Three-dimensional Discrete Element Research Articles

A comprehensive numerical investigation of multi-scale particle shape effects on small-strain stiffness of sands

The effects of multi-scale particle shape characteristics on the small-strain stiffness of granular soils remain controversial. This study revisits this topic using well-calibrated three-dimensional discrete-element simulations incorporating a particle roughness–embedded contact model and particles of realistic shape. Based on the numerical simulation results, the multi-scale particle shape effects on the small-strain stiffness of sands and the magnitude of Hardin's equation parameters are systematically investigated, and the underlying micro-mechanisms are also thoroughly explored. Results indicate that the small-strain stiffness increases with the increase of particles’ overall irregularity due to the increased mechanical coordination number, but decreases with the increase of particle surface roughness because of the decreased contact normal stiffness. In addition, the constant term parameter and void ratio term parameter of Hardin's equation increases and decreases linearly, respectively, with the particles’ overall regularity, but reduces and grows with the particle surface roughness, respectively. Furthermore, the stress exponent is almost unchanged with the particles’ overall regularity, but increases with the particle surface roughness, which determines the relative proportions of contacts under asperity-dominated, transitional and Hertzian stages. The study helps to advance the cross-scale understanding of multi-scale particle shape information in relation to small-strain stiffness of sands.

Research on creep modeling and simulation of cotton fiber assembly based on the three-dimensional discrete element

In order to analyze the mesoscopic contact characteristics of cotton fiber creep, this paper uses the meso-particle discrete element method to establish a cotton fiber assembly creep microscopic model based on the existing cotton fiber creep experiment. The microscopic parameters of the Particle Flow Code contact constitutive model are calibrated according to the macroscopic mechanical indexes of cotton fiber creep obtained from experiments, and the creep strain of the cotton fiber in the instantaneous creep stage under different compressive stresses and the contact force between particles are analyzed. According to the time–temperature equivalence principle, the calculation efficiency of the viscoelastic discrete element model is optimized, which improves the time scale of the simulation test. The results show that the numerical simulation results accord with the variation characteristics of the experimental creep variables, which verifies the feasibility of the method in this paper. The research results can provide a certain reference for the deeper study of cotton fiber creep characteristics and its mechanism.

Discrete element modelling of mechanical response of crumb rubber-modified asphalt pavements under traffic loads

ABSTRACT This study aims to predict the mechanical response of crumb rubber-modified (CRM) asphalt pavements under traffic loads using three-dimensional (3D) discrete element method (DEM). First, irregular-shaped aggregates were generated in Python language, and discrete element models of six different asphalt layers, which considered the temperature gradient and fatigue damage, were established using 3D DEM. Then, model parameters were obtained through the uniaxial creep test for asphalt mastics at different temperatures. The fatigue damage was implemented by introducing a damage factor into the Burgers model. Finally, three mechanical response parameters (namely, permanent deformation, shear stress and transverse strain) of the six asphalt layers under traffic loads were analysed and compared. Results show that the mechanical response of CRM asphalt layers under traffic loads is significantly influenced by number of repeated loads, temperature, and asphalt layer materials. The shear stress and transverse strain at the wheel centre vary along the depth of asphalt layers, and the middle layer is the principal area of shear stresses in asphalt pavements.

Erosion dynamics of wet particle agglomerates

We study the erosion dynamics of wet particle agglomerates inside a simple shear flow of noncohesive granular materials by relying on the three-dimensional discrete-element simulations. The simulation model is discretized by assembling of wet and dry spherical particles. By systematically varying different parameters related to the shear flow of dry particles (the shear rate), the wet agglomerates (the amount of the binding liquid in the “pendular” state, the liquid viscosity, and the liquid–vapor surface tension), and the relative dry–wet density as well as the initial position of wet agglomerates, we measure the erosion of these agglomerates on their surface by quantifying the cumulative number of eroded particles. We show that the erosion rate increases proportionally to the inertial number and the height of the agglomerates decreases linearly with the liquid content and the liquid viscosity and decreases nonlinearly with the cohesion index (or liquid–vapor surface tension) for each value of the inertial number, whereas this rate is nearly independent to the relative dry–wet density with a low shear rate. It is worth noting that the normalized erosion rate by the shear rate collapses well on a master curve as a cutoff function of the erosion scaling parameter (combining the inertial number, the cohesion index, and the Stokes number), thus providing clear evidence for the unified description of the material and flow parameters on the erosion of wet agglomerates.

