3D Discrete Element Model Research Articles

DEM study of the alteration of the stress state in granular media around a bio-inspired probe

Soil penetration is a ubiquitous energy-intensive process in geotechnical engineering that is typically accomplished by quasi-static pushing, impact driving, or excavating. In contrast, organisms such as marine and earthworms, razor clams, and plants have developed efficient penetration strategies. Using motion sequences inspired by these organisms, a probe that uses a self-contained anchor to generate the reaction force required to advance its tip to greater depths has been conceptualized. This study explores the interactions between this probe and coarse-grained soil using 3D discrete element modeling. Spatial distributions of soil effective stresses indicate that expansion of the anchor produces arching and rotation of principal effective stresses that facilitate penetration by inducing stress relaxation around the probe’s tip and stress increase around the anchor. Spatial strain maps highlight the volumetric deformations around the probe, while measurements of both stresses and strains show that the state of the soil around the anchor and tip evolves toward the critical state line. During subsequent tip advancement, the stresses and strains are similar to those during initial insertion, leading to the remobilization of the tip resistance. Longer anchor and shorter anchor-to-tip distance better facilitate tip advancement by producing greater stress relaxation ahead of the tip.

Formation and evolution of a giant old deposit in the First Bend of the Yangtze River on the southeastern margin of the Qinghai-Tibet Plateau

The formation of the First Bend of the Yangtze River is an important event in the fluvial geomorphological evolution on the southeastern margin of the Qinghai-Tibet Plateau. After the formation of the First Bend, the water system in the Jinsha River (Upper Yangtze River) changed, resulting in a key period shaping geomorphology in this area. A large number of deposits (e.g., avalanches, landslides and debris flows) formed in the valley. Analysis of these deposits enables an understanding of paleoenvironmental, paleoclimatic and paleotectonic characteristics. Remote sensing interpretation and on-site investigation have confirmed the existence of the Sangushui old deposit in the First Bend of the Yangtze River. To understand the formation and evolution of this deposit, the genetic mechanism of the deposit was analyzed using field research, topographic interpretation and dating, combined with paleoclimate and paleostructure data. Results show that the deposit was formed at around 33.5 ± 1.4 ka BP by moraine destruction caused by an earthquake with the synergistic action of climate. By using a 3D discrete element model based on terrain reconstruction, the dynamic process of the landslide was simulated. Simulation results indicate that the sliding process lasted 150 s and the speed of the main sliding body reached 72 m/s. A giant deposit about 3.6 km long, l.4 km wide and 105 m thick was formed during this process. Finally, the peak acceleration of the paleoearthquake that induced the landslide was obtained by seismic parameter inversion. In addition, the formation of the Sangushui old deposit and subsequent valley evolution is proposed, providing information for the formation of the First Bend of the Yangtze River.

High compression of granