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In this study, the effect of particle size and shape parameters on shear strength of sand–rubber mixture was investigated using repose angle, as an analogy to the constant volume response, maximum dilatancy, and peak internal friction angles for various mixture conditions. For this purpose, four different granulated rubber and sand sizes were used to include the particle size effect. The size ratio of rubber and sand was maintained as unity in the mixtures. The influence of particle shape was reflected using three different shape classes for sand and two different particle forms for rubber granules. The shape properties of particles were quantified using overall regularity parameter with image processing. The percentage of rubber in the mixtures was varied at five different rates by weight of total sample. As a result, it was observed that the soft-rubber inclusion affected the shear strength parameters distinctively. The geometrical properties of sand and rubber particles have leading roles in the evolution of shear strength. [ABSTRACT FROM AUTHOR]

The energy consumption of particle breakage is added to the Cambridge energy balance equation so that the energy balance equation for coarse-grained soil is obtained. To reasonably measure the energy consumption of particle breakage, a function of friction coefficient relating to the axial strain is proposed to replace the friction coefficient as a constant in the energy balance equation based on the evolution rule of particle breakage. Then, according to the energy balance equation of coarse-grained soil, the energy consumption of particle breakage is calculated, and the particle breakage energy increases with axial strain increasing, which satisfies the thermodynamic law. Based on this energy balance equation, a simple stress dilatancy relationship is developed. In this stress dilatancy relationship, the relationship between d E b / p d ε s and M / M f can be described by a simple function with acceptable accuracy. This stress dilatancy relationship is validated with the satisfactory capability to predict the dilatancy behavior of coarse-grained soils, which can be the effective choice to build the constitutive model. [ABSTRACT FROM AUTHOR]

A granular gripper is a device used to hold objects by taking advantage of the phenomenon of Reynold's dilatancy. A membrane containing a granular sample is allowed to deform around the object to be held and then vacuum is used to jam the granular material inside the membrane. This allows to hold the object against external forces since deformation of the granular material is prevented by not allowing the system to increase its volume. The maximum holding force supported by the gripper depends on a number of variables. In this work, we show that in the regime of frictional holding (where the gripper does not interlock with the object), the maximum holding force as a function of the penetration of the object in the gripper does not depend on the granular material used to fill the membrane. Results for a variety of granular materials can be collapsed into a single curve if maximum holding force is plotted against the penetration depth achieved. The results suggest that a robotic arm capable of sensing the penetration depth can use this master curve to estimate the maximum holding force at each gripping operation. [ABSTRACT FROM AUTHOR]

This study presents a rate-independent constitutive model for granular materials which is based on the analysis of discrete element modelling (DEM) results. We examine the elastic and plastic strain rate at any state of shear, and explore how the plastic strain rate and the stress rate are related to some state variables (e.g. stress, void ratio, etc.). The elastic and plastic strain rate are evaluated by assuming that the rate of conservative energy defined in term of elastic strain rate equals the energy rate from work–energy analysis of granular assemblies. The verification of the theoretical work–energy analysis and the acquire of data regarding the mechanical rates at contacts between grains are done through DEM. It is shown that for a representative element volume of granular materials, the external work rate equals the sum of an elastic energy rate and a dissipation rate at contacts. The work rate is either the product of stress and strain rate or the sum of discrete work rate of external forces. Equations regarding the dilatancy, plastic deviatoric strain rate and stress rate are proposed. In the model, a yield surface is not explicitly defined and the deviatoric stress rate is related to not only the elastic deviatoric strain rate, but also a rate of dilatancy. Simulations show that the predictions match both virtual and Toyoura sand satisfactorily in various shear tests within a board range of densities. Additionally, the calibration of parameters is extremely simple. The treatment of cyclic loading condition is also presented. [ABSTRACT FROM AUTHOR]

This paper aims at studying the shear behavior of glass beads samples through experimental triaxial tests as regards the case of 4 mm diameter glass beads. Repeated tests are systematically carried out, together with the evaluation of the repeatability. Study of numerous experimental conditions is performed: compactness state (high/medium density), variable effective confining pressure ( $$p'_0$$ = 50-200 kPa), saturation state (saturated/dry) and surface state (rough/smooth). Specifically, the influence of these parameters on the stick-slip phenomenon is reported. Equivalent study of perturbation on the mechanical response by larger size spherical inclusions is also performed. The experimental findings received from drained triaxial tests classically throw light on the shear behavior of granular media directly dependent on compactness, in addition to the application of effective confining pressure, with a more dilatant behavior with the decrease of the confining pressure. The Rowe's law is truly verified in experiments. The modification of the surface state of glass beads results into observably different responses on both the deviatoric and the volumetric aspects. It is quite evident that the intensity of stick-slip phenomenon experienced increase with the application of confining pressure and depends on the surface roughness. No noteworthy impact on the mechanical response is taken into observation between the states of entire saturation and dry. Calculation of friction and dilatancy angles are made as well as reported in respect of different experimental configurations. [ABSTRACT FROM AUTHOR]

