Search

The friction behavior of widespread clay minerals is a major concern in many geo-engineering problems, such as the stability of soft soil foundations and the induction of seismic fault zones. The present work aimed to study the friction behaviors of kaolinite at the particle level using the molecular dynamics method. The effects of normal force (F n), shear velocity (v), and interfacial water film on nanofriction were discussed. The "stick-slip" phenomenon and periodic evolution of friction forces (F f) were observed in dry friction and became less pronounced with water lubrication. The dry F f of kaolinite was found to be insensitive to F n. However, wet F f exhibited a linear increase with F n and then transitioned to a non-linear relationship as slip displacement increases due to the continuous loss of water molecules from the interface during friction. Notably, at high loads (F n ≥ 30 nN), the peak friction of wet kaolinite showed characteristics similar to dry friction. A velocity-strengthening behavior of kaolinite at high velocities was observed in both dry and wet conditions. The macroscale friction coefficients of kaolinite were predicted from nanofriction data and results showed good agreement with experimental values. This study lays the foundation for bridging micro- and macro-mechanical behaviors, suggesting a new pathway for acquiring precise macroscopic friction of minerals through cross-scale studies. • Interparticle friction behavior of kaolinite is investigated using MD simulations. • The effects of normal force, shear velocity, and water lubrication on friction behavior are discussed. • Kaolinite exhibits both velocity-strengthening and weakening friction. • Nanofriction coefficient of hydrated kaolinite is estimated at high speed. • Macroscopic friction coefficients are predicted from microscale friction. [ABSTRACT FROM AUTHOR]

The aim of this paper is to develop an experimental procedure to measure contact friction between granular particles. The contact friction is a micro-property needed in the micromechanical modeling of a granular medium. The proposed method can measure the interparticle friction of idealized spherical particles using the conventional direct shear apparatus in soil testing. In preparation for the test, the test specimen is made of four steel balls embedded halfway in a sulfaset paste plate positioned in a statically determinant configuration to provide point contacts among the steel balls. The upper half of the shear box contains one steel ball, which is supported by three steel balls in the lower shear box, ensuring contact points at all times during the test. Shear force and shear displacement are measured under a specific normal force during the test. An analytical equation is developed based on the geometrical configuration of the balls to calculate the interparticle friction angle. The test is shown to be repeatable, and the calculated interparticle friction angle agrees well with experimental measurements with a high degree of accuracy and consistency. [ABSTRACT FROM AUTHOR]

The present paper addresses the influence of shape, size, and frictional characteristics of granular materials by utilizing X-ray computed tomography (CT) imaging and FEM. High-fidelity triaxial shearing simulations are conducted on a realistic assembly of sand grains. Applicable boundary conditions are represented, including the latex membrane for applying confining pressure. For this research, two poorly graded clean sands (SP) with distinct grain morphologies of round and angular particle shapes are considered. The effect of the particle size is naturally embedded in two materials with a difference of small size fraction grains present in angular sand. The deviatoric stress and volume change response increase up to the value of friction coefficient (μ) equal to 0.5 for rounded sand, and the strength-deformation response is unaffected by a further increase in friction. The axial strain, corresponding to peak deviatoric stress, did not depend on the friction value for identical initial microstructure and boundary conditions. The microscale investigation suggests a similar mean coordination number and percentage of grains with a coordination number less than two for friction values producing similar responses. The angular sand resulted in higher strength and low dilation compared to the rounded sand for friction value that best reproduces experimental results. The strength increase due to the particle shape effect becomes less pronounced for smooth grains. The grain-scale analysis indicates angular sand exhibits less dilation in contrast to the general observation in the literature due to smaller grain fractions absent in rounded sand, highlighting the need to introduce additional subclassifiers in classifying SP sands. The localized deformation pattern remains unchanged with friction and varies with grain shape. An attempt is made to link micromechanical insights to the macroscale response. [ABSTRACT FROM AUTHOR]

