Granular Matter最新文献

Cemented granular materials, as unique granular substances possessing both permeability and load-bearing characteristics, have found extensive applications in chemical catalysis and geological engineering, and other fields. Given the significant impact of skeleton particle shape on the mechanical properties of cemented granular materials, this paper proposes a bonded polyhedral discrete element method adaptable to arbitrary skeleton particle shapes. Within this method, the adhesive surface is constructed from the contact geometry, and the interaction between particles of different shapes is described by employing an energy-conserving contact model based on strain energy density. The spring-damping model and bilinear constitutive model are utilized to characterize the elastic behavior and damage fracture behavior of cement, respectively. Moreover, the influence of skeleton particle shape on cemented granular materials is elucidated through both mesoscopic and macroscopic analyses using the proposed model. Mesoscopic results indicate that the area of the adhesive surface is a critical factor influencing the destructive force of bonding units. Variations in particle shape cause particles with identical volume and density to form adhesive surfaces with differing shapes and areas under the same conditions, leading to varied destructive forces in the bonding units. The macroscopic results reveal that both the sphericity and aspect ratio of the skeleton particles impact the strength of the cemented granular material. This effect predominantly arises from the differences in the coordination number of the accumulation bodies formed by skeleton particles of varying shapes.

Graphical Abstract

Particle size and shape are critical for characterizing gravel soils, typically quantified through the analysis of two-dimensional (2D) images and three-dimensional (3D) models. However, obtaining 3D particle features can be challenging and time-consuming, often resulting in low efficiency. To address this, many researchers have recently attempted to estimate 3D morphological features from 2D data by establishing relationships between 2D image features and their 3D counterparts. Nevertheless, these methods generally focus on specific particle categories within a limited region, limiting their broader applicability. In response, this study proposes a method for acquiring extensive morphological data for gravelly soils in both 2D and 3D formats through multiple collections. Additionally, it introduces and validates a practical approach for deriving 3D information from 2D image analysis, offering a series of new equations that are compared with previously published models. The result demonstrates that 3D morphological features, including particle size and shape, can be effectively estimated from 2D data using linear and polynomial correlation equations.

This paper examines the simultaneous impact of strain rate, aggregate fragmentation, and free water on the dynamic behavior of concrete in mesoscale uniaxial compression conditions. A concrete specimen measuring 50 × 50 mm² and having a low porosity of 5% was the subject of extensive two-dimensional (2D) dynamic investigations (that is, a research limitation). Its mesostructure was based on laboratory micro-CT images. Concrete’s fracture patterns, strength, brittleness, and fluid pressure distributions were all investigated. A mesoscopic pore-scale hydro-mechanical model based on a unique fully coupled DEM/CFD technique with breakable aggregate particles was utilized to study the behavior of partially or fully saturated concrete. A four-phase material comprising aggregate, mortar, ITZs, and macropores was used to replicate concrete. Groups of small spherical particles were used to simulate the fragmentation of aggregate particles with various shapes and sizes, allowing for intra-granular fracturing among them. A network of fluid channels was assumed in a continuous region between discrete elements. A two-phase laminar compressible fluid flow (air and water) in pores and cracks was suggested for wet concrete. The accurate volumes of pores and cracks were computed for tracking the liquid/gas content. Dynamic numerical compressive tests were performed with strain rates ranging between 1 1/s and 1000 1/s. Strain rate, aggregate fragmentation, and free water flow increased the dynamic compressive strength. Because of free water confinement in pores and cracks, the pore fluid pressures retarded a fracture process, enhancing the concrete dynamic strength.

Asteroid impacts are a significant area of study in astronomy; however, the specific impact properties of binary systems at close distances have not been extensively explored. This study employs a dual approach, combining low-velocity impact experiments with numerical simulations, to investigate the dynamic characteristics of binary projectiles at varying separation distances. Special focus is placed on how the initial separation distance affects the repulsion and attraction phenomena of the projectiles within granular media. Empirical evidence shows that smaller initial separation distances lead to significant repulsion between projectiles upon impact. Once a specific separation distance is reached, the binary projectiles exhibit attractive behavior post-impact. Quantitative simulations clarify the observed repulsive and attractive phenomena by considering the force chain, thereby providing a deeper understanding of the dynamic impact process.

Graphical Abstract

The strength-displacement behaviour of soil-structure interface should be carefully considered during slope stabilisation using soil nail. Experimental evidences have demonstrated that tangential displacement between the soil and structure could deteriorate the microstructure within the interface, resulting in a strength degradation of the soil-structure system. To capture such responses, an elastoplastic model is developed by adopting particle probabilistic entropy to characterise the evolution of particle rearrangement within the interface, where a microstructure-dependent plastic flow rule and a kinematic hardening law are proposed. The capability of the model is verified by simulating a series of interface direct shear tests, where the normal-dilatancy response and strain softening of the interface under low normal stress as well as the distinct normal-contraction under cyclic loads are well simulated. Then, the model is further implemented through FRIC subroutines for finite element (FE) simulation of the pull-out tests on a soil–nail under different overburden pressures. It is found that the FE model can reasonably simulate the pull-out behaviour of a soil nail. The stress and strain fields around the soil nail as well as the pull-out force and displacement response can be reproduced.

Graphical Abstract

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.