Impact of particle degradation on repose angle of railway ballast: insights from experimental and DEM analysis

The microstructure characteristics of slip zone soils and the variation law of microstructural parameters during shearing are the focus of the study of landslide mechanism, which is of great importance for the study of landslide prevention and control technology. Providing satisfactory explanations at the macro view is impossible because many phenomena occur after the soil is stressed in a slip zone. At present, the microscopic and meso analyses of slip zone soils are single and emphasize the macroscopic strength and deformation characteristics of slip zone soils during shearing. This study takes the landslide on the south slope of west open-pit mine in Fushun as the research object. The mineral composition and micro-characteristics of slip zone soils were studied with an X-ray diffractometer and a scanning electron microscope. Discrete element method was used to simulate the direct shear test. The meso parameters of slip zone soils were obtained by matching the direct shear test curve. The comparative analysis of meso parameters, such as the meso displacement vector field, distribution of maximum and minimum principal stresses, contact force chain, and evolution of normal and tangential contact forces, with four groups of particle gradations under different weathering conditions was conducted. Results show that the main mineral compositions of slip zone soils are original mineral-quartz and secondary mineral-clay minerals. The content of montmorillonite in clay minerals is relatively high, reaching 43.8%. The particles of slip zone soils are mainly flakes, with compact structure, and have obvious characteristics of oriented arrangement. Slip zone soils with a low degree of weathering contain coarse particles (quartz). The shear stress-displacement curves of slip zone soils with four groups of particle gradations show a strain hardening trend. Particle gradation affects the macroscopic and microscopic geotechnical and mechanical properties of slip zone soils. The displacement zone of the displacement field after shearing under poor gradation is characterized with large particles as the boundary, and its dilatancy is pronounced under the same normal stress. When the gradation is good, the particles have contact force chain, normal direction, and shear. The distribution of directional contact force before shearing is relatively uniform, the grading of large particles is mostly poor, and the stress concentration is considerable. In the case of poor grading, the distribution range of normal and tangential contact forces after shearing is smaller than that of good grading. The distribution of contact force in the same direction angle shorter than that in the case of good grading.

Taking the discontinuity and random character of granular system into account,a kinematic model of particles was established by three-dimensional discrete element method in order to investigate the relationship between angle of repose and gravity. Accumulation process of granular pile was simulated using the model under variable gravity. The angle of repose and probabilistic distribution of contact forces were obtained in the model. Results show that the force-chain formed from contact forces within particle pile has the characteristics of structure of irregular mesh. Contact forces within particle pile are approximately log-normal distributed and there are about 65% of contacts carrying a force lower than the mean. Moreover,there are about70% of contacts whose friction force are fully mobilized and the ratio of tangential force to normal force among the rest of contacts is uniformly distributed. The distributions of contact force of granular piles in different gravity field have similar properties. The contact forces,which are normalized with respect to particle's gravity,of piles with variable gravity have nearly the same distribution. The angle of repose is not affected by the gravity,though granular pile has a randomness property of microscopic structure.

In the discrete element method analysis, the number of particle elements affects the calculation time. Because of this disadvantage, a method called coarse-grained method is sometimes used to improve the calculation speed by replacing multiple particle elements as a single particle element. In this study, a contact force model based on Hertzian contact theory is implemented in a contact force calculation between coarse-grained elements, and the contact force is used as input force for vibration analysis of a ball mill. Compared to the analysis results obtained by the conventional coarse-graining method using a contact force model based on linear springs, the proposed contact force calculated by the proposed mode closes the one of the original model.

Discrete element method (DEM) is applied to study the granular accumulation problem. Using Herz-Mindlin (no slip) model to simulate particles and container model is also established by software. When the container elevates, the process of granular falling and collision can be ob-served. Detailed analysis of that the impact of static and rolling friction coefficient for particles - particles, particles - flat on angle of repose is accomplished. The variation law is also further val-idated from the energy point of view. The results show that rolling friction has a greater impact on angle of repose than static friction, and rolling friction coefficient among particles play the more prominent role in the two kinds of rolling friction. The research method and results provide a the-oretical reference for the granular movement and DEM analysis.

