Granular Matter最新文献

Laboratory-scale research on railway ballast often fails to produce parameters reflecting real-world conditions, while real-scale research incurs high costs. Advancements in computational capacity allowed for discrete element method (DEM) to simulate ballast behavior with three-dimensional, irregularly shaped particles. This research focuses on developing virtual 3D particles for DEM based on digital image processing (DIP) from the use of the Aggregate Imaging Measurement System (AIMS). This can potentially provide a rationale for taking full advantage of databases of aggregate properties obtained with this equipment over more than a decade across various regions worldwide. Quarry-produced aggregates were characterized in terms of shape properties in three orthogonal positions using AIMS. Virtual 3D particles were generated from one, two, or three real 2D images, with strong correlations between real and virtual particles results obtained for sphericity, flatness, elongation, and flatness/elongation ratio. This study shows that generating virtual 3D particles from one single real 2D image from AIMS is an effective and time-efficient process. Furthermore, shape properties classification of virtual particles closely matched real ones, with minimal variation near classification boundaries, confirming the method’s consistency. This approach can be an alternative to more computationally expensive 3D modeling, as well as allowing for the virtual reproduction of aggregates not locally available by sharing AIMS databases. Finally, numerical simulations were proven to be sensitive to real particle shapes, allowing for better understanding of ballast performance, leading to optimization of maintenance and reducing track wear and elements’ failure.

The stress dip, a local minimum in the vertical stress distribution beneath granular piles, has captured the interest of many researchers. Studying stress dips in granite residual soil is of critical importance due to its relevance to engineering projects, soil mechanics, and particle behaviors. The purpose of this study is to confirm the existence of the stress dip in granite residual soil and explore its evolution during accumulation. In this work, granite residual soil conical piles were formed by the localized source piling method in experiments. During the experiment, Teflon film was placed below the piles to hinder the formation of stress dips, while the vertical stress distribution beneath each pile at varying heights was measured to monitor the evolution of stress dips. Besides, DEM simulations were employed to analyze the formation and evolution mechanism of the stress dips. The experimental and simulation results showed that stress dips can be formed in granite residual soil piles, occurring both in the center and locally. Stress dips evolve gradually through accumulation rather than being intrinsic properties of the piles. From a spatial perspective, no clear pattern is observed in the location of the stress dips. Quantitatively, as pile size increases, stress dips become more prevalent throughout the entire scope, although individual dips may dissipate. The normalized analysis of the central stress dip suggests that the normalized stress distribution pattern of the central stress dip is independent of pile size. The formation and evolution of stress dips are influenced by the force chain network, which consists of arch and ring force chains that are promoted by the supporting effect of the base plate and the particle squeezing effect.

This study examines particle statistics using simulated particle assemblies derived from mathematical models. This approach serves as a complement to investigations that analyze samples of real particles to assess the accuracy of measurement and statistical methods. Three mathematical particle models, all based on tessellations of three-dimensional space, including the well-known Voronoi tessellation, are employed to generate random convex polyhedra. A key advantage of this approach is that the true statistical properties of the particles and particle assemblies are well understood, allowing for a realistic evaluation of statistical methods. Furthermore, the analyses performed can be easily replicated or verified by other researchers in parallel studies. The approach is applied to the evaluation of two commonly used statistical methods: estimating the volume-weighted particle size distribution function from image analysis data, and estimating the specific surface area when particle volumes are measured. The simulation results indicate that image analysis methods yield accurate results for particle size distributions. Additionally, estimating the specific surface area using particle size distributions produces acceptable results when incorporating the mean sphericity of the aggregates, without accounting for particle roughness, which is not a significant factor for the particles under consideration.

To investigate the breakage characteristics of particle groups, the particle bed compression tests were conducted with feed groups. Three compression stages were determined based on the force–displacement curves of feed pellets with different aspect ratios. The occurrence of the stage ascent curve was explained by the microstructure of the feed pellet and the filling of pores in the particle bed. The influence of aspect ratio and moisture content on the mass-specific compressive energy was investigated, revealing that the energy increased with the increase of aspect ratio and water content. Additionally, the impact of these factors on the pulverization rate was assessed, demonstrating that the pulverization rate rises with the aspect ratio but diminishes with water content. The PSD (particle size distribution) of the feed pellet after compression was fitted with the Weibull function. The R² values are higher than 0.97 and 0.93 for different aspect ratios and water content, respectively. Furthermore, new models were established to represent the PSD, taking aspect ratio and water content into consideration. The R² values are higher than 0.95 and 0.96, respectively. These findings enhance the understanding of breakage characteristics of feed pellets under compression and provide a theoretical model for quality assessment of agricultural products.

Particle breakage is an important factor affecting the mechanical properties of granular materials. In this study, the influence of particle breakage under different fine particle content is investigated by DEM. Through 3D scanning and Voronoi tessellations, the breakable particle model with realistic shape is constructed. A series of confined cyclic loading tests were performed at different fine particle content. Then, the particle breakage characteristics, including the degree of breakage and the breakage pattern, were evaluated. In addition, the compaction deformation was analyzed according to the evolution of porosity. Finally, the influence mechanism of particle breakage is explained from two perspectives of particle contact and particle motion. On the one hand, with the increase of fine particle content, the number of contacts on the coarse particles is increasing. Hence, the coarse particles can withstand greater forces without breaking. On the other hand, the displacement of coarse particles and the porosity decrement have very similar evolution curve. This indicates that the Z-axis displacement of coarse particles can directly reflect the variation of sample porosity. In addition, particle breakage has little effect on particle rotation. The effect of particle breakage on porosity is mainly realized through the effect of particle translation rather than particle rotation.

