Granular Matter最新文献

Roundness/angularity is a vital shape descriptor that significantly impacts the mechanical response of granular materials and is closely associated with many geotechnical problems, such as liquefaction, slope stability, and bearing capacity. In this study, a series of biaxial shearing tests are conducted on dual-size aluminum circular and hexagonal rod material. A novel image analysis technique is used to estimate particle kinematics. A discrete element model (DEM) of the biaxial shearing test is then developed and validated by comparing it with the complete experimental data set. To systematically investigate the effect of roundness/angularity on granular behavior, the DEM model is then used to simulate eight non-elongated convex polygonal-shaped particles. Macroscopically, it is observed that angular assemblies exhibit higher shear strengths and volumetric deformations, i.e., dilations. Moreover, a unique relationship is observed between the critical state stress ratio and particle roundness. Microscopically, the roundness shows a considerable effect on rotational behavior such that the absolute mean cumulative rotation at the same strain level increases with roundness. A decrease in roundness results in relatively stronger interlocking, restricting an individual particle’s free rotation. Furthermore, the particles inside the shear band exhibit significantly higher rotations and are always associated with low coordination numbers. Generally, the geometrical shape of a particle is found to have a dominant effect on rotational behavior than coordination number.

Graphical Abstract

The finite-discrete element method (FDEM) can be used to simulate brittle materials such as polycrystalline ice with specific geometric information. However, most previous studies treat ice as intact and nonporous, ignoring the effect of internal porosity. In this study, an FDEM model of polycrystalline ice with specific porosity is built by using the cohesive interface element and the method of randomly deleting elements. Comparison with experimental results confirms that the model can capture the strength properties and deformation patterns of polycrystalline ice. The fracture failure patterns and mechanical responses of ice specimens and their relationships with porosity are investigated by uniaxial compression tests and Brazilian splitting tests. The results show that with increasing porosity, the fracture failure patterns of the specimens in the uniaxial compression test evolve into three types: global shear failure, local shear failure and local tensile‒shear failure. There is no obvious difference in the failure patterns of the specimens in the Brazilian splitting test. In addition, as the porosity increases, the material exhibits a transition from brittleness to ductility, and the porosity also affects the local fragmentation characteristics inside the polycrystalline ice, significantly weakening the strength of the specimens.

Graphic Abstract

It is a common phenomenon that feed pellet is broken by compression and impact during the processing of manufacturing and production. At present, the breakage characteristics of feed pellet under repeated loading are not clear. In order to predict the breakage probability of feed pellet accurately, the compound feed for piglets feeding was selected to conduct repeated compression and repeated impacts tests in this paper. Firstly, the quasi-static repeated compression tests were conducted, and it was found that the cyclic stiffening occurred due to the densification of feed pellet during the repeated compression. Secondly, the quasi-static repeated compression tests in radial and axial direction were performed under different loading forces. And the results showed that the compressive energy required for feed pellet breakage increased with the decrease of loading force. Then, two-parameter Weibull function was used to fit the relationship between mass-specific compressive energy and breakage probability. And the fitting results R² were all greater than 0.9 and the fitting effect was good. Finally, dynamic repeated impacts tests with different impact velocities were conducted. The results showed that the impact times required for feed pellet to reach the same breakage probability decreased, with the increase of impact velocity. Three-parameter Weibull function was used to fit the relationship between mass-specific impact energy and breakage probability. Good fitting effect was obtained and R² was greater than 0.95. The fitting results can predict the breakage probability of feed pellet in the process of repeated loading, and provide guidance for the feed pellet production and transportation.

Graphical abstract

The breakage characteristics of feed pellet under reteated loading

The granular temperature, which is the mean kinetic energy per unit mass of velocity fluctuations, is an important rheological parameter for granular flows. Artoni and Richard (Phys Rev E 91: 032202, 2015) showed that accurate calculation of the fluctuation velocity during coarse graining requires the subtraction of the mean velocity at the position of the particle since the variation of the mean velocity over the coarse graining domain may be of the same order as the fluctuation velocity. This requires a two-step calculation. In this study, a single-step method of calculating granular temperatures is proposed. The values obtained by this method for a simulated system of inclined surface flow of granular materials are compared with those obtained using earlier methods and are shown to be accurate.