Calendering is an essential process step to manufacture electrodes for lithium-ion batteries. The relationship between the various component material properties and calendering conditions has a large impact on the battery performance. In this work, Discrete Element Method (DEM) was used to investigate the electrode structure evolution under different calendering conditions. The initial positions of active material (AM) particles were obtained from an uncalendered electrode microstructure characterised experimentally by X-ray tomography and then imported to DEM simulations. Simulated structures under different processing conditions were obtained by compression tests in DEM. The Edinburgh elasto-plastic adhesive (EEPA) model and bond model were used to describe the mechanical response of AM particles and binder phase during compression. Detailed stress and structural evolutions at microscopic scale were further analysed. For the first time, the results demonstrate a promising way to predict and design battery electrode structures by combining X-ray tomography and DEM analysis.

The performance and longevity of lithium-ion batteries (LIBs) are significantly influenced by the electrode microstructure. Traditional electrochemical models, such as the Pseudo-Two-Dimensional (P2D) model, often assume homogeneous properties across the electrode, thereby neglecting the localized variations in transport properties, reaction rates, and porosity. However, in real electrodes, the microstructure exhibits non-uniform characteristics due to variations in particle size distribution, packing density, and electrode processing conditions. These microstructural heterogeneities critically impact the effective transport of lithium ions, electronic conductivity, and electrochemical reaction kinetics, leading to spatially varying performance and degradation patterns. To address these complexities, this study presents a novel multiscale modeling framework that integrates a heterogeneous Pseudo-Four-Dimensional (P4D) model with microstructure-based Discrete Element Method (DEM) analysis to accurately capture local porosity variations and their impact on battery performance.The heterogeneous P4D model extends the traditional P2D approach by incorporating spatial variations in key electrochemical parameters, derived from microstructural analysis. Instead of assuming uniform porosity, the model considers localized porosity distributions obtained from high-fidelity DEM simulations. The DEM framework simulates the electrode microstructure by modeling individual active material particles as discrete elements, considering their interactions during electrode fabrication processes such as calendaring, mixing, and compaction. By employing particle-scale physics, the DEM analysis provides realistic insights into porosity variations, particle connectivity, and tortuosity, which are critical factors influencing ion transport and reaction kinetics.By coupling the heterogeneous P4D model with DEM-based microstructure characterization, this approach enables a more accurate representation of the electrode's spatially varying properties. The local porosity data extracted from the DEM simulations is incorporated into the P4D model to determine the effective transport properties of lithium ions and electrons, as well as the electrochemical reaction kinetics within different regions of the electrode. This coupling allows for the investigation of spatially resolved performance metrics such as local overpotentials, state-of-charge (SOC) distribution, and lithium plating propensity.One of the key advantages of this integrated approach is its ability to bridge the gap between microstructural heterogeneity and macroscale electrochemical performance predictions. Traditional P2D models fail to capture the intricate role of particle-scale features in governing battery behavior, leading to potential inaccuracies in predicting degradation mechanisms. By incorporating microstructure-resolved porosity variations, the heterogeneous P4D model provides enhanced accuracy in predicting transport limitations, enabling improved optimization of electrode designs. Additionally, the approach offers a valuable tool for assessing the impact of manufacturing processes on battery performance by linking process-induced microstructural variations to electrochemical behavior.This work presents a comprehensive approach for modeling lithium-ion battery electrodes by integrating heterogeneous P4D analysis with microstructure-resolved DEM simulations. By explicitly considering local porosity variations and their effects on ion transport and reaction kinetics, this framework offers a significant improvement over traditional modeling approaches that assume uniform electrode properties. The insights obtained from this study are expected to aid in the development of next-generation lithium-ion batteries with optimized microstructures, improved performance, and enhanced cycle life.Also, non-uniform porosity distributions across multiple cells can lead to localized uneven degradation and capacity fade, ultimately affecting the reliability and lifespan of the entire battery pack. In future, this work can provide critical insights for optimizing battery module/pack design by integrating porosity-dependent transport and reaction kinetics.

