Granular Matter最新文献

The evolution of particle breakage in sand with different mineralogy under cyclic shear loading is explored. The work focuses on the impact of factors such as the cyclic stress ratio (CSR), confining pressure, amplitude of shear stress and number of cycles. Direct shear tests were carried out at increasing stress levels and numbers of cycles. Specimens were recovered after each test and subjected to dynamic image analysis (DIA), which permitted capturing not only changes in the particle size distribution (PSD) but also evolution of particle shapes for approximately 4% of all particles tested at a fine scale. Detailed analysis of the PSD curve combined with an analysis of the evolution of particle shapes, demonstrates how soil gradation evolves during cyclic loading and how this impacts the mechanical behavior of sand. The study presents a novel framework for predicting particle breakage in sands subjected to cyclic loading using readily available stress–strain data, eliminating the need for complex and costly fine-scale particle size analyses. The method adapts the existing Loading Intensity (L_I) framework, incorporating an efficiency factor that accounts for the diminishing effect of cyclic loading as the number of cycles and cyclic stress ratio increase. A strong correlation was established between the Particle Partition Potential (P₃) and Hardin's Breakage Index (Br), enabling the prediction of particle breakage with generally small errors (< 2%) and remarkable accuracy at higher breakage levels. This framework offers a reliable and practical tool for assessing soil degradation under cyclic loading.

Graphical Abstract

The flow of granular particles is characterized by particle-size sorting called “inverse-grading transport”, and it is important to carry out a series of basic studies on the inverse-grading transport behavior of coarse particles for disaster prevention and mitigation and related theoretical study of particle separation. In order to investigate the influence of shape on the inverse-grading transport characteristics of a single coarse particle, a series of cyclic shear tests were conducted utilizing 3D sand printing technology alongside a self-constructed two-dimensional cyclic shear test device. Using the YOLO target detection algorithm, the inverse-grading transport trajectory, rotation characteristics, and local structure were analyzed. A kinematic equivalent analysis method classified transport behaviors of coarse particles, revealing correlations between single coarse particles of different shapes and macroscopic segregation patterns. The results indicate that: (1) Single coarse particles slowly ascend from the bottom center, with their vertical transport rate increasing until they reach the surface. (2) Particle shape significantly affects the inverse-grading transport of single coarse particles. The closer the coarse particles are to the free surface, the lower is the local volume fraction above them, while the volume fraction below them increases. (3) The inverse-grading transport of coarse particles is significantly correlated with their own rotation and with changes in the local structure of the granular medium around them. Our experiments thus show that the inverse-grading phenomenon of landslide-debris flow is mainly caused by changes in the local structure of the granular medium around the coarse particles.

Coral gravel soil, coral sand, and coral-derived mixed soil are common construction and building materials in coastal areas and islands, which are characterized by biologically formed fossilized sediments such as coral gravels (CG). The unique pore structures and irregular particle shapes of CG result in high porosity and significant breakage potential, influencing their mechanical properties and hydraulic behavior in engineering practice. However, the evolution of pore structures in CG during particle breakage and its impact on permeability remains poorly understood. This study employs a multi-scale analysis method, combining X-ray computed tomography and seepage simulations, to quantitatively investigate the evolution of pore structure and permeability in four types of CG: rod-shaped, branchlet, massive, and flaky during the particle breakage process. Test results categorized the internal pores of particles into intraparticle, blind, and through pores and demonstrated that as particle breakage occurs, intraparticle and blind pores decrease while through pores increase, leading to enhanced permeability. In the branchlet and flaky CG samples, intraparticle porosity decreases from 74.43% and 72.88% to 22.32% and 12.2%, respectively, while through porosity significantly increases with the progression of particle fragmentation. In addition, an exponential correlation between through porosity and permeability is established, supported by a regression model. This study proposes a framework for understanding multiscale pore evolution during particle breakage by analyzing changes in porosity and seepage behavior, improving the comprehension of the pore structure and hydraulic performance of fragmented granular materials. It provides valuable insights for the design and performance prediction of biological materials in offshore and geotechnical engineering applications.