The evolutions of meso structures of granular materials play important roles during shearing toward the critical state. This study aims to establish an extended analytical model based on a specific upscaling procedure considering the distributions of meso structures in granular materials. A general Gaussian distribution is applied when determining the relationship between meso critical state parameters and order n, and then a quantitative model of macro CSL is developed by taking the discrete case into a continuous function. The quantitative model is then related to interparticle friction μ and contact stiffness k for predicting the macro CSLs for ideal granular assemblies, and the analytical results have good agreement with DEM results. Furthermore, the influence patterns of contact properties on both meso and macro critical states are discussed in detail, which provides a roughly explanation on the variations of critical friction angle φcs with μ. [ABSTRACT FROM AUTHOR]

The aim of this paper is to develop an experimental procedure to measure contact friction between granular particles. The contact friction is a micro-property needed in the micromechanical modeling of a granular medium. The proposed method can measure the interparticle friction of idealized spherical particles using the conventional direct shear apparatus in soil testing. In preparation for the test, the test specimen is made of four steel balls embedded halfway in a sulfaset paste plate positioned in a statically determinant configuration to provide point contacts among the steel balls. The upper half of the shear box contains one steel ball, which is supported by three steel balls in the lower shear box, ensuring contact points at all times during the test. Shear force and shear displacement are measured under a specific normal force during the test. An analytical equation is developed based on the geometrical configuration of the balls to calculate the interparticle friction angle. The test is shown to be repeatable, and the calculated interparticle friction angle agrees well with experimental measurements with a high degree of accuracy and consistency.

We investigated experimentally the tribological behavior of tailing grain contacts using a micromechanical apparatus which allows high precision of force and displacement measurements to derive contact stiffness. A technique was developed to apply biopolymer coating to the grain surfaces and the emphasis of the study was placed on the investigation of the influences of saturation conditions, the presence of polymer-based coating, and the abrasion on the frictional behavior of the grains. Material characterization was based on interferometry, micro-indentation and elemental composition analyses. Elastoplastic displacements dominated the first cycles of normal loading and the Young’s modulus was interpreted based on different contact models. The tailing grains showed significantly higher inter-particle friction compared with that of quartz grains. Three major characteristics which influenced the frictional behavior of the grain contacts were the abrasion, which was more dominant in the first loading cycles, the high roughness of the grains and the presence of the biopolymer coating which increased significantly the friction. [ABSTRACT FROM AUTHOR]

We developed a 2D discrete element model to simulate shearing response of elongated round and angular particles. Model was calibrated and validated by comparing numerical results with corresponding biaxial shearing tests on circular (round) and hexagonal (angular) rods at microscopic and macroscopic scales. Then, the systematic effect of particles aspect ratio was investigated on quasi-static shearing response of round and angular granular materials. Macroscopically, we observed a nonlinear tendency wherein as the aspect ratio decreased from 1, the critical state stress ratio initially increased, reaching a maximum, followed by a decreasing trend. This effect was more prominent in round samples. Microscopically, decreasing the aspect ratio from 1 reduced particle rotations and increased the mean coordination number. Elongated particles exhibited significant contact anisotropy, forming irregular force chains, facilitating interparticle sliding, and reducing overall strength. Additionally, we explored the impact of the interparticle friction coefficient. A high value of interparticle friction coefficient led to a monotonically increasing strength with elongation, underscoring the importance of accurately calibrating microscopic friction coefficients. [ABSTRACT FROM AUTHOR]

