in granular aggregates. It is shown that: a) The contact network of a noncohesive granular aggregate becomes exactly isostatic in the limit of large stiffness-to-load ratio. b) Isostaticity is responsible for the anomalously large susceptibility to perturbation of these systems, and c) The load-stress response function of granular materials is critical (power-law distributed) in the isostatic limit. Thus there is a phase transition in the limit of intinitely large stiffness, and the resulting isostatic phase is characterized by huge instability to perturbation.; Comment: RevTeX, 4 pages w/eps figures [psfig]. To appear in Phys. Rev. Lett

Sound propagation and force chains in granular materials

Owens, Eli T.; Daniels, Karen E.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
56.33%
Granular materials are inherently heterogeneous, leading to challenges in formulating accurate models of sound propagation. In order to quantify acoustic responses in space and time, we perform experiments in a photoelastic granular material in which the internal stress pattern (in the form of force chains) is visible. We utilize two complementary methods, high-speed imaging and piezoelectric transduction, to provide particle-scale measurements of both the amplitude and speed of an acoustic wave in the near-field regime. We observe that the wave amplitude is on average largest within particles experiencing the largest forces, particularly in those chains radiating away from the source, with the force-dependence of this amplitude in qualitative agreement with a simple Hertzian-like model of particle contact area. In addition, we are able to directly observe rare transient force chains formed by the opening and closing of contacts during propagation. The speed of the leading edge of the pulse is in quantitative agreement with predictions for one-dimensional chains, while the slower speed of the peak response suggests that it contains waves which have travelled over multiple paths even within just this near-field region. These effects highlight the importance of particle-scale behaviors in determining the acoustical properties of granular materials.

Thermodynamic Theory of Weakly Excited Granular Materials

Hayakawa, Hisao; Hong, Daniel C.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 07/03/1997
Relevância na Pesquisa
56.24%
We present a thermodynamic theory of weakly excited two-dimensional granular systems from the view point of elementary excitations of spinless Fermion systems. We introduce a global temperature T that is associated with the acceleration amplitude \Gamma in a vibrating bed. We show that the configurational statistics of weakly excited granular materials in a vibrating bed obey the Fermi statistics.; Comment: 12 pages, 1 figure, To Appear in Phys. Rev. Lett. April, 1997

Spreading of viscous fluids and granular materials on slopes

Takagi, Daisuke
Fonte: University of Cambridge; Department of Applied Mathematics and Theoretical Physics Publicador: University of Cambridge; Department of Applied Mathematics and Theoretical Physics
Tipo: Thesis; doctoral; PhD
EN
Relevância na Pesquisa
56.41%
Materials can flow down a slope in a wide range of geophysical and industrial contexts, including lava flows on volcanoes and thin films on coated surfaces. The aim of my research is to provide quantitative insight into these forms of motion and their dependence on effects of the topography, the volume and the rheology of the flowing structure. Numerous different problems are investigated through mathematical models, which are developed analytically and confirmed by laboratory experiments. The initial advance of long lava flows is studied by considering the flow of viscous fluid released on sloping channels. A scaling analysis, in agreement with analog experiments and field data, offers a practical tool for predicting the advance of lava flows and conducting hazard analysis. A simple and powerful theory predicts the structure of flows resulting from any time-dependent release of fluid down a slope. Results obtained by the method of characteristics reveal how the speed of the advancing front depends importantly on the rate of fluid supplied at an earlier time. Viscous flows on surfaces with different shapes are described by similarity solutions to address problems motivated by engineering as well as geophysical applications. Pouring viscous fluid out of a container can be a frustratingly slow process depending on the shape and the degree of tipping of the container. The discharge rate of the fluid is analysed in simple cases...

Flow and Jamming of Granular Materials in a Two-dimensional Hopper

Tang, Junyao
Fonte: Universidade Duke Publicador: Universidade Duke
Tipo: Dissertação
Publicado em //2012
Relevância na Pesquisa
66.34%

Flow in a hopper is both a fertile testing ground for understanding fundamental granular flow rheology and industrially highly relevant. Despite increasing research efforts in this area, a comprehensive physical theory is still lacking for both jamming and flow of granular materials in a hopper. In this work, I have designed a two dimensional (2D) hopper experiment using photoelastic particles ( particles' shape: disk or ellipse ), with the goal to build a bridge between macroscopic phenomenon of hopper flow and microscopic particle-scale dynamics. Through synchronized data of particle tracking and stress distributions in particles, I have shown differences between my data of the time-averaged velocity/stress profile of 2D hopper flow with previous theoretical predictions. I have also demonstrated the importance of a mechanical stable arch near the opening on controlling hopper flow rheology and suggested a heuristic phase diagram for the hopper flow/jamming transition. Another part of this thesis work is focused on studying the impact of particle shape of particles on hopper flow. By comparing particle-tracking and photoelastic data for ellipses and disks at the appropriate length scale, I have demonstrated an important role for the rotational freedom of elliptical particles in controlling flow rheology through particle tracking and stress analysis. This work has been supported by International Fine Particle Research Institute (IFPRI) .

; Dissertation

Nonlinear Dynamics and Network Properties in Granular Materials under Shear

Ren, Jie
Fonte: Universidade Duke Publicador: Universidade Duke
Tipo: Dissertação
Publicado em //2013
Relevância na Pesquisa
66.39%

Granular materials are hard to understand due to their discrete and a-thermal nature. The mechanical response of a granular packing under external deformations, although highly relevant in industrial processes, is still poorly understood, partly due to the difficulty to generate a homogeneous granular packing. In this thesis, I present a novel shear apparatus that avoids the formation of inhomogeneities known as shear bands. This apparatus provides quasi-static, quasi-uniform simple shear deformation to a 2D model granular system under fixed packing fraction &phi. The position, orientation and forces for each particle are obtained at each shear step, using the photo-elastic technique. This model granular system exhibits coupling between the shear strain, &gamma, and the pressure, P, which we characterize by the `Reynolds pressure', and a `Reynolds coefficient', R(&phi) = (&partial^2 P/ &partial &gamma^2)/2. Under cyclic shear, this system evolves logarithmically slowly towards limit cycle dynamics, which we characterize in terms of pressure relaxation at cycle n: &Delta P &simeq - &beta ln(n/n_0). &beta depends only on the shear cycle amplitude, suggesting an activated process where &beta plays a temperature-like role. In addition...

On Capillary Bridges in Wet Granular Materials

Kohonen, Mika; Gazeas, K; Scheel, Mario; Schier, Christof; Herminghaus, Stephan
Fonte: Elsevier Publicador: Elsevier
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
66.46%
The properties of wet granular materials are significantly influenced by the presence of microscopic liquid (capillary) bridges between the constituent granules. However, despite the progress which has been made in recent years the dependence of the mechanical properties of such materials on the amount and type of liquid present is still not completely understood. At a very basic level, a better understanding of the distribution of liquid between particle contacts and particle surfaces is required. Here we describe some of our observations on liquid bridge coordination numbers and liquid bridge volumes in model granular materials consisting of glass beads. We also describe experiments on the dynamic deformation of a wet granular material, which reveal a type of shear-thinning behavior that we attribute to the shear-rate dependence of liquid bridge volumes.