Graphical Abstract

A computational framework that utilizes the discrete element method (DEM) was developed to conduct triaxial compression tests on lithium-based pebbles. In addition, the process for determining optimized simulation parameters for performing stress-controlled numerical hydrostatic and triaxial compression tests using LIGGGHTS has been outlined. This framework was employed to study the influence of the polydispersity of pebbles on the Drucker-Prager (D-P) parameter, which is the friction angle. The study revealed that the effect of polydispersity, measured by the polydispersity index ((lambda )) was negligible, as the value of friction angle ((beta )) remained constant at (approx 29^circ ) for (lambda le 0.5). However, in highly polydisperse samples with (lambda =0.8), (beta ) increased to (approx 31^circ ). Additionally the dilatancy angle ((psi )) decreases as (lambda ) increases. The difference between (beta ) and (psi ) increases with (lambda ), and thus, the associative flow rule is not suitable for highly polydisperse samples. To conduct a more detailed analysis of the mesoscopic mechanics, three parameters based on the mean number of contacts, which governed the microstructural similarity of the sample and the extent of particle participation, were examined. Additionally, two local parameters based on Voronoi tessellation were investigated. These parameters highlight how changing (lambda ) influences the local deformation and characterizes the local structural variation in the granular samples. In particular it was found that, the participation of particles to the total deformation was higher in samples with high polydispersity.

The streamwise and vertical wind speed fluctuations predicted by a multi-scale wind speed fluctuation prediction model were introduced into a one-dimensional windblown sand model to simulate windblown sand under different friction velocities and particle sizes. And the scaling relation of major physical statistics was studied. Our results show that the mean and standard deviation of sand transport rate and average saltation velocity are proportional to u_*³/g. The mean and standard deviation of average saltation length are proportional to u_*²/g. The mean and standard deviation of speed and angle distribution of lift-off sand particles over time also need to consider friction velocity (u_*) and particle size. Here, g represents the gravitational acceleration. Empirical representations of mean and standard deviation of these five statistics were obtained based on the scaling relation, and the variations of coefficients with dimensionless particle size were mainly considered. Empirical representations provide a basis for simulating windblown sand over a large spatial range and introducing turbulent motions in the study of aeolian landforms.

Graphical abstract

Distribution of the mean of lift-off speed under different frictional wind speeds.

In order to elucidate the lubrication mechanism of dense granular flows under vibrational conditions, the force chain characteristics of dense particulate systems were systematically investigated. A parallel inter-plate model for dense granular flow lubrication was established using the discrete element method (DEM). Subsequently, the effects of vibration frequency and amplitude on the dynamic fluctuation, load-bearing capacity, distribution, and overall directionality of force chains were analyzed in detail. When the frequency is below mid-frequency (10,000 Hz), the dynamic fluctuations of force chains in the same-phase direction are weaker, and the overall fluctuation properties of the force chains tend to stabilize. The distribution and load-bearing capacities of weak force chains increase, and the directionality of the total force chain gradually aligns with the x-axis. Conversely, when the frequency exceeds 10,000 Hz, stronger fluctuations of force chains occur in both phases, leading to more intense overall fluctuations. Simultaneously, the distribution and load-bearing capacities of weak force chains decrease, and the directionality of the total force chain shifts gradually from the x-axis to the y-axis. As the vibration frequency and amplitude increase, the distribution and load-bearing capacities of weak force chains reach their maximum values more rapidly under mid-frequency conditions. Additionally, the average and fluctuating velocities increase continuously at low frequencies, peak at mid-frequencies, and then decrease at high frequencies. The primary contribution of this study is to establish a theoretical foundation for the lubrication mechanism of granular flows under vibrational influences.

