Granular Matter最新文献

Understanding the stick-slip instability of granular materials is crucial to studying the quasi-periodicity of fault slip. The macroscopic frictional responses have inferred that particle size is one of the notable variables. To reveal the micro-mechanical cause, we analyzed the frequencies of Acoustic Emission (AE) events induced by the glass bead deformation through the compression tests and examined the effects of particle diameter on the stick-slip mechanism. We found that AEs distribute in the three frequency bands during the direct shearing process, with the probability density reflecting the three-stage evolution of force chains. At the microscopic scale, particle friction within the force chains induces micro-failures, generating low-frequency (0–100 kHz) AEs, as evidenced by environmental scanning electron microscopy. Meanwhile, high-frequency (200–350 kHz) AEs concentrate at the yield points. The probability density of AE events was used to quantify force chain deformation. The results show a negative correlation between AE rate and amplitude, with the micro-mechanical cause attributed to fewer frictional failures and an increase in critical elastic stiffness. The established AE-based experimental framework can provide insights into the micro-mechanisms of stick-slip instability.

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As a key operation in the bulk processing, particle mixing plays a crucial role in multiple industrial fields. In this study, the discrete element method (DEM) is used to investigate the mixing characteristics of non-spherical particles in a horizontal high shear mixer. Through quantitative analysis of Lacey mixing index, the effects of key parameters such as aspect ratio, particle shape, and rotational speed on mixing performance are discussed. The results show that the radial mixing efficiency of particles is significantly better than the axial mixing efficiency, and both increase with the increase of rotational speed. In addition, the mixing efficiency has a significant correlation with the flow resistance and energy transfer efficiency of particles. Specifically, in the axial direction, the hexahedral particles have the highest mixing efficiency, and the oblate spheroidal particles with AR = 0.25 have the lowest efficiency. In the radial direction, the hexahedral particles still maintain the highest efficiency, while the spherical particles with AR = 1 have the lowest efficiency.

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This paper carries out extensive simulations of granular columns composed of frictional-pentagonal grains, collapsing on a horizontal plane. Various two-dimensional columns are used and the interparticle friction coefficient is systematically varied in a broad range of values, aiming to comprehensively highlight and universally describe the runout distance, deposition height, area of top-deposition surface, kinetic energy, and apparent friction coefficient. We show that these physical quantities observed in this work are consistent with previous findings and are affected with the degrees depending differently on the initial column aspect ratio and interparticle friction coefficient. Remarkably, we nontrivially unveil a unified power-law scaling behavior for runout distance, deposition height, area of top-deposition surface, kinetic energy, and apparent friction coefficient by defining an effective aspect ratio, inversely incorporating the complex competition between initial aspect ratio and interparticle friction coefficient. This universal power-law description may confirm a unified competition of frictional and inertial effects on geophysical mass flows, providing a better understanding of the behavior of natural hazards such as rock avalanches and landslides.

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Collapse model and unified scaling behavior of thedeposition morphology and collapse mobility.

Understanding grain fragmentation in fault gouge is essential for capturing the mechanical behavior and evolution of fault zones under shear. In this study, we present a 2D Discrete Element Method (2D-DEM) framework that simulates comminution using angular, breakable grains, overcoming limitations of traditional models based on spherical particles. Our approach incorporates realistic fracture mechanics and grain geometries to better represent microstructural evolution during shearing. A series of numerical experiments, including Brazilian, oedometric, and shear tests, were conducted to calibrate the model and examine the roles of grain strength, friction, and Young’s modulus. The simulations reproduce key numerical observations such as strain localization, force chain evolution, and grain rounding through chipping mechanisms. Results show that the model captures the onset and progression of fragmentation, as well as its impact on fault strength and mechanical stability. A comparison with a dedicated laboratory experiment is provided. This work provides a robust numerical tool for studying fault gouge behavior and lays the foundation for future studies exploring the influence of initial grain size and material properties on fault mechanics.

