Granular Matter最新文献

This study investigates the impact of particle plasticity on the mechanical behaviour of a model granular system under plane stress conditions simulated with the Discrete Element Method. A contact model transitioning from nonlinear elasticity to linear plastic deformation is integrated to analyse its effects on a 576-ball-bearing assembly subjected to varying loads. Simulations were conducted using Yade, comparing them with experimental results and traditional elastic models. The findings show that incorporating plastic deformation improves the accuracy of simulated force distributions and the material’s frictional response, particularly under high external loads. These results underscore the need for plasticity-inclusive models in realistic granular simulations, providing valuable insights for practical applications in industries handling high-stress granular systems.

In this article, lentils are used as a case study to characterise the particulate behaviour of soft granular materials. Experiments were carried out on a single lentil particle or a pair of lentils. The single particle or a pair of particles in contact were compressed vertically to crushing or to a fixed vertical load. Then, in inter-particle tests, pairs of particles were slid over each other at a constant vertical load, and the tangential stiffness and coefficient of friction were estimated. The pairs of lentil particles in contact were also subjected to repeated normal and tangential loading. The presence or absence of the cover of lentil particles (shell) was found to affect their behaviour significantly under these loading conditions. The lentil particles have a very compliant shell and stiff core in normal loading; the stiffness of the shell is constant, and the core follows Hertz contact law. The lentil particles show less variability in their crushing strength, with high Weibull modulus (~ 10), in comparison to other natural granular materials like sand. With repeated cycles of vertical loading, the contact between a lentil pairs of particles becomes more stiff and less damp. In tangential loading, the coefficient of friction between lentil particles decreases with normal force while the contact stiffness increases. Further, in cyclic tangential loading, the coefficient of friction decreases and the contact stiffness increases with cycles. A simple contact model is also proposed to use in discrete element simulations.

Graphical abstract

The heterogeneity of a dense sand specimen in triaxial compression has been revealed in many different studies using tools such as x-ray computed tomography. It has been shown that a significant variation of the soil variables already exists at the initial state and that, if shear banding occurs, all variables localise inside the specimen. To resolve the discrepancy between such observations and the assumption of a homogeneous specimen, which is commonly made in the interpretation of triaxial tests, one could either extract the local soil behaviour rather than the global one or use the initial distribution of the variables as the initial state of a boundary value problem. For both purposes, the size of a representative elementary volume (REV) is determined regarding the void ratio, two contact fabric descriptors, the volumetric and deviatoric strain. The size of the REV is either determined for individual loading states or by considering the evolution of deforming elements throughout the triaxial test. At the final loading state, a REV size of 3.6 (d_{50}) is identified, which is also the size where the statistical distribution of the variables becomes independent of the element size. The same size is determined for the deforming elements and is therefore used to extract the soil behaviour from the evolving shear band. The local soil behaviour is found to be much simpler than the global one, which suggests that the complexity of the global behaviour mainly results from homogenising the highly different zones inside the specimen.

Graphical Abstract

Extraction of the soil behaviour inside the evolving shear band with the help of deforming representativeelementary volumes. The volumetric behaviour is represented by the evolution of the void ratio and the evolution ofthe contact fabric anisotropy is closely connected to the stress-strain behaviour. The soil behaviour on the REVscale might form the basis for an alternative approach for the development and calibration of constitutive modelsconsidering the heterogeneity of a soil specimen.

This investigation delves into the scaling laws governing pressure and key mean variables throughout the first and second jamming transitions previously observed in asymmetric bidisperse granular packings. Motivated by a theoretical model integrating crucial parameters—size ratio, (delta), concentration of small particles, (X_{mathrm{S}}), packing fraction, (phi), mean contact number, (langle Z rangle), mean overlap, (langle alpha ^{c}_{n} rangle), and mean branch vector length (langle ell ^{c}_{n} rangle)—we employ molecular dynamics simulations to validate the model. Our findings reveal a non-linear relationship between pressure and (phi) stemming from the dynamic interaction of mean variables with (phi) during compression. Regardless of (X_{mathrm{S}}) for δ = 0.73, the scaling exponent (c_{Z}) characterizing (langle Z rangle) with (phi) consistently approximates 0.5, holding true for δ = 0.73 and high (X_{mathrm{S}}) values. Intriguingly, for δ = 0.15 and low (X_{mathrm{S}}), where the two jamming transitions are observed, (c_{Z}) exhibits distinct values. At the first transition, where large particles jam, (c_{Z}) slightly exceeds 0.5, while it diminishes to approximately 0.3 at the second transition following the jamming of small particles. Additionally, the exponents associated with the scaling of (langle alpha ^{c}_{n} rangle) and (langle ell ^{c}_{n} rangle) with (phi) consistently converge around (c_{alpha } = c_{ell } sim 0.92) varying with changes in (X_{mathrm{S}}) and (delta). Moreover, the pressure model aligns seamlessly with simulation trends, exhibiting a consistent exponent around (c_{p} sim 1.1)–1.3 throughout the first and second jamming transitions. These results offer valuable insights into the compression behavior of highly asymmetric bidisperse packings, emphasizing the substantial influence of (delta) and (X_{mathrm{S}}) on the system’s macroscopic properties.