This paper presents the effect of particle morphology (grain shape) on the mechanical response of granular materials. Two model systems with extreme differences in morphology were selected (spherical glass ballotini and angular sand) for this experimental programme. A series of hollow cylinder torsion tests were conducted in this programme under monotonic drained conditions on specimens reconstituted to the same relative density. Tests were conducted under different intermediate principal stress ratio ( b) on both the model materials. The glass ballotini shows increased dilation at the outset of the test, however, at large strains, the particle rearrangement in the sand and the increased interlocking leads to higher strength at the critical state. The effect of individual particle morphology is manifested in both the increased friction angle and a larger sized failure locus in stress space with increase in angularity. The stresses developed in these two model materials are also accompanied by intriguing volume change behaviour. The glass ballotini despite a lower strength presents a predominantly dilative response immaterial of the ' b' value, while the angular sand shows increased strength at large strains, while showing a contractive response. These results allow incorporation of particle morphology effects at the ensemble level in plasticity based constitutive models. [ABSTRACT FROM AUTHOR]

Soft, disordered, micro-structured materials are ubiquitous in nature and industry, and are different from ordinary fluids or solids, with unusual, interesting static and flow properties. The transition from fluid to solid-at the so-called jamming density-features a multitude of complex mechanisms, but there is no unified theoretical framework that explains them all. In this study, a simple yet quantitative and predictive model is presented, which allows for a changing jamming density, encompassing the memory of the deformation history and explaining a multitude of phenomena at and around jamming. The jamming density, now introduced as a new state-variable, changes due to the deformation history and relates the system's macroscopic response to its micro-structure. The packing efficiency can increase logarithmically slow under gentle repeated (isotropic) compression, leading to an increase of the jamming density. In contrast, shear deformations cause anisotropy, changing the packing efficiency exponentially fast with either dilatancy or compactancy as result. The memory of the system near jamming can be explained by a micro-statistical model that involves a multiscale, fractal energy landscape and links the microscopic particle picture to the macroscopic continuum description, providing a unified explanation for the qualitatively different flow-behavior for different deformation modes. To complement our work, a recipe to extract the history-dependent jamming density from experimentally accessible data is proposed, and alternative state-variables are compared. The proposed simple macroscopic constitutive model is calibrated from particles simulation data, with the variable jamming density-resembling the memory of microstructure-as essential novel ingredient. This approach can help understanding predicting and mitigating failure of structures or geophysical hazards, and will bring forward industrial process design and optimization, and help solving scientific challenges in fundamental research. [ABSTRACT FROM AUTHOR]

Shearing stresses can change the volume of a material via a nonlinear effect known as shear dilatancy. We calculate the elastic dilatancy coefficient of soft sphere packings and random spring networks, two canonical models of marginal solids close to their unjamming transition. We predict a dramatic enhancement of dilatancy near rigidity loss in both materials, with a surprising distinction: while packings expand under shear, networks contract. We show that contraction in networks is due to the destabilizing influence of increasing hydrostatic or uniaxial loads, which is counteracted in packings by the formation of new contacts. [ABSTRACT FROM AUTHOR]

Drained triaxial axisymmetric compression tests are performed on water-saturated short cylindrical samples of nearly monodisperse glass beads, initially assembled in a loose state by a moist tamping technique. Both deviator stress $$q$$ and volumetric strain $$\epsilon _v$$, measured as functions of axial strain $$\epsilon _a$$, for different strain rates, are affected by stick-slip events of very large amplitude, while the classical behavior of loose, contractant granular assemblies, approaching the critical state for large $$\epsilon _a$$, corresponds to the upper envelop of the stress-strain behaviour. Those events consist in $$(i)$$ a very fast ( slip) part in which a drop of $$q$$ coincides with a jump of $$\epsilon _v$$ (contraction), while loss of control of $$\epsilon _a$$ and generation of pore pressure signal a dynamic collapse of the material structure triggered by an instability; and then $$(ii)$$ a quasi-static ( stick) part in which the sample regains its strength and, over a short strain interval, behaves similarly to a denser system that dilates before reaching its critical state. A unique stress-dilatancy relation applies to all stick-slip events. Apparent internal friction angles and effects of strain rate and confining pressure are discussed, and it is argued that stick-slip instabilities originate in physico-chemical aging phenomena coupled to contact mechanics. [ABSTRACT FROM AUTHOR]