In this study, the micromechanical interparticle contact behavior of "De NoArtri" (DNA‐1A) grains is investigated, which is a lunar regolith simulant, using a custom‐built micromechanical loading apparatus, and the results on the DNA‐1A are compared with Ottawa sand which is a standard quartz soil. Material characterization is performed through several techniques. Based on microhardness intender and surface profiler analyses, it was found that the DNA‐1A grains had lower values of hardness and higher values of surface roughness compared to Ottawa sand grains. In normal contact micromechanical tests, the results showed that the DNA‐1A had softer behavior compared with Ottawa sand grains and that cumulative plastic displacements were observed for the DNA‐1A simulant during cyclic compression, whereas for Ottawa sand grains elastic displacements were dominant in the cyclic sequences. In tangential contact micromechanical tests, it was shown that the interparticle friction values of DNA‐1A were much greater than that of Ottawa sand grains, which was attributed to the softer contact response and greater roughness of the DNA‐1A grains. Widely used theoretical models both in normal and tangential directions were fitted to the experimental data to obtain representative parameters, which can be useful as input in numerical analyses which use the discrete element method. Plain Language Summary: Lunar regolith simulants comprise natural soils found on Earth or artificially created materials which mimic the properties of the real lunar surface soil. Understanding the behavior of these simulants can help researchers to prepare for further explorations and settling of facilities on the Moon. In this study, an attempt is made to examine in the laboratory the behavior of the lunar regolith simulant "De NoArtri" (DNA‐1A), and the results are compared with a standard soil of quartz grains to understand the differences and obtain insights into the properties of the lunar simulant. The behavior of regolith simulant is compared with Ottawa sand grains to understand the differences between these two materials in terms of material properties as well as micromechanical behavior. This micromechanical behavior gives a fundamental understanding of the mechanical response of the material and can provide important parameters to be further utilized in computer simulations so that settling of facilities on the Moon surface can be designed safely. Key Points: The interface properties of DNA‐1A lunar regolith simulant and Ottawa sand were studied with an advanced grain‐scale apparatusDNA‐1A had much lower normal contact stiffness but higher interparticle friction angles compared with Ottawa sandAnalytical model by Yimsiri and Soga (2000, https://doi.org/10.1680/geot.2000.50.5.559) in the normal direction fits well the experimental curves for the entire span of displacements [ABSTRACT FROM AUTHOR]

[Display omitted] • A method for constructing a breakable particle model with realistic shape. • Particle breakage under different interparticle friction in drained triaxial test. • Macro and micro mechanical behavior under different interparticle friction. • The mechanism of interparticle friction on the critical friction angle. Interparticle friction coefficient μ is an important factor affecting the mechanical properties of granular materials. In this study, the influence of μ on the mechanical behavior of granular materials is investigated by DEM. Through 3D scanning and Voronoi tessellations, the breakable particle model with realistic shape is constructed. A series of drained triaxial tests were performed at different μ values. Then, the particle breakage characteristics, including the degree of particle breakage and the breakage mode, were evaluated. In addition, microscopic characteristics, including coordination numbers and contact forces, were examined. Finally, the underlying mechanisms of φ c evolution are explained from two perspectives of particle movement and fabric anisotropy. First, the rolling behavior of the particles increases with μ values, leading to a larger average displacement of particles. The influence of particle movement and friction coefficient balance each other, resulting in φ c remaining constant when μ ⩾ 0.4. Second, the initial increase of φ c is due to the increase of the contact normal anisotropy a c , the normal contact force anisotropy a n and the tangential contact force anisotropy a t. Subsequently, the constant φ c is the result of the decrease of a c and a n being compensated by the increase of a t. [ABSTRACT FROM AUTHOR]

The method of calculation of slip plane angles in compacting dispersed materials for combined action of interparticulate and external (wall) friction with method of Mohr stresses is developed. The variation range for this angles in dependence at relationship for interparticulate and external (wall) friction is determinate.