Analysis on the Dynamic Pressure of a Cylindrical Silo

In this paper, the silo wall and the storage material in the silo are considered as a whole. It is proposed that there is energy conversion between them in the process of loading and unloading. The interaction between them is analyzed and the mechanism of dynamic pressure increase is explored by referring to the method established by the earth pressure theory. At the same time, the energy principle and three-dimensional discrete element are used for theoretical analysis, and a simplified calculation method of silo lateral dynamic pressure is proposed to effectively predict the lateral dynamic pressure field and the range of pressure coefficient.

Algorithms for Generating Air-Void Structures of Idealized Asphalt Mixture Based on Three-Dimensional Discrete-Element Method

AbstractThe air-void structure of an asphalt mixture is a very important feature because it significantly impacts the strength and modulus of the material. Consequently, in-depth research into an a...

A Three-Dimensional Discrete Element Model for an NbTi/Cu Composite Strand

The mechanical behaviors of a composite superconducting strand, which is composed of a variety of materials with different mechanical properties, are difficult to be accurately predicted using traditional mechanical methods due to the complex modeling process and difficulties in dealing with the interactions among different materials as well as material failure. In this paper, a three-dimensional equivalent discrete element model of an NbTi/Cu composite strand is established by twisting process first, and its mechanical response under external load is successfully achieved by introducing different material constitutive relations into contact models discriminatively and taking the filament fracture into consideration. This model can reduce the modeling process largely and acquire more accurate results, which can also provide a new thinking in predicting the mechanical performance of other superconducting strand or composite material.

Three-Dimensional Discrete-Element Modeling of Aggregate Homogeneity Influence on Indirect Tensile Strength of Asphalt Mixtures

AbstractThis study aims to investigate the influence of aggregate homogeneity on the indirect tensile (IDT) strength of asphalt mixtures using imaging and discrete-element method. An aggregate homo...

Three-Dimensional Discrete-Element Method Analysis of Stresses and Deformations of a Single Geogrid-Encased Stone Column

Three-Dimensional Discrete-Element Method Analysis of Stresses and Deformations of a Single Geogrid-Encased Stone Column

Simulation of dynamic response of projectile and granular target

In the present study, the three-dimensional elastic discrete element method (DEM) and the elastic finite element method (FEM) analysis were, respectively, applied to the randomly distributed identical spheres as a granular target and to the cylindrical projectiles in order to clarify dynamic response during penetration. In the target, it was found that highly densified region was formed just ahead of the projectile and began to propagate spherically at much higher velocity than that of projectile leaving relatively rarefied region. It was also found that the peak resistance during penetration was closely related to the initial formation of densified region and was expressed in terms of momentum change of target particles accelerated by the projectile. Stress wave propagation in the projectiles with various body lengths was investigated during penetration and was discussed in connection with dynamic response of target particles ahead of the projectile. Effect of mechanical properties of projectile on the peak resistance was also investigated and was understood in connection with dynamic behavior of target particles.

3D mode discrete element method with the elastoplastic model

The three-dimensional mode-deformable discrete element method (3MDEM) is an extended distinct element approach under the assumptions of small strain, finite displacement, and finite rotation of blocks. The deformation of blocks is expressed by the combination of the deformation modes in 3MDEM. In this paper, the elastoplastic constitutive relationship of blocks is implemented on the 3MDEM platform to simulate the integrated process from elasticity to plasticity and finally to fracture. To overcome the shortcomings of the conventional criterion for contact fracturing, a new criterion based on plastic strain is introduced. This approach is verified by two numerical examples. Finally, a cantilever beam is simulated as a comprehensive case study, which went through elastic, elastoplastic, and discontinuous fracture stages.

Quantitative Comparison of Two-Dimensional and Three-Dimensional Discrete-Element Simulations of Nominally Two-Dimensional Shear Flow

AbstractResults obtained from numerical simulations of direct (or ring) shear tests on ASTM standard graded (Ottawa) sand using the discrete-element method with periodic boundary conditions in both two and three dimensions are reported. The data obtained from these simulations are quantitatively compared to experimental data for ASTM standard graded sand. The results show that the three-dimensional (3D) effects of nonplanar interparticle contact forces and particle motion are significant in the 3D discrete-element method simulations, even during nominally two-dimensional (2D) shear flow. Moreover, the 3D discrete-element method simulations accurately predict the peak and residual friction angles of ASTM standard graded sand. On the other hand, the 2D discrete-element method simulations fail to accurately predict the peak and residual friction angles. It is argued that the failure of the 2D discrete-element method simulations and the success of the 3D discrete-element method simulations in providing quanti...

