Granular Matter最新文献

In order to eliminate the undesirable characteristics of carbonaceous mudstone roadbed fillers, cement and fly ash are used to modify the pre-disintegrated carbonaceous mudstone, and the stress–strain relationship of pre-disintegrated carbonaceous mudstone before and after modification are analyzed by a series of conventional unconsolidated undrained triaxial compression tests at different confining pressures and different ages. Based on the microscopic modification mechanism of carbonaceous mudstone and the concept of binary medium model, the products from hydration reaction of pre-disintegrated carbonaceous mudstone, cement, and fly ash are regarded as bonded elements, and the pre-disintegrated carbonaceous mudstones without hydration reaction are regarded as frictional elements, and the binary medium model of modified pre-disintegrated carbonaceous mudstone is established. The results show that the stress–strain curve of pre-disintegrated carbonaceous mudstone is strain-hardening type, and the stress–strain of pre-disintegrated carbonaceous mudstone modified by fly ash and cement is strain-softening type, and the mechanical properties of modified pre-disintegrated carbonaceous mudstone are significantly improved. The deformation and damage mechanism of modified carbonaceous mudstone is investigated by applying the concept of binary medium model from a mesoscopic perspective, and the stress-bearing mechanism of bonded elements and frictional elements in external loading and stressing processes are analyzed. Finally, the measured data reveals that the binary medium model can simulate both the stress–strain softening characteristics of modified pre-disintegrated carbonaceous mudstone and the stress–strain hardening characteristics of organic material-modified expansive soils reasonably well.

Granular materials may undergo static liquefaction under undrained shearing, which is related to many natural hazards, such as landslides. Despite great efforts, the overall process of static liquefaction remains largely unclear. Numerical undrained shear tests on granular assemblies are performed using the discrete element method, and network-based methods are introduced to investigate the evolution of the contact network. The occurrence of static liquefaction is attributed to the collapse of the contact network induced by contact loss. The weak subnetwork is broken before reaching the liquefaction point, while the strong contact subnetwork remains relatively unchanged. The failure of the strong subnetwork is further investigated by the mechanical features of two important mesoscopic structures, namely force chains and contact loops. The buckling events with buckling ratio exceeding the envelope line and the transition from small loops to large loops significantly destroy the stability of force chains, which causes the failure of force chains and eventually the occurrence of static liquefaction. The relationship of macroscopic stress with microscopic and mesoscopic structures is also identified. The evolution of node degree and global efficiency versus macroscopic stress presents a two-stage development mode, and the buckling events accelerates the transition of the development mode. Our analysis elucidates the occurrence of static liquefaction from microscopic and macroscopic perspectives, which are essential for better prediction and modeling of the catastrophic failures under undrained loading path of granular materials.

Graphical abstract

A snow slab avalanche releases after failure initiation and crack propagation in a highly porous weak snow layer buried below a cohesive slab. While our knowledge of crack propagation during avalanche formation has greatly improved over the last decades, it still remains unclear how snow mechanical properties affect the dynamics of crack propagation. This is partly due to a lack of non-invasive measurement methods to investigate the micro-mechanical aspects of the process. Using a DEM model, we therefore analyzed the influence of snow cover properties on the dynamics of crack propagation in weak snowpack layers. By focusing on the steady-state crack speed, our results showed two distinct fracture process regimes that depend on slope angle, leading to very different crack propagation speeds. For long experiments on level terrain, weak layer fracture is mainly driven by compressive stresses. Steady-state crack speed mainly depends on slab and weak layer elastic moduli as well as weak layer strength. We suggest a semi-empirical model to predict crack speed, which can be up to 0.6 times the slab shear wave speed. For long experiments on steep slopes, a supershear regime appeared, where the crack propagation speed reached approximately 1.6 times the slab shear wave speed. A detailed micro-mechanical analysis of stresses revealed a fracture principally driven by shear. Overall, our findings provide new insight into the micro-mechanics of dynamic crack propagation in snow, and how these are linked to snow cover properties.