In the present study, ejecta dynamics techniques are used to investigate the ballistic response of shear thickening particle suspensions as a means of assessing the particle hardness and the role of interparticle friction during penetration. Through particle material variations, the role of particle material strength is discussed primarily through the ratio of the total lateral to total axial kinetic energy of the ejecta field at increasing impact velocities. Two dominant trends are observed in the relation between this ratio of kinetic energy and impact velocity, which are attributed to the properties of the suspended particles. A qualitative model of particle fracture and deformation is proposed to account for the experimental observations. The results of analytical particle strain estimates and computational discrete element modelling of impact ejecta are used to inform the model and discuss the role of interparticle friction in the ejecta field. [ABSTRACT FROM AUTHOR]

This paper presents an experimental study on the shear behavior of granular materials, focusing on the effects of interparticle friction and particle size, which are of fundamental importance but are not yet well understood. The experimental program consisted of a large number of direct shear tests on glass beads of varying sizes and interparticle friction conditions, performed under a range of packing densities and normal stress levels. Test data were interpreted in terms of the stress-dilatancy relationship and shear strength parameters. The study indicates that under otherwise similar testing conditions, oil-lubricated glass beads tend to have substantially lower shear strength as compared with water-lubricated, water-flooded, and dry glass beads. It has also been found that at similar particle size uniformity, increasing mean particle size (d50) leads to more dilatant shear response and higher shear strength. A generalized stress-dilatancy relation is proposed, which introduces a variable dilatancy coefficient that reflects on the effects of interparticle friction and particle size. It is shown that classical stress-dilatancy relations can be regarded as special cases of this generalized case, with the dilatancy coefficient being taken as a constant. Further explanations for the observed effects on macroscopic behavior are provided from the micromechanics perspectives. [ABSTRACT FROM AUTHOR]

The effect of the internal (interparticle) and external friction on the limiting state generation has been analyzed in terms of the shear-induced failure criterion of the surface layers in the compacted disperse materials. The proposed design scheme based on the Mohr-Coulomb combined diagram has made it possible to develop an original method and derive analytic dependences for determining the normal and tangential stresses in the interlayer shear sites along with the orientation angles of these sites. It has been shown that, with increasing coefficient of external friction, the pressure in the interlayer shear sites reduces until tensile stresses are generated. This is accompanied by the nucleation of the crack at a right angle close to the side surface perpendicular of the formed item. [ABSTRACT FROM AUTHOR]

SUMMARY Understanding the extent to which discrete element method (DEM) simulations can capture the critical state characteristics of granular materials is important to legitimize the use of DEM in geomechanics. This paper documents a DEM study that considered the sensitivity of the critical state response characteristics to the coefficient of interparticle friction ( μ) using samples with gradings that are representative of a real soil. Most of the features that are typically associated with sand behaviour at the critical state were seen to emerge from the DEM simulation data. An important deviation occurs when high μ values ( μ ≥ 0.5) are used, as has been the case in a number of prior DEM studies. While there is a systematic variation in the critical state behaviour with μ for μ < 0.5, when μ ≥ 0.5, the behaviour at the critical state seems to be insensitive to further increases in μ. In contrast to observations of conventional soil response, when μ ≥ 0.5, the void ratio at the critical state initially increases with increasing mean effective stress ( p′). Analysis of the DEM data and use of simple models of isolated force chains enabled some key observations. When 'floating' particles that do not transmit stress are eliminated from the void ratio calculation, the void ratio at the critical state decreases consistently with increasing p′. There is a transition from sliding to rolling behaviour at the contact points as μ increases. Beyond a limiting value of μ, further increases in μ do not increase the buckling resistance of individual strong force chains. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

This study presents the influence of the interparticle friction angle on the cyclic behavior of granular materials using the two-dimensional (2D) discrete-element method (DEM). The numerical sample was modeled with oval-shaped particles, whereas the isotropically compressed dense sample was prepared from the initial sparse sample using periodic boundaries. Biaxial cyclic shear tests were simulated with different interparticle friction angles. It was noted that the width of the stress-strain cyclic loops becomes thin when the interparticle friction angle increases. It was also noted that the induced fabric anisotropy is more pronounced during unloading than loading. Moreover, a strong correlation between macro- and microquantities was observed for strong contacts during cyclic loading. [ABSTRACT FROM AUTHOR]

