Granular Matter最新文献

The deformation of rockfill materials is mostly caused by particle breakage and subsequent skeleton adjustment. To investigate the effect of size and strain rate on particle strength under seismic load, a series of single particle crushing tests with different sizes and loading rates were conducted. The results show that the particle strength increases with the loading rate, while the size effect on particle strength gradually weakens. Furthermore, within the framework of the weakest chain theory, the failure probability per unit volume and the spatial location distribution of microcracks are discussed, and a statistical model for quasi-static particle strength is established. The spatial location of microcracks follows a power law distribution, and there is a specific power exponent at different strain rates, so that the compound parameters of the particle volume and failure probability are gathered on a master curve determined by the weakest chain statistics. The strain rate effect reduces the failure probability per unit volume and makes the spatial location distribution of microcracks sparser.

The spreading behaviour of cohesive sand powder is modelled by Discrete Element Method, and the spreadability and the mechanical jamming are focused. The empty patches and total particle volume of the spread layer are examined, followed by the analysis of the geometry force and jamming structure. The results show that several empty patches with different size and shapes could be observed within the spread layer along the spreading direction even when the gap height increases to 3.0D₉₀. Large particles are more difficult to be spread onto the base due to jamming, although their size is smaller than the gap height. Size segregation of particles occurs before particles entering the gap between the blade and base. There are almost no particles on the smooth base when the gap height is small, due to the full-slip flow of particles. The difference of the spread layer and spreadability between the cases with rough and smooth base is reduced by the increase of the gap height. An interesting correlation between jamming effect and local defects (empty spaces) in the powder layer is identified. The resistance to particle rolling is important for the mechanical jamming reported in this work. The jammed particles with a larger size ratio tend to be more stable.

The study of gravity retaining wall supporting cohesionless soil is performed using the discrete element software Particle Flow Code (PFC), with emphasis on three-dimensional analysis at the grain scale, covering the transition from the initial state to the active state. The soil particles are represented by spherical balls and the rolling resistance linear contact model is used to include the shape effect. The rigid balls are connected by linear parallel bond contact model with a specified strength and stiffness to replicate the physical characteristics of retaining wall. The domain size is reduced by using high g criteria and scaling laws to enhance computational efficiency. The earth pressure coefficient obtained from the present study is compared with the existing analytical and experimental solutions. It is concluded that widely used Coulomb and Rankine methods underestimate the active earth pressure. The total earth thrust acting on the gravity wall is 67.3 kN/m acting at 0.301H above the wall base. The initial state shows a decrease in average coordination number from 5.0 to 3.3 at wall top indicating the debonding of grains and simultaneous decrease in density. In addition, the force chain distribution, porosity, lateral displacement, and axial displacement are investigated. A correlation between the earth pressure coefficient and lateral displacement is also established. The discrete analysis provided valuable insights into the particle-level mechanisms underlying the overall behavior of the retaining wall, contributing to a better understanding of its continuum response.

Graphical Abstract

In DEM simulations of triaxial tests, modelling a flexible lateral membrane is crucial and challenging. It is essential for the correct application of a uniform lateral pressure and for an accurate measurement of sample volume. Here, we introduce a membrane made of triangular facets, and model it as a continuum; we then compare this approach with a well-established method that uses a layer of bonded spheres. With either method, it is also possible to assess the additional stress applied by the membrane as it deforms, i.e. the difference between the stress applied at the boundary and the actual stress within the sample. It is shown that this difference has two origins: the tension developed in the membrane, as it deforms; and the curvature of the membrane, since this causes a vertical component of the confining pressure which can be significant. These findings may be used to inform and improve the membrane correction commonly used in experiments, where similar effects occur.

Graphic abstract

We modified the inelastic bouncing ball model (IBBM) to account for the role of collision time ({tau }_{c}) in defining the dynamics of vertically vibrated confined granular systems. Although ({tau }_{c}) was surmised to be consequential for dissipative systems, previous studies on the accuracy of IBBM did not formally incorporate ({tau }_{c}) as a dynamical variable of the model, focusing instead on other factors during flight, such as air friction. We utilized the discrete element method (DEM) to study the role of ({tau }_{c}) in the granular dynamics, and to cross-validate the efficacy of our reformulation of IBBM to account for the effect of collisions. When the ({tau }_{c}) value is greater than that of ({t}_{0}), which is the first instance that the container acceleration exceeds the gravitational acceleration (g), the time-of-flight decreases, and the location of the bifurcation point shifts in the bifurcation diagram (time-of-flight versus dimensionless acceleration). We model ({tau }_{c}) as representing the range of uncertainty in the occurrence of ({t}_{0}). Assuming a separation of timescale between the dynamics of the collision between the center-of-mass (CM) of the granular system and the container, and the time-of-flight of the CM itself, we propose a supporting but separate model for the dependence of ({tau }_{c}) on Γ. The time-of-flight duration is determined when ({tau }_{c}) is known in the modified IBBM that now produces bifurcation diagrams which are in closer agreement with the DEM simulation results.

Graphical Abstract

Producing dense and homogeneous powder layers with smooth free surface is challenging in additive manufacturing, as interparticle cohesion can strongly affect the powder packing structure and therefore influence the quality of the end product. We use the Discrete Element Method to simulate the spreading process of spherical powders and examine how cohesion influences the characteristics of the packing structure with a focus on the fluctuation of the local morphology. As cohesion increases, the overall packing density decreases, and the free surface roughness increases, which is calculated from digitized surface height distributions. Local structural fluctuations for both quantities are examined through the local packing anisotropy on the particle scale, obtained from Voronoï tessellation. The distributions of these particle-level metrics quantify the increasingly heterogeneous packing structure with clustering and changing surface morphology.

The mechanical properties of rockfill materials are not only influenced by microscopic factors such as particle morphology and gradation, but also closely related to different loading stress paths. It is of great significance to study the microscopic mechanical properties of rockfill materials under different stress paths for revealing the macroscopic mechanical properties as well as the microscopic deformation and failure mechanisms of rockfill materials. In this paper, based on the results of triaxial tests, a series of numerical triaxial simulation tests under different stress paths were carried out using the discrete element particle flow method, and the deformation, strength change rules, and fine structure evolution mechanism under three stress paths were explored. The results demonstrated that there were significant differences in the effects of stress paths on the stress–strain and strain-volume change characteristics of the rockfill materials. Stress paths exhibited little effect on the strength characteristics. The anisotropy of strong contact number and strong contact force was the microscopic source of macroscopic strength. The contact situation between the particles was the main microscopic factor affecting the macroscopic deformation. The intrinsic mechanism of macroscopic deformation properties could be revealed by the average coordination number and porosity. The stress path affected the growth rate of the number of bond failures and the total number of failures. The relationship between macroscopic mechanical properties and microstructural evolution under different stress paths was also discussed. The findings can provide meaningful insights into the deformation control and stability analysis of rockfill engineering.

Graphical Abstract