Discrete element numerical simulations can help researchers find potential problems in the design phase, shortening the development cycle and reducing costs. In the field of agricultural engineering, more and more researchers are using discrete element methods (DEM) to assist in designing and optimising equipment parameters. Model parameters calibration is a prerequisite for discrete element numerical calculations, and the angle of repose (AoR) is commonly used to calibrate the parameters. However, the measurement of AoR in DEM was not seriously considered in industrial or academic fields. In practice, AoR is measured manually, using 2D digital image processing or using a 3D scan. However, reliable and consistent measurements of AoR in DEM are rarely mentioned. This study suggests an accurate and consistent way to measure AoR in DEM using a novel method to read particle coordinate information directly from the data file; then, the AoR is calculated by linearly fitting the centre coordinates of the outermost particles. Influences of input variables on AoR acquisition are discussed through several examples using customised templates with known angles. Then a comparative study of the accuracy of the measurement of AoR in DEM and the reliability of the parameter calibration results by the manual measurement, 2D digital image processing, and algorithm proposed in this paper was conducted. In case studies with four seed materials, this method prevented the subjective selection of AoR, improved the identification accuracy, and increased the precision and accuracy of DEM calibration. In addition, the time consumption for obtaining AoR using the novel method for measurement is much less than that of 2D.

Micro–macro properties of quartz sand: Experimental investigation and DEM simulation

This paper presents a new code for the two-dimensional discrete element method (DEM) and relevant simulations to quantitatively characterize the contact force behavior of the Nb3Sn strands in the ITER CICC cross-section under a transverse electromagnetic load. In order to obtain the essential parameters in the contact force model employed in the DEM, a simulation of the experiments conducted by Nijhuis et al (2004 IEEE Trans. Appl. Supercond. 14 1489–94) is first performed, where the load–displacement curve predicted by the code is in good agreement with the measurements. After that, the contact force chain between strands and its distribution is quantitatively analyzed by the code. It is found that the contact force distribution among strands is heterogeneous and strongly anisotropic. In other words, the force chain distribution, which determines the behavior of the assembly of strands with discrete media, and the distribution of area average magnitude of the contact force are obviously inhomogeneous. To describe this inhomogeneity, here, the probability density function (PDF) is used in the statistical analysis. The numerical results show that the PDFs of the magnitudes of the resultant contact force, normal contact force, and tangential contact force all decay with an exponential law, and that PDFs of the directions of the contact forces are all anisotropic and exhibit about six periodic changes in which the peak values in the direction parallel to the applied electromagnetic load are appreciably larger than the other peaks.

The angle of repose (AOR) is one of the key parameters used to comprehensively characterize the basic mechanical properties and stacking properties of granular materials. In this research, the influence of temperature and moisture on the stacking performance of railway ballast was studied through tests of the AOR. Additionally, the parameters of the discrete element method (DEM) for ballast beds with various temperatures and moisture levels were calibrated and combined with the response surface method. Then, the AORs of the ballast in different environments were simulated with the DEM, and the micromechanical properties of the ballast in dry, wet, low-temperature, and frozen environments were compared. The results showed that the AOR of the ballast in a frozen environment was smaller than that in a dry environment at the same low temperature (the difference could reach 8.48° at −30°C). The AOR of the ballast increased with the decrease of the temperature in a dry environment (18.44% increase between 20°C and −30°C). The AOR of the ballast decreased with the decrease of the temperature in a wet (frozen) environment (22.86% decrease between 20°C and −30°C). In addition, the validity tests of the AOR simulations of ballast proved that the obtained parameters could be used in the DEM models of ballast for the corresponding temperature and moisture. At the same time, it was found that with the change of temperature and moisture, the force chain network of the ballast would also change.