Graphical Abstract

A robust numerical tool with fragmentation for studying fault gouge behavior

Three phase granular mixes consisting of soft (recycled tyre aggregates) and rigid (crushed rock) materials bonded with flexible or semi-flexible binders are gaining momentum as a solution for waste tyre crisis. Experimental works suggest macro-scale responses of these mixes are dependent on both soft and binder content in the mix. However, the underlying mechanisms governing these responses remain unclear. The present study focuses on exploring the contact mechanics of three-phase granular mix composed of soft and rigid particles bonded with flexible binders. Conventional contact laws for particle-scale study of bonded materials have limitations, as they are primarily formulated for rock-like materials. Therefore, advanced contact models are necessary to understand the macroscopic behaviour of these three-phase granular media. Given the flexible nature of the binder in this study, the ‘softbond model’ is employed to simulate the behaviour of the three-phase mix. However, the softbond model overestimates the strength of these granular mixes by assuming identical contact stiffness for bonded and unbonded conditions. The compressibility of unbonded contact is significantly different from the bonded contacts due to the presence of soft particles. To address this contact stiffness disparity, the softbond model is enhanced to better simulate the contact behaviour of these mixes. The new modified model can accurately predict the constrained modulus evolution against axial stress as found in one-dimensional compression experiments in the literature. Microstructural analysis of these mixes provides valuable insights into bond breakage, force distribution, and strong force chains. Bond breakage alters the force chain distribution in these mixes, and despite the presence of bonds with higher stiffness, unbonded contacts begin to dominate the force chain. The variation in microstructural properties indicates that the behaviour of these mixes depends not only on the binder content but also on the proportion of soft particles in the mix. The new microstructural understanding will ultimately help proposing better hypothesis to explain the complex material response for different soft and binder contents under applied loading.

Firework oxidizer powders are inorganic compounds that supply active oxygen within pyrotechnic systems. Due to their fine particle size and tendency to leak during handling, accurately simulating their flow behavior is essential for optimizing powder-filling equipment. However, limited research exists on their discrete element method (DEM) parameters, and many key contact properties remain undocumented. This study systematically calibrated six critical DEM contact parameters for firework oxidizer powders-particle density, elastic modulus, Poisson’s ratio, static friction coefficient, rolling friction coefficient, and shear modulus-using a combination of experimental measurements and numerical simulations. The Hertz-Mindlin contact model was adopted, and a Plackett–Burman design was used to identify the most influential parameters affecting the static angle of repose. Results showed that the static friction coefficient, rolling friction coefficient, and shear modulus had the most significant effects. Their optimal ranges were further refined via steepest ascent testing. A Box-Behnken design was employed to develop a regression model for the angle of repose, yielding final calibrated values of 0.288, 0.035, and 65.076 MPa for the three dominant parameters, respectively. These parameters were then applied in DEM simulations of powder flow involving a T-shaped scraper, which pushes the powder toward the dosing holes, allowing it to fall under gravity. Physical filling tests demonstrated that powder leakage was effectively controlled when the scraper speed did not exceed 0.3 m/s. This study provides valuable insights for equipment optimization and offers a reliable reference for DEM-based modeling and design of firework oxidizer powder handling systems.

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Baffles is an effective means of preventing and controlling rock avalanches, but the blocking performance of particle splashing in rock avalanches is not yet clear. And the particle shape has a significant impact on the dynamic characteristics of particle splashing. This study reviewed the research and application of baffles in avalanches, debris flows, and rock avalanches and adopted Discrete Element Method (DEM) software to conduct numerical simulation experiments, comparing the motion characteristics and energy evolution of particle splashing in rock avalanches with various shape particles under baffles protection, analyzing the motion mode of particle splashing, and assuming the internal mechanism of shape affecting particle splashing. The results showed the splashing particle mass, peak particle velocity, maximum splashing distance, and average splashing height of particles with different shapes are roughly linearly and negatively correlated with the particle shape factor with the maximum difference of 2166.0, 25.2, 98.5 and 10.2% respectively, while the relative energy consumption of particles during the sliding motion is roughly linearly positively correlated. There are two modes of particle splashing, inter-particle collision and deposit body collision, changing the motion direction and velocity of splashing particles. The differences among the splashing motion characteristics of particles with different shapes result from the distinct collision modes induced by the collision angle, velocity, and spin, which can influence the direction and velocity of particles after collision. Based on our previous studies in baffle-net structures, the optimal engineering height of protective net was confirmed as 0.94 times of baffle height for the interception of particle splashing in rock avalanches reducing 86.5% splashing particles than the baffles with the protective net of 0.5 times baffle height, as reference for particle splashing prevention.

