Granular Matter最新文献

This study investigates the interplay between particle shape, grading and initial sample density, three of the most important factors influencing the mechanical behaviour of sheared granular assemblies. Using the discrete element method (DEM), two-dimensional assemblies of varying initial sample density, particle aspect ratio, (AR), and coefficients of uniformity, ({C}_{u}), were prepared and subjected to drained biaxial shearing until the critical state was reached. We assessed the interplay between each of these parameters by evaluating whether the effect of any given parameter on a mechanical quantity is influenced by any other parameter. Our analyses show that the effect of some of these key parameters on mechanical response, can indeed be influenced by other key parameters. The effect of the particle (AR) on the peak shear strength for the initially dense assemblies differs when compared with the medium-dense assemblies. The mechanical coordination number of the assemblies at the initial state correlates with the peak strength thereby explaining the interplay between particle (AR) and initial sample density on the peak shear strength. The linear relationship established between strength and dilatancy for a combination of all assemblies studied suggests that the strength-dilatancy relationship is a unique characteristic of granular assemblies. The dilatancy of the assemblies correlates strongly with the amount of contacts lost during shearing. The interplays found between particle shape, grading and initial sample density in this study show that to develop robust constitutive models for the prediction of granular material behaviour, the effects of multiple factors must be considered.

Graphic abstract

This study aims to suggest a new method for predicting 3D sphericity through traditional 2D image processing through a novel sieving analysis. The 3D sphericity indices of grains (over 3000 particles for each material) from seven irregular granular materials are determined using μCT slices. These indices are then compared with existing 2D indices obtained through SEM image processing. Additionally, seven synthetic materials (semi-regular in size and shape) are also assessed to account for unusual particle shapes. The findings shed light on the role of sphericity in the rate at which particles pass through sieve openings. The results indicate that the initial passing rate of grains is strongly correlated with the 3D sphericity indices, which significantly decrease as sphericity decreases. The proposed method involves a sequential sieving test, performed similarly to the conventional sieving test but conducted sequentially at different time steps. Several correlations between 3D sphericity and its 2D counterparts are presented, which can successfully predict the 3D sphericity indices. Additionally, two empirical equations are proposed to predict the most frequent flatness and elongation aspect ratios, used in the Zingg diagram. Furthermore, the grading analysis derived from both 2D and 3D image processing is compared with sieve analysis. The results show that, unlike the 2D results, the grading curves obtained from 3D image processing are in excellent agreement with the sieve analysis. A corrected grading curve, derived from traditional 2D image processing, is proposed to align with 3D grading curves.

Granular materials used for vibration reduction often show dense granular clusters in engineering practice. Nevertheless, there are great differences in the damping effect between different dense granular clusters. In this work, discrete element simulations are performed to investigate the evolution of dense granular cluster in dissipation behavior by vertically vibrating a quasi-2D granular container with constant excitation frequency but different excitation amplitude, which reveals nine different granular motion patterns. Simulation results indicate that, with the increase of excitation amplitude, the internal configuration of dense granular cluster in granular container evolves gradually from static-disordered to dynamic-disordered and then dynamic-ordered, and finally becomes loose. The scope of high damping granular phases (HDGPs) is finalized based on the friction dissipation mechanism of granular balls in four dynamic-ordered dense granular clusters, where there may be reversible granular jamming transitions. The universal dynamical behavior of dense granular clusters in HDGPs is revealed, which contributes to obtaining the optimal granular damping effect by controlling the motion pattern of vibrated granular materials.

Graphical abstract

We consider two (2D) and three (3D) dimensional granular systems exposed to compression, and ask what is the influence of the number of physical dimensions on the properties of the interaction networks that spontaneously form as these systems evolve. The study is carried out based on discrete element simulations of frictional disks in 2D and spheres in 3D. Within the constraints of the considered simulation protocols, the main finding is that both the number of physical dimensions and the type of particle-particle interaction significantly influence the properties of interaction networks. These networks play an important role in bridging the microscale (particle size) and macroscale (system size), thus both aspects (the interaction model and dimensionality) are carefully considered. Our work uses a combination of tools and techniques, including percolation study, statistical analysis, as well as algebraic topology-based techniques. In many instances, different techniques and measures provide complementary information that, when combined, allow for gaining better insight into the properties of interaction networks in compressed particulate systems.

Graphic Abstract

Impact loads can exacerbate the deterioration and deformation of railway ballast, leading to changes in the mechanical properties of the track bed. Geogrid and rubber granules (RG) have been widely used in research to enhance the performance of ballast, however, the effects and mechanisms of these two materials working together are not yet clear. Therefore, in this study, a series of drop hammer impact tests were carried out on ballast aggregates with geogrids and RG. The tests were set up with different geogrid placement locations and RG contents on rigid and flexible subgrades. During the tests, the deformation, impact force and impact time of specimens were measured and recorded, the ballast specimens were sieved after the tests to investigate the breakage of the ballast, and the mechanical properties of the ballast specimens were analyzed after the impact using stiffness and damping ratio. It was found that the deformation and breakage of ballast specimens were significantly reduced by the combination of geogrid and RG, which was better than the geogrid or RG alone, and that the RG improved the damping ratio of ballast, while the geogrid reduced the reduction of stiffness of ballast caused by the addition of RG. Comparing and analyzing the results of each group of tests, the study confirmed that RG with 10% by volume and geogrid placed at 100 mm from the subgrade were the best combination to enhance the ballast performance.