Mechanical behavior of granular soils is a classic research realm but still yet not completely understood as it can be influenced by a large number of factors, including confining pressure, soil density, loading conditions, and anisotropy of soil etc. Traditionally granular materials are macroscopically regarded as continua and their particulate and discrete nature has not been thoroughly considered although many researches indicate the macro mechanical behavior closely depends on the micro-scale characteristics of particles. This paper presents a DEM (discrete element method)-based micromechanical investigation of inter-particle friction effects on the behavior of granular materials. In this study, biaxial DEM simulations are carried out under both 'drained' and 'undrained' (constant volume) conditions. The numerical experiments employ samples having similar initial isotropic fabric and density, and the same confining pressure, but with different inter-particle friction coefficient. Test results show that the inter-particle friction has a substantial effect on the stress-strain curve, peak strength and dilatancy characteristics of the granular assembly. Clearly, it is noted that apart from the inter-particle friction, the shear resistance is also contributed to the dilation and the particle packing and arrangements. The corresponding microstructure evolutions and variations in contact properties in the particulate level are also elaborated, to interpret the origin of the different macro-scale response due to variations in the inter-particle friction. [ABSTRACT FROM AUTHOR]

We present different aspects of dense granular flows in a Couette geometry using a variety of particulate materials with shape and size distributions. Tracer studies point to an apparent coupling of particle size with flow and stress field gradients. While there is a clear industrial motivation to use 'real' materials as a means to expand basic physical and engineering research in granular dynamics, the current study suggests additional academic motivations. Indeed, particles with distributed characteristics uncover rich interactions between flow and stress fields that might otherwise go un-noticed with model materials such as spherical glass beads. Distribution of size and shape play a strong role in how stress is transmitted in granular media (Kheiripour Langroudi et al. in Powder Technol 203:23-32, ) and how particle pattern arrangements evolve. Direct solid fraction measurements, using a capacitance probe, show that dense particle flows exhibit significant variations in solid fraction in both sheared and stagnant layers. Furthermore, these measurements also show different dependence of the solid fraction on shearing rate: solid fraction decreases in sheared layers and increases in stagnant layers as the shear rate is increased. From these results the thickness of the shear band could be estimated and was found to vary as a function of particle shape and the roughness of the container walls. The main result is that shear stress (or torque) (see also Kheiripour Langroudi et al. in Powder Technol 197:91-101, ) and solid fraction profiles depend on particle shape and whether or not an extra degree of freedom in their movement is provided so that the system can dilate under various shear states in the Couette cell. This extra degree of freedom is assured in the present experimental work by allowing a slight axial outflow from the Couette device while the driven shear fields are in the radial and tangential directions. [ABSTRACT FROM AUTHOR]

Non-coaxiality occurs when the directions of the principal plastic strain increments and the principal stresses deviate. Extensive experimental data have now conclusively shown that plastic flow in granular soils is non-coaxial particularly during loadings involving rotation of the principal stress directions. One way to integrate the effects of non-coaxiality is by modifying the expressions for energy dissipation and stress-dilatancy used in modeling plastic deformation of granular soils. In this regard, the paper’s main objective is to derive a non-coaxial version of Rowe’s stress-dilatancy relation, thereby making it more general and applicable to loadings involving principal stress rotation. The paper also applies Rowe’s non-coaxial stress-dilatancy equation in the determination of the effects of principal stress rotation in granular soils during simple shear loading conditions. Previous experimental data from simple shear tests on sand are used to validate the proposed non-coaxial version of Rowe’s stress-dilatancy relation. [ABSTRACT FROM AUTHOR]

The resistance to shear deformation developed by a granular material layer in contact with a topographically rough natural or manufactured solid material surface is critical to the stability of a variety of composite systems. By using discrete-element method numerical simulations, we show that evolution of fabric and contact force anisotropy at the boundary between the surface and the granular media controls shear behavior. Full mobilization of granular material strength occurs when the contact force anisotropy developed at the interface is equal to the maximum contact force anisotropy of the granular media. [ABSTRACT FROM AUTHOR]