The air entrainment caused by the transportation, loading and unloading of industrial bulk materials is the main cause of dust diffusion. Local exhaust is the most effective means to control the diffusion of industrial pollutants. In order to study the flow field disturbance and control effect of the exhaust hood on the entrained air. First, a numerical model was established for the exhaust hood to control the air entrainment caused by the particle flow falling and hitting the wall. Secondly, the numerical model was verified using experimental data. Finally, the control of entrained air by the exhaust hood was analysed using a coupled CFD-DEM method, with the exhaust air velocity, the exhaust hood size and the exhaust hood position as variables. The results showed that the best effect on entrained air control was achieved when the exhaust air velocity was 7.5 m/s, the exhaust hood diameter was 125 mm, and the position of the exhaust hood was flush with the secondary wall impact point of the particles flow. The relative velocity recovery coefficient is pioneered to analyze the degree of influence of the three variables on the flow field of entrained air. It was found that the exhaust hood size has the greatest influence on the entrainment air velocity distribution, followed by the exhaust wind speed, and the least impact is the position of the exhaust hood.

Graphical Abstract

A granular damper consists in a container partially filled with solid particles that attenuate vibrations thanks to the multiple dissipative particle–particle collisions. These dampers have been investigated for decades because they are affordable, require low maintenance and can operate in harsh environments where viscous dampers fail. However, the nonlinear response of granular dampers, which includes chaotic dynamics and sharp transitions in loss factor, makes it difficult to predict their final performance when attached to the primary vibrating structure. To some extent, this has hampered widespread utilization of granular dampers in the industry. We show that a cone-in-cone design of such dampers can lead to an essentially linear response, which is compatible with a simple force law.

Cohesive granular materials are found in many natural and industrial environments, but experimental platforms for exploring the innate mechanical properties of these materials are often limited by the difficulty of adjusting cohesion strength. Granular particles levitated in an acoustic cavity form a model system to address this. Such particles self-assemble into free-floating, quasi-two-dimensional raft structures which are held together by acoustic scattering forces; the strength of this attraction can be changed simply by modifying the sound field. We investigate the mechanical properties of acoustically bound granular rafts using substrate-free micro-scale shear tests. We first demonstrate deformation of rafts of spheres and the dependence of this deformation on acoustic pressure. We then apply these methods to rafts composed of anisometric sand grains and smaller spheres, in which the smaller spheres have a thin layer of air separating them from other grain surfaces. These spheres act as soft, effectively frictionless particles that populate the interstices between the larger grains, which enables us to investigate the effect of lubricating the mixture in the presence of large-grain cohesion.

Graphical Abstract

Vibrofluidization in monodisperse granular materials is a hierarchical phenomenon involving different spatial and temporal behaviors, known to produce macroscopic structures with well-defined properties and high reproducibility. However, as witnessed by the paucity of relevant results in the literature, investigating the collective organization of particles across such different length and time scales becomes particularly challenging when multi-component systems are considered, i.e. if the considered vibrated material is not monodisperse. In this work, this problem is addressed through numerical simulation of the governing equations accounting for (dissipative) inelastic and frictional effects in the framework of a DEM (Discrete Element Method) method. Binary and ternary particle distributions are considered and, in order to filter out possible density-driven particle segregation or mixing mechanisms, particles are assumed to be iso-dense. The problem is initially analyzed through the coarse-grained lens of patterning behavior (supported by a Voronoi analysis for many representative cases) and then from a micromechanical level in which statistical data based on particle collisions and related dissipative effects are used to gain additional insights into the observed macroscopic trends. It is found that, starting from the initial traditional monodisperse case, the addition of particles with smaller sizes (while keeping the overall mass and depth of the considered layer almost unchanged) generally leads to a corrugation in the otherwise perfect symmetry of the original patterns, which is similar to that already seen in companion situations related to viscoelastic fluids. Moreover, while in the case of an initially hexagonal pattern, this topology is generally retained, in other situations, the initial perfection is taken over by less regular waveforms. Specific circumstances also exist where the initial square symmetry is lost in favor of a triangular symmetry. In all cases, segregation effects simply manifest as a preferential concentration of particles with larger size in an intermediate layer, which apparently behaves as a cohesive entity during each vibration cycle.

Using glass beads as an ideal material analogous to soil particles makes it feasible to explore the effects of particle interactions on the mechanical behavior of the material. In this study, 2 mm high-precision spherical glass beads were selected as the raw material, and three test samples with varying surface roughness were produced using sandblasting technology. After quantifying the surface roughness of the particles, samples were prepared, and a series of laboratory triaxial consolidation drainage tests were conducted to investigate the shear behavior of particle materials with varying roughness levels. This investigation explores the effects of variations in particle surface roughness on the stress–strain characteristics, shear strength, critical state, and stick–slip behavior of triaxial samples. The experimental results indicate that an increase in particle surface roughness significantly raises the peak deviatoric stress, and the stress–strain curves predominantly exhibit strain softening behavior. Additionally, the slope of the critical state line increases, and the stick–slip behavior becomes less pronounced. The variation trend of the roughness index is similar to peak friction angle (φ_max), peak deviatoric stress growth rate, slope (k) of the critical state line, and the maximum deviatoric stress drop (Δ_qmax) during stick–slip process.

Graphical Abstract