Granular Matter最新文献

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

This work examines how crushable sand responds to the different quasi-static constant rates of pile penetration. A breakage mechanics-based viscoplastic constitutive model simulates plane strain pile driving in sand, focusing on how the penetration rate affects particle crushing at the pile tip. Finite element modelling (FEM) is used to simulate the pile granular media interaction in 2D. The model, which links the macro and micro aspects of granular media, predicts the behaviour of particle crushing and material strength at different rates of pile penetration. Input parameters are calibrated based on experimental sand samples. The results show that piles driven at higher rates have greater strength and less particle breakage. In contrast, piles with slower penetration rates show more breakage and reduced strength, with stress and breakage accumulating most at the pile tip corners. Also, the impact of the penetration rate on shear resistance force is more evident along the pile length, but it is reduced at the ends because of crushing-induced particle rearrangement and resultant loss of contacts. This study provides important insights into the behaviour of granular media in geotechnical applications like pile driving, highlighting how different penetration rates can influence crushable granular media response.

The discrete element method (DEM) is widely used to investigate the micromechanical behavior of granular materials. The accuracy of numerical modelling using this method depends greatly on the correct selection of the components of the rheological model. On the other hand, the rheological model is affected by the geometric shape, movement, and constituent particular materials. In this study, the stored and dissipated energy variations in the granular media in linear and non-linear contact models, as well as the change of the damping coefficient at the contact points under cyclic loading, were studied. The numerical model with linear and non-linear contact models was studied in four cases including the application of the normal and shear damping coefficient, and both normal and shear dashpots with the same and different values. The results showed that in the linear contact model, when the damping coefficient was applied only in the normal direction, the energy level was lower than the other three cases. However, in the non-linear model, all four cases had almost the same behavior. In the linear model, the amount of dissipated energy due to viscous damping was more than dissipated energy due to the friction sliding. However, in the non-linear model, dissipated energy due to sliding was more than the dissipated energy due to viscous damping.

Graphical Abstract

An experimental study is made to understand the deformation characteristics and failure mechanism of sands subjected to severe plastic deformation in the ploughing model setup of in-plane orthogonal cutting. The cutting experiments were performed on sands over 3 orders of strain rates. High-speed imaging and concomitant image analysis were performed using the Particle Image Velocimetry algorithm to obtain the whole field velocity measurements of the material flow. The velocity field maps of the near tool tip region demonstrate a sharp change in the motion of sand particles along with the formation of a dead zone. The effective strain rate maps show regions of intense localized plastic deformation- termed “shear bands”. The inclination angle of these bands evolved periodically with time and showed a decreasing trend due to an increase in the surcharge and effective depth of cut. The morphology and overall characteristics of these mesoscale structures (shear bands) do not change significantly with strain rate. The cutting force signatures were oscillatory and suggested cyclic material softening (dilation)—hardening (compaction) ahead of the tool, which is also reflected in the periodic repositioning of shear bands. The limit equilibrium-based model was adequate to predict the tool-cutting forces well, even with the significant variation in strain rates.

Graphical Abstract

Comminution is an energy intensive process. In SAG-mills, it is achieved by rotating a drum in which large metal balls crush ore particles. In-situ monitoring of particle size would be of considerable interest to optimize their operation. However, there is no established solution to measure particle size in such a harsh mechanical environment. We show here that the acceleration of the grinding media, which can be monitored using embedded accelerometers, can be used to sense the particle size and size distribution during operation. In DEM simulations, we find that a machine learning classifier is able to detect the size and distribution of small particles solely based on the knowledge of the acceleration of larger grinding media particles. Results show that this kinematic sensing is effective over a wide range of particle size ratios, size distribution, mixture ratio and mill charge. Beyond their potential applications in mineral processing, these results point out that the kinematics of large particles is affected by the size of the smaller particles, an observation which can help advance rheological models for bi-disperse granular flows.

Graphical Abstract

To improve the working efficiency of in-situ soil remediation equipment, this paper designs a new type of chain plate soil remediation equipment based on the working principle and technical requirements. The mixing process of soil and chemicals under different parameters was investigated using the discrete element method and the orthogonal test method. The experimental designs were all based on horizontal movement speed, chain knife speed, screw speed, and homogeneous mixing pitch as test factors and discrete coefficient and soil fragmentation rate as indices. The test method uses a unidirectional test to determine the value of the reference centre level for the orthogonal test and a combined balancing method to determine and validate the optimum parameters of the soil remediation device. The optimised parameters were determined as follows: the horizontal movement speed of the mechanism is 0.15 m/s, the rotational speed of the chain knife is 5.25 m/s, the rotational speed of the screw is 187.5 rpm, and the homogeneous mixing pitch is 98 mm, respectively. The dispersion coefficient was reduced by 7.43% and the soil fragmentation rate increased by 5.45% compared to the operating parameters of the baseline group.

Graphical abstract