Graphical Abstract

This paper investigates the response of Ottawa sand to cyclic loading using virtual oedometer tests and the level-set discrete element method. We study both the macroscopic and the micromechanical behavior, shedding light on the grain-scale processes behind the cyclic response observed in crushable sand, namely stress relaxation under strain control and ratcheting under stress control. Tests without particle breakage first show that asymmetrical frictional sliding during loading-unloading induces these cyclic-loading effects. Then, tests considering particle breakage reveal more pronounced stress relaxation and ratcheting, which decrease in rate over cycles, accompanied by increased frictional sliding and reduced particle contact forces. It is found that the broken fragments unload the most and promote an enhanced cushioning effect. These micromechanical processes contribute to a decrease in breakage potential as the cycles progress, implying that cyclically loaded materials may become more resistant to breakage when compared to the same material loaded monotonically at the same strain level. These new insights highlight the main contributions of the present work, factoring in real particle shapes from 3D X-ray tomography and notably contributing to the existing literature on the topic, where most studies rely on idealized particle shapes and rarely consider crushable grains.

The corn variety “Zhenghong 507”, which is widely cultivated in hilly and mountainous areas of Southwest China, was assigned as the research object. The discrete element model of the mid-section of the corn ear that can be threshed was established by integrating the Hertz-Mindlin with the bonding V2 contact model, and the crucial bonding parameters were simulated and calibrated. With the measured normal threshing force (6.34 N) and tangential threshing force (4.75 N) of a single kernel as target values, the parameters of bonding characteristics between the kernel and the cob of corn ear were screened and optimized for significance via the Placket-Burman test, steepest ascent test, and the central composite design. The results indicate that the optimal parameter combinations for the normal stiffness and shear stiffness per unit area, normal strength, shear strength, contact radius between kernels, contact radius between cobs, and bonded disk scale were 3.4 × 10⁸ N·m⁻³, 2.238 × 10⁸ N·m⁻³, 0.6 × 10⁶ Pa, 0.364 × 10⁶ Pa, 1.87 mm, 16.5 mm and 1.321. Finally, the accuracy of the corn ear DEM model was validated by comparing the simulation to the physical test using the threshing rate as an evaluation index combined with the quality distribution of kernels after threshing.

Graphical Abstract

Calibration and validation of a corn ear bonded model.

The spring-dashpot-slider is a common way to include solid friction for discrete element method simulations of granular matter. However, the most popular model that is currently in use has a number of problems, including the spontaneous creation of energy. The main cause for these problems is the discontinuous evolution of the spring displacement. In this paper, we derive a differential equation for the displacement that yields a continuous time evolution, that fixes the problems of the discontinuous model and is simpler to implement.

Suffusion severely threatens the stability of granular soils supporting infrastructure. While the geometric conditions of the granular soils are intrinsic to their mechanical behavior, their influences on the suffusion characteristics have not been fully understood. This study presents a micro-macro investigation of the suffusion characteristics of gap-graded soils with different fines contents and particle size ratios using the coupled computational fluid dynamics and discrete element method (CFD-DEM). The severity of the suffusion was quantified by both the loss of fines by mass and the volumetric deformation of the specimen. Meanwhile, Voronoi tessellation and weighted Delaunay method were employed to analyze the evolution of pore structures. The evolution of different contact types was used to analyze the rearrangement of the specimen skeleton. The simulation results show that suffusion is aggravated under otherwise identical conditions with an increase in particle size ratio and fines content. The particle size ratio influences the local pore difference between coarse and fine particles, while the fines content influences the fines’ contribution to the soil skeleton. The evolution of the distribution of local void fractions, constriction size distributions, stress-reduction factors, different types of coordination numbers, and different types of contact forces provides useful insights into the microscopic mechanism of the suffusion process.