A series of tests was carried out to study the effect of particle size distribution on the mechanical behavior and particle breakage of coral sand. The tested materials had the same origin, and ten different particle-size distributions were used in the tests. Notably, oedometric and isotropic compression tests and monotonic drained and undrained triaxial tests were conducted. Additionally, a simple particle breakage model considering particle size distribution based on the input energy was proposed. The test results showed that the oedometric and isotropic compressibilities of coral sand decreased with increasing uniformity coefficient C_u but increased with increasing mean particle size D₅₀ and curvature coefficient C_c. The effective internal friction angle φ, maximum dilation angle ψ_max and secant modulus E₅₀ of coral sand all increased with increasing coefficient (sqrt {C_{u} /C_{c} }) but decreased with increasing mean particle size D₅₀. The relative density, particle breakage, confining pressure and particle size distribution had negligible influences on the residual friction angle of coral sand. In addition, under the same input energy, the relative breakage B_r decreased with increasing uniformity coefficient C_u, increased with increasing mean particle size D₅₀ and was basically independent of the curvature coefficient C_c. The proposed particle breakage model could effectively predict the particle breakage trends of coral sands from the same source but with different particle gradations at the stress level used in this study.

Graphical Abstract

We study the local structural changes along the jamming transitions in asymmetric bidisperse granular packings. The local structure of the packing is assessed by the contact orientational order, (tilde{Q}_{ell }), that quantifies the contribution of each contact configuration (Large–Large, Small–Small, Large–Small, Small–Large) in the jammed structure. The partial values of (tilde{Q}_{ell }) are calculated with respect to known ordered lattices that are fixed by the size ratio, (delta ), of the particles. We find that the packing undergoes a structural transition at (phi _J), manifested by a sudden jump in the partial (tilde{Q}_{ell }). Each contact configuration contributes to the jammed structure in a different way, changing with (delta ) and concentration of small particles, (X_{textrm{S}}). The results show not only that the packing undergoes a structural change upon jamming, but also that bidisperse packings exhibit local HCP and FCC structures also found in monodisperse packings. This suggests that the jammed structure of bidisperse systems is inherently endowed with local structural order. These results are relevant in understanding how the arrangement of particles determines the strength of bidisperse granular packings.

Graphic abstract

This work introduces a scaling analysis of sub-aerial axisymmetric column collapses of glass beads and Newtonian glycerol-water solutions mimicking some of the behaviours of debris flows. The beads were in a size range where their inertia partly decouples their collapse behaviour from the water column. Experiments explored a range of viscous, surface tension and particle inertia effects through systematic variation of particle size and fluid viscosity. Crucially a geotechnical centrifuge was used to access elevated effective gravitational accelerations driving the collapse, allowing field-scale viscous and surface tension effects to be replicated. Temporal pore pressure and run out front position evolution data was extracted using a pressure transducer and high speed imaging, respectively. A least-squares fitted scale analysis demonstrated that all characteristic dimensionless quantities of the acceleration phase could be described as a function of the column-scale Bond number (text{ Bo }), the Capillary number (text{ Ca }), the system size (r^*), and the grain-fluid density ratio (rho ^*). This analysis demonstrated that collapses as characterised by pore pressure evolution and front positions were controlled by the ratio of (text{ Bo}/text{Ca}). This indicates that grain-scale surface tension effects are negligible in such inertial column collapses where they may dominate laboratory-scale granular-fluid flow behaviour where geometric similarity between grain and system size is preserved.

Graphical abstract

The closed-form displacement field equations for the case of a disc subjected to self-equilibrated loads of arbitrary magnitude and direction are systematically derived from the existing closed-form stress field expressions. The proposed equations are validated with the help of numerical simulation carried out for the case of a disc under diametral compression and a disc subjected to four arbitrary loads. The derived expressions open the way for the extensive use of displacement measuring experimental techniques in the inverse analysis of granular materials for determining the particle forces.

Graphical abstract