Graphical Abstract

In this study, we examined at the grain-scale the normal contact behavior of rigid-soft interfaces via a series of micromechanical experiments, in which “rigid” refers to quartz particle and “soft” refers to polymeric granules composed of recycled rubber. Emphasis was placed on the influence of creep by quantifying the creep deformations at the composite interface subjected to long-term loading. The experimental data suggested that the creep behavior of the sand-rubber interface subjected to different normal loads is not deterministic and that there is a correlation between creep and elastic deformations. We also examined the applicability of available creep models to the specific creep problem and the parametric study highlighted the heterogenous features of the creep contact behavior of the sand-rubber samples, which is dependent on the elastic properties of the rubber and the irregular geometry of the contact area. The ground-truth dataset suggested the Burgers model is the most suitable contact model for the creep problem at the sand-rubber interface. The parameters of the Burgers model were also calibrated for further exploration of constitutive models to be used in discrete-based computer analyses. This modeling also provided fundamental insights to understand the physics of the problem.

Abstract

Multi-sized mixtures are frequently encountered in soils, industry and construction. In many cases, the particle-size distribution (PSD) must be optimized to achieve a certain packing density, the estimation of which must be as precise as possible. For this purpose, three packing density models are evaluated from published data on PSDs of spheres, angular flint, crushed aggregates: the Compressible Packing Model (CPM), the 3-parameter Particle Packing Model (3PPM), the Theoretical Packing Density Model (TPDM). The PSDs exploited are complementary: power-laws, truncated power-laws, uniform distributions by volume and fractal models. They then make it possible to focus the estimates of packing density on reference PSDs: the century-old power-law of Fuller & Thompson (FT) and the Caquot’s distribution for concrete, the “well-graded” size distribution in soil classification. The conclusions are as follows. The 3PPM underestimates packing densities due to an overvaluation of the geometric interactions. The CPM overestimates packing densities due to an underestimation of the loosening effect in a certain range of size ratios. The TPDM provides the most homogeneous estimates with prediction error values almost all below 2%. Due to its efficiency, the TPDM is used on model materials whose granular extent varies between 1 and 10,000 to determine the preferred domain of each of the reference PSDs.

Graphic abstract

Particle shape variability is a key to understanding the rich behavior of granular materials. Polyhedra are among the most common particle shapes due to their ubiquitous origins in nature such as rock fragmentation and mineral crystallisation. Because of their faceted shape, polyhedral particles tend to assemble in jammed structures in which face-face and face-edge contacts between particles control the packing-level properties. In this paper, we use tri-periodic particle dynamics simulations to derive for the first time a generic analytical expression of the elastic moduli of polyhedral and spherical particle packings subjected to triaxial compression as a function of two contact network variables: (1) a “constraint number" that accounts for the face-face and edge-face contacts between polyhedra and is reduced to the coordination number in the case of spherical particles, and (2) the contact orientation anisotropy induced by compression. This expression accurately predicts the simulated evolution of elastic moduli during compression, revealing thereby the origins of the higher elastic moduli of polyhedral particle packings. We show that particle shape affects the elastic moduli through its impact on the contact network and the level of nonaffine particle displacements is the same for the simulated shapes. Its nearly constant value during compression underlies the constant values of our model parameters. By connecting the elastic moduli to the contact network through parameters that depend on particle shape, our model makes it possible to extract both the connectivity and anisotropy of granular materials from the knowledge of particle shape and measurements of elastic moduli.

Graphic abstract

The Flip-Flow Screen is extensively utilized in the vibrating screening process for the treatment of particle matter. In this study, a four-degree-of-freedom (4-DOF) Flip-Flow Screen was proposed. The granules in the 4-DOF Flip-Flow Screen were modeled using the discrete element method. The screening process of the 4-DOF Flip-Flow Screen was simulated by multi-flexible body dynamics-discrete element method (MFBD-DEM) coupling. The impact of vibration frequency and amplitude on the sieving effect of the Flip-Flow Screen was studied. The vibration parameters of the 4-DOF Flip-Flow Screen were optimized using the response surface method to improve sieving performance. Analysis of variance (ANOVA) was employed to assess the simulation findings. The results show that the variables with the greatest influence on the screening efficiency are z-direction frequency and x-direction frequency, respectively. The best outcome corresponding to the maximum screening efficiency is found to occur as the z-direction frequency, x-direction frequency, x-direction amplitude, and y-direction amplitude are all at 7.5 Hz, 15 mm, and 3 mm, respectively.

Graphical abstract