Recent decades have witnessed considerable research effort in the development and application of the discrete element method (DEM) for modeling granular materials. One key aspect that appears to have not been addressed adequately is the choice or determination of the DEM model parameters to produce satisfactory quantitative predictions. In many DEM studies, the input parameters were often not measured and the values were sometimes assumed without any proper justification. This article describes a careful validation study in which DEM was used to model confined compression of spherical (glass beads) and nonspherical (corn grains) particles. The key properties of the particles were independently measured. The experimental data on the bulk compressibility and the corresponding load transfer to the cylindrical wall and the bottom platen provide quantitative information for direct comparison with the DEM results. Initial particle packing structure is expected to influence the bulk behavior of the assembly, so several methods of particle generation were used to investigate the sensitivity of particle generation in DEM. An investigation of the influence of interparticle friction and particle stiffness in DEM simulations provided an insight into the bulk response of the granular assemblies under confined compression. Comparison between the simulations and the experiments revealed several useful observations, including the roles of these model parameters in the predicted bulk responses. [ABSTRACT FROM AUTHOR]

Abstract: Three different methods of attaching polymers to sand are compared: in situ graft polymerization, chemical grafting of preformed polymers, and admicellar polymerization which uses a surfactant template. Sample characterization includes Raman spectroscopy, gravimetric analysis, contact angle measurements, and high solids rheology tests. In all cases, the presence of polymer on the surface was confirmed, but differences in terms of amount of polymer adsorbed, surface chemistry, and interparticle friction were found. The effect of polymer molecular weight for the chemically grafted material was studied as well. An optimum molecular weight around 600,000g/mol was found for maximum friction reduction in preformed poly(vinylpyrrolidone)-modified sand samples. [Copyright &y& Elsevier]

A microdynamic study of the particle flow in a vertical axis mixer with slowly rotating flat blades has been performed by means of a modified discrete element method. The conditions are comparable to recent experiments conducted using positron emission particle tracking, with a mixer being 249 mm in diameter, filled by 16,000 monosized spheres of 5 mm diameter, and two blades rotating at a speed of 19 rpm. The dependence of flow behaviour on particle–particle and particle–wall sliding and rolling frictions is quantified and the results are used to establish the spatial and statistical distributions of microdynamic variables related to flow and force structures such as velocity, porosity, coordination number, particle–particle and particle–wall interaction forces. While the geometry and operational conditions are relatively simple, the particle flow is shown to be very complicated. There is a three-dimensional zone in front of a blade where particles have a strong recirculating flow. Increasing sliding friction coefficient or decreasing rolling friction coefficient can promote the formation of this zone. The flow and force structures of particles in the mixer are not uniform, although macroscopically steady flow is reached readily. The results show that increasing the rolling friction coefficient and, in particular, the sliding friction coefficient can increase the bed porosity and decrease the mean coordination number. The recirculating flow and the mixing kinetics are promoted by increasing the sliding friction coefficient or decreasing the rolling friction coefficient. Furthermore force arching is strong in the particle bed, with large inter-particle forces concentrating near the bottom corner just in front of the blade and propagating into the bed. Increasing the sliding or rolling friction coefficient increases the potential energy of particles in the mixer, but the kinetic energy is not sensitive to these coefficients. The increased potential energy gives increased particle–particle and particle–wall interaction forces and hence an increased torque required to drive the system. The results highlight the capacity and usefulness of numerical simulation in developing an understanding of the interplay of structure, forces, velocities and mixing in granular systems. [Copyright &y& Elsevier]