In this work, we performed experiments with spheres, rice-shaped particles with different aspect ratios, and macaroni-shaped particles in a quasi-two-dimensional hopper, where the thickness was adjusted to the minor dimensions of the particles such that a mono-layered system is created. We quantitatively investigate the vertical velocity and solid fraction profiles at the orifice and determine how these are influenced by the slope of the hopper. Interestingly, where the hopper angle hardly influences the velocity profile for rice-shaped particles, the magnitude of the velocity profile increases for spherical particles and decreases for macaroni particles with the steepness of the hopper. The spheres have flat solid fraction profiles for all hopper angles, but a transition from flat to dome-shaped profiles is observed with decreasing hopper steepness for all non-spherical particles. The discharge rate determined by integrating the product of the velocity and solid fraction profiles has good agreement with the experimentally measured discharge rate for all particle shapes.

Graphical Abstract

Experimental images of discharge of (a) spheres, (b) rice (a_s), (c) rice (a_l), and (d) macaroni particles. (e) Discharge rate of different particle shapes with hopper angles.

Existing porosity predictors for fluvial sediments are mainly derived from laboratory-generated, randomly packed sediment samples. However, such predictors could not adequately describe beds with non-random grain arrangements that occur widely in fluvial deposits. In this work, the effect of grain imbrication on non-uniform gravel-bed porosity has been quantified using fluvial sediment samples showing imbrication and no imbrication collected from the river Buëch, France. The in-situ porosity of the undisturbed samples was directly measured on-site, while the ex-situ porosity was measured by randomly packing the particles of a sample in a cylindrical container in the laboratory. The in-situ porosity and the ex-situ porosity of the same sample were compared. Apart from the porosity measurement, a relatively new and simple workflow was applied to quantify the degree of bed imbrication based on the X-Ray Computed Tomography images of frozen sediment samples. For samples showing no imbrication, the in-situ and the ex-situ porosity showed similar values, indicating that sediment samples randomly packed in the laboratory (with shaking) are well representative of the fluvial sediment with random grain orientation formed under natural conditions. For samples showing imbrication, the in-situ porosity values were about 30% lower than their ex-situ porosity values, indicating denser packing structure due to imbrication. This increase in structural compactness is believed to stem from the ordered arrangement of sediment particles, thereby reducing the formation of large pores.

Graphical Abstract

(a) Comparison of the n_in-situ and n_ex-situ of samples showing imbrication and no imbrication, (b) Dip direction distribution, (c) Imbricated particles

An adaptive discretization method is introduced to develop numerical models for boundary value problems using the Discrete Element Method. This method involves discretizing a domain, with smaller particles in regions of interest—where the material undergoes large displacements and irreversible deformations—while continuously increasing particle sizes elsewhere, as distance from these regions of interest increases. While maintaining uniform mechanical properties within the whole simulation domain through appropriate scaling of contact model parameters, this approach presents the main benefit to substantially reduce the number of particles in the model, thereby lowering computational costs, without making the numerical method itself more cumbersome. This method is applied and assessed in the context of modeling a Cone Penetration Test within a calibration chamber.

Gap-graded soils, extensively utilized in geotechnical and hydraulic engineering, exhibit diverse strength characteristics governed by their distinctive particle size distribution (PSD). To investigate the influence of PSD on the shear strength of gap-graded soils, this study utilizes the Discrete Element Method (DEM) to reproduce drained conventional triaxial tests of gap-graded soils across a wide range of fine particle content (FC = 1-40%) and particle size ratio (SR = 2.5-6.0). The simulation results reveal that the peak shear strength follows a characteristic unimodal curve versus FC, attaining its maximum value at about FC = 25%. SR governs peak strength through critical FC thresholds: negligible impact at FC < 10%, whereas significant enhancement occurs at FC = 25%. Micromechanical analysis reveals that branch anisotropy evolution controls strength behaviour. Shear strength inversely correlates with peak branch anisotropy as reduced branch anisotropy promotes homogenized contact force distribution. FC and SR collectively regulate macroscopic strength through coupled control of branch anisotropy evolution, where their synergistic interaction governs force chain reorganization and stress distribution homogeneity. Based on these insights, a novel predictive formula for peak strength incorporating both SR and FC were proposed, providing guidance for optimized deployment of gap-graded soils in engineering practice.

Quantification of particle morphology plays a crucial role in studying the physical properties of materials. Current methods for quantifying particle morphology using image analysis technology have many limitations. To address this issue, we propose a morphology quantization approach based on the principle of altitude-to-chord ratio, referred to as the ACR morphology quantization approach. This approach calculates corresponding descriptors for particle surface texture, angularity, and form across three different scales of morphological characteristics. It has established a multi-scale quantitative method to describe particle morphology. The surface texture descriptor calculated therein is unaffected by macroscopic scale variations and exhibits strong stability. Utilization of angularity descriptor results in sorting outcomes that are completely identical with manual visual assessments when applied to Krumbein’s standard particle chart and Powers’ angularity grading chart. It can also distinguish particles with different levels of angular grades within these charts quite distinctly. The form descriptors focus on how close the particles approximate to a circle along with the macroscopic scale of the particles. And it is possible to measure the distance between the concave boundary and the opposite edge in concave particles, which is a piece of information that is often overlooked in existing descriptors. Through the calculation of actual particles, it was demonstrated that the ACR quantification approach provides a complete and objective characterization of particles and the quantified results are consistent with human subjective perceptions.

Graphical Abstract