This study focuses on the dynamics of granular flow similar to landslides and avalanches using a controlled environment. The work experimentally investigates the gravity-driven granular flow to mitigate the threat of landslides using both dry and wet surfaces. For dry conditions, different friction surfaces are used, while for wet conditions, different water levels are employed. Granular flow is studied for different angles of inclination and compared using spread area, run-off distance, and height of the terminated flow. At lower angles, the run-off distance experiences a reduction of approximately 50% for the frictional surface. The study reveals the formation of distinct deposit patterns on the ground table, ranging from a mustache-like structure to a tongue-shaped structure for low to high friction conditions respectively. Additionally, it’s observed that a wider spread area is formed under both low and high friction conditions, but a narrow spread is observed for the medium friction surface. For medium friction, there exists a balance between energy dissipation due to particle –surface collisions and particle momentum loss affecting the spread formation. The granular flow entering the water is divided into a leading wave at the top surface and a turbidity current, which is propagated along the bottom surface of the water. The medium friction surface presents a viable strategy for enhancing resilience against landslide. Additionally, the potential use of water bodies as a control measure shows promising results. These findings contribute to a deeper understanding of granular flow behavior and the development of effective control measures in landslide-prone regions.

Graphical abstract

Deposition pattern and spread area comparison

This article explores the hysteretic behavior and the damping features of sheared granular media using discrete element method (DEM) simulations. We consider polydisperse non-cohesive frictional spherical particles, enclosed in a container with rigid but moving walls, subjected to a cyclic simple shear superimposed on a confining pressure. The mechanical response of the grains is analyzed in the permanent regime, by fitting the macroscopic stress–strain relation applied to the box with a Dahl-like elasto-frictional model. The influence of several parameters such as the amplitude of the strain, the confining pressure, the elasticity, the friction coefficient, the size and the number of particles are explored. We find that the fitted parameters of our macroscopic Ansatz rely qualitatively on both a well-established effective medium theory of confined granular media and a well-documented rheology of granular flow. Quantitatively, we demonstrate that the single degree-of-freedom elasto-frictional reduced model reliably describes the nonlinear response of the granular layer over a wide range of operating conditions. In particular, we show that the mechanical response of a granular slab under simple shear depends on a unique dimensionless parameter akin to an effective Coulomb threshold at low shear/high pressure. Furthermore, exploring higher shear/lower pressure, we evidence optimal damping at the crossover between a loose unjammed regime and a dense elastic regime.

Graphical Abstract

Optimizing vibrations mitigation by confined elasto-frictional particles with a DEM-based Dahl-like reduced model

The principle of detailed balance (DB) states that every kinetic transition in a system with many micro-states, (mu ), is balanced, on average, with the opposite transition, (mu _ileftrightharpoons mu _j). The current perception is that, on the scale of the most elementary degrees of freedom, DB is satisfied only in equilibrium systems, although a rigorous proof exists only for thermal systems. It is believed that, on this scale, non-equilibrium steady states can only be balanced by cycles, such as (Arightarrow Brightarrow Crightarrow A). We report here experiments on a family of out-of-equilibrium quasi-statically cyclically sheared granular systems, which appear to show robust DB. We then analyse in detail the concept and interpretation of DB and show that our systems are the exact equivalent of chemically reactive systems in thermal equilibrium. We therefore conclude that our non-equilibrium systems do indeed satisfy this principle. We further study the approach to DB as a function of system size and time. Given the significant progress to which this principle has led in equilibrium systems, these observations may pave the way for better models of the dynamics and statistical mechanics of these and potentially other non-equilibrium systems.