Investigation of wall stress and outflow rate in a flat-bottomed bin: A comparison of the DEM model results with the experimental measurements

Repose angle is an important property of granular materials and is usually simulated using Discrete Element Method (DEM). However, DEM simulation is computationally intensive and is thus unsuitable for online applications where parameters are frequently changed. To solve this problem, we propose a DEM data-driven modeling method for fast prediction of repose angle. Firstly, variables affecting the repose angle are analyzed; by Latin hypercube sampling of parameter spaces, 100 sets of DEM simulations are performed to generate data of repose angle. Based on these data, a support vector machine (SVM) model is then established and trained for fast prediction of repose angle under various conditions. Tests and comparison show that the reposed angle predicted by the SVM model is close to the DEM simulation result while the required computing time is greatly decreased (from 43.8 hours to 0.17 seconds), and it outperforms BP neural network and Kriging interpolation method in terms of prediction accuracy. The SVM model for repose angle is also verified by physical experiments, with prediction error less than ± 1 °. The established model can replace DEM, and is suitable for applications where fast prediction of repose angle is required.

After potash ore has been mined and processed it is stored and shipped as bulk granular particles. If these particle beds are exposed to a relative humidity that is above 50%, moisture will accumulate in the bed. This causes the formation of concentrated brines on the surfaces of the particles that migrate toward the contact points between particles. If drying occurs crystal bridges at or near the contact points will form between particles. If large numbers of bridges are formed per unit volume of bed, the bed is said to be caked. In this paper, the Discrete Element Method (DEM) is used to analyze the micro-mechanical behaviour of ideal spherical or ellipsoidal particles inside a potash bed and these results can be used to determine the cake strength. Hertz contact compressive stress theory is applied at each contact point between particles to resolve the stiffness matrix in the DEM analysis. The results of the DEM analysis compare well to those obtained using continuum mechanics, especially when Poisson's ratio has a low value. After potash ore has been mined and processed it is stored and shipped as bulk granular particles. If these particle beds are exposed to a relative humidity that is above 50%, moisture will accumulate in the bed. This causes the formation of concentrated brines on the surfaces of the particles that migrate toward the contact points between particles. If drying occurs crystal bridges at or near the contact points will form between particles. If large numbers of bridges are formed per unit volume of bed, the bed is said to be caked. In this paper, the Discrete Element Method (DEM) is used to analyze the micro-mechanical behaviour of ideal spherical or ellipsoidal particles inside a potash bed and these results can be used to determine the cake strength. Hertz contact compressive stress theory is applied at each contact point between particles to resolve the stiffness matrix in the DEM analysis. The results of the DEM analysis compare well to those obtained using continuum mechanics, especially when Poisson's ratio has a low value.

Experimental and numerical study on the force transmission in granular packings under the hypergravity

Particle Size Distribution (PSD) exerts a substantial influence on the mechanical properties of geological materials such as rocks and soils, which can be viewed at a microscale as an assembly of discrete particles. An exploration into the effects of particle gradation on the properties of these materials provides valuable insights into their nature. In the study, the Discrete Element Method (DEM) was used to conduct numerical shear tests on eight distinct groups of slip zone soil, each characterized by a different particle gradation. The aim was to examine the meso-mechanical properties and shear evolution laws of slip zone soil numerical samples with both optimal and sub-optimal PSDs. Findings underscore the pivotal role that PSD plays in various aspects, including dilatancy, the evolution of the displacement field, the network of contact force chains, the principal stress, and the distribution of normal and tangential contact forces within the slip zone soil. It was observed that the network of contact force chains in the numerical samples with an optimal PSD was more complex than in those samples with a sub-optimal PSD. Additionally, the distribution of principal stresses before and after shear was more uniformly balanced. This particle size-based study offers significant reference value for future investigations into the impact of PSD on the macroscopic and meso-mechanical properties of slip zone soil. By augmenting this knowledge, a more comprehensive understanding of the fundamental behavior of these materials can be attained, leading to improved prediction and management of geological risks.