Three-Dimensional Discrete Element Simulation of Asphalt Concrete Subjected to Haversine Loading

ABSTRACT Limited by the current computing power, it is impossible to simulate a full scale asphalt pavement with a three-dimensional (3D) microstructure-based discrete element (DE) model, when the time-dependent behaviors of the materials are considered. Researchers currently focus on modeling the microstructure of asphalt mixture materials and have not attempted pavement modeling. In addition, existing studies are limited to modeling asphalt mixtures without considering the time-dependent properties. In this paper, 3D DE models of asphalt mixtures under dynamic loads were developed with the assistance of X-ray Computed Tomography (X-ray CT). The time-dependent model was considered in the simulations. To shorten the computation, a frequency-temperature superposition technique was employed. X-ray CT images were used to reconstruct the microstructure of asphalt concrete, while a time-dependent model was built by taking into consideration micro-scale interactions within the models. Through this study, it was observed that the dynamic modulus and phase angles of asphalt mixtures could be well-predicted from the component properties of those mixtures. It is anticipated that the techniques developed in this study can be used to simulate full scale pavements when the computation power is available.

Insights into ground shock in jointed rocks and the response of structures there-in

Ground shock in rock masses is created by dynamic events such as rock-bursts, coal bumps or man-made explosions. This article examines two aspects of the phenomenology: the propagation of ground shock, and its effects on underground structures. Numerous experiments, at all scales, as well as numerical modeling clearly show that geologic discontinuities have a large influence on ground shock propagation through their actions as wave-guides or as decoupling features. Three-dimensional discrete element (DE) numerical models provide additional insights into the dynamic response of underground structures as influenced by joint orientation, joint spacing, characteristics of the loading pulse (duration, peak velocity, peak displacement), and repeated loadings. It is also shown that in hard rocks the deformability of rock blocks has a negligible influence on tunnel dynamic structural response compared to that of the joint sets or bedding. When viewed together with pictures of underground failures, the 3-D DE models are shown to capture quite well the kinematics of jointed rocks, and their power is illustrated through a very large simulation of an underground facility under dynamic loading.

Stimulation for Methane-Gas Recovery From Coal

Summary During the past several years, a combination of data from field and laboratory experiments, published data from commercial treatments, and concurrent analysis and model development activities has led to a better understanding of the processes occurring in coal during stimulation operations. At the same time, better modeling capabilities have been developed. For example, a new hydraulic fracture design model that includes pressure-dependent nonlinear leakoff, multiple interacting fractures, and failure in the coal near the fracture plane as a result of changes in effective stress is being developed and is being used to study fracture propagation in coal. In addition, two- and three-dimensional (2D and 3D) discrete element and hybrid finite element models that couple the mechanical and multiphase fluid response of the coal have been applied to analyzing problems arising from cavity completion operations. In this paper, we compare data obtained from field trials in coal seams with model simulations, and we discuss the implications for stimulation design and execution.

Evolution and breakup of vortex rings in straining and shearing flows

A study of the effect of external straining and shearing flows on the evolution and form of breakup of vortex rings has been performed. Two orientations each of straining and shearing flows are considered. A theoretical analysis of the ring motion for small strain and shear rates is performed, and it is found that for shearing and straining flows in the plane of the ring, the ring may oscillate periodically. For a straining flow with compression normal to the initial plane of the ring, the linear theory predicts that the ring radius will expand with time. For shearing flow normal to the initial plane of the ring, the linear theory predicts tilting of the ring in the direction of the shear flow rotation.Numerical calculations are performed with both single vortex filaments and with a three-dimensional discrete vortex element method. The numerical calculations confirm the predictions of the linear theory for values of strain and shear rates below a certain critical value (which depends on the ratio R/σ0 of initial ring to core radii), whereas for strain and shear rates above this value the ring becomes very elongated with time and eventually pinches off. Three distinct regimes of long-time behaviour of the ring have been identified. Regime selection depends on initial ring geometry and orientation and on values of strain and shear rates. These regimes include (i) periodic oscillations with no pinching off, (ii) pinching off at the ring centre, and (iii) development of an elongated vortex pair at the ring centre and wider ‘heads’ near the ends (with pinching off just behind the heads). The boundaries of these regimes and theoretical reasons for the vortex behaviour in each case are described. It is also shown that the breakup of stretched vortex rings exhibits a self-similar behaviour, in which the number and size of ‘offspring’ vortices, at the point of pinching-off the ring, may be scaled by the product of the strain rate e (or shear rate s) and the oscillation period τ of a slightly elliptical ring with mean radius R.