Wet agglomerates deform plastically until they break through crack propagation. On the particulate level, liquid bridges are responsible for the strength of the wet agglomerate as they hold the particles together. Recent micro-scale studies have identified the role of liquid surface tension, bridge Laplace pressure and liquid viscosity, which, in combination, explain the axial strength of pendular liquid bridges. Different situations exist depending on the degree the liquid wets the particles and on the saturation of the agglomerate mass.On the wet agglomerate level, the hardness is related to three factors: the liquid binder surface tension and viscosity and the interparticle friction. A simple model is developed in this paper, based on the powder and liquid binder properties, which shows that the forces due to interparticle friction are generally predominant in wet agglomerates made from non-spherical particles. Although mechanical interlocking is not accounted for, the model yields accurate prediction of wet agglomerate hardness independently measured on wet masses of varying composition. This theoretical hardness could prove an interesting tool for wet granulation research and technology. [Copyright &y& Elsevier]

The successful production of P/M (powder metallurgy) components depends to a large extent on the tooling used for powder compaction. While designing the tool, the complexities arise from the interaction of the parameters such as powder characteristics, expected green density, the size and geometry of the product, and to whom the properties of the tool materials during compaction should be addressed. Floating type of compaction tooling set (die, punches, and core rod) was designed and fabricated. Pure electrolytic copper powder was compacted in the above-mentioned tool to obtain P/M copper bushes. Compaction pressure–density relationship and their influence on green strength are analyzed. [ABSTRACT FROM AUTHOR]

In this study, the coupled influence of contact time and rate of loading on the interface behavior of quartz grains was examined experimentally. The rate of loading is considered not just a tangential (or sliding) rate but a combination of both normal and tangential loading rates, which leads to the target normal load and tangential displacement for a given contact time. Two classes of tests were conducted; in the first class, steady-state sliding was first allowed to be reached by shearing the grains at a constant normal load before the application of the simultaneous normal load and tangential displacement variation, whereas in the second class, the normal load and tangential displacement were varied simultaneously from the start of the test. The experimental results from monotonic and cyclic micromechanical tests highlight the significant influence of the coupled load rate and contact time on the interparticle friction, tangential stiffness, and energy loss of the contacted interfaces. The results are compared with standard micromechanical experiments that were conducted under a constant normal load during the shearing of the grain contacts. Indirect qualitative analysis was performed to substantiate the increase in contact area with increasing contact time, which is not expected in constant normal load sliding. [ABSTRACT FROM AUTHOR]

This letter examines the effect of interparticle friction (μ) on the critical state friction angle (φ cs) of granular materials via the discrete element method. A series of drained triaxial tests are conducted on multisphere ellipsoids. The results indicate that φ cs first increases with μ and then reaches a plateau when μ ≥ 0.3. Anisotropic analysis provides new insights into the underlying mechanisms of the φ cs evolution from two perspectives. First, the initial increase of φ cs results from the increases of contact normal anisotropy a c , normal contact force anisotropy a n and tangential contact force anisotropy a t ; subsequently, the saturation of φ cs is a consequence of the decrease of a n being compensated by the increase of a t. Second, the relationship between the stress ratio q / p and the contact normal anisotropy within the strong and nonsliding contacts a c s n is nearly linear for ellipsoids (i.e., q / p = k a c s n ). Accordingly, the initial increase of φ cs is related to the increase of k and a c s n at low μ. The plateau of φ cs at high μ can be attributed to the decrease of a c s n being compensated by the increase of k. [ABSTRACT FROM AUTHOR]

Abstract: In comminution modeling, breakage rate (selection) functions are generally influenced more by the comminution machine than are breakage distribution functions. Linear grinding kinetics can be expressed either in terms of grinding time or specific energy consumption. Nonlinearities in comminution are caused by energy transfer mechanisms in the comminution machine whereby coarser particles might be ground preferentially or are protected by fines, by energy dissipation through interparticle friction in compressed bed comminution, and sometimes from heterogeneities produced in the feed particles. This paper discusses modifications of breakage rate functions for a number of situations and compares simulated and experimental results. [Copyright &y& Elsevier]