Graphical abstract

The energy generated by the explosion of the arc-shaped charge spreads relatively evenly in all directions, when the density of the explosive charge is uniform and compact, the energy released by the reaction is more concentrated and stable, thereby achieving a more efficient explosion effect. By establishing a simulation model of the powder compaction process, the powder displacement, relative density variation, and stress distribution of the formed explosive charge during its compaction process were analyzed, and the effects of process parameters such as pressing rate, pressure holding time, number of compaction times, and mold roughness on the density of arc-shaped charges were studied. The results show that: The stress of the formed explosive column is mainly concentrated at the bottom end of the upper die punch; From top to bottom, the relative density of the explosive charge shows a wave-like decreasing trend, and the minimum density appears at the interface between the lower die punch and the female mold; In the complete compaction process, the influence of different compaction rates and pressure holding times on the density of the formed explosive column does not follow an obvious pattern. However, it is certain that the pressure holding process is crucial, as it can enhance the density and uniformity of the explosive column; With an increase in the number of compaction times, the relative density of the explosive charge gradually increases; Reducing the roughness of the mold can make the density distribution of the formed explosive column more uniform.

Graphical Abstract

To enhance the mechanical seeding performance of sunflowers and improve seed delivery efficiency, this study measured the adhesion force and angle of repose between sunflower seeds and soil at varying moisture contents through physical experiments. A discrete element model (DEM) was developed to analyze the interaction between soil and sunflower seeds, with the angle of repose as the response variable. The Plackett–Burman (PB) Design was utilized to identify significant influencing factors, and a combination of Response Surface Methodology (RSM) and Feedforward Neural Network (FNN) was employed for optimization. The results indicated that FNN provided higher prediction accuracy and stability. Specifically, at soil moisture contents of 10%, 14%, 18%, and 20%, the static friction coefficients were 0.67, 0.74, 0.66, and 0.63; dynamic friction coefficients were 0.45, 0.46, 0.38, and 0.36; surface energies were 1.18, 2.11, 3.6, and 4.99; and angles of repose were 37.58°, 40.22°, 41.56°, and 41.81°. The absolute errors from physical experiments were 0.59%, 0.6%, 0.82%, and 0.46%, respectively. These findings demonstrate that the FNN model can effectively predict simulation parameters for sunflower seeds and soil under varying moisture conditions, providing a theoretical foundation for field crop seeding processes.

The mechanical behaviour of dry granular materials is strongly influenced by the effects of cyclic accelerations caused by vibrations. While the shear strength is temporarily reduced for the duration of the vibration, the bulk density is permanently increased. Although these phenomena have been known for a long time, the underlying mechanisms are not yet fully understood. This work contributes to a better understanding of the compaction of dry granular materials by vertical vibrations by introducing a new dimensionless parameter in the form of the ratio of dynamic to static stresses inside the granulate. With the help of numerical simulations using the discrete element method, a parametric study shows that this stress ratio is better suited than the conventionally used acceleration ratio to characterise the essential compaction properties of the material. An analytical model for the stress distribution in the granulate is presented and employed alongside with laboratory tests to validate the numerical model. The problem is treated here in one dimension, but an extension to three dimensions is possible.

Graphical abstract

A granular sample subject to gravity, a static surcharge and vibrational accelerations (left). The dynamic to static stress ratio grows non-linearly with depth and correlates to the local void ratio after vibrations (right).

The two most commonly used methods to model the behaviour of granular flows are discrete element and continuum mechanics simulations. These approaches concentrate on the deterministic description of particle or bulk material motion. Unlike these approaches, this paper introduces an alternative model that describes the stochastic dynamics of the void spaces under the action of gravity. The model includes several key phenomena which are observed in deforming granular media, such as segregation, mixing, and an angle of repose. These mechanisms are modelled heterarchically using both spatial and microstructural internal coordinates. Key aspects of the model include its ability to describe both stable and flowing states of granular media based on a solid fraction cut-off, and the influence of particle size on flow, segregation, and mixing. The model is validated with simulations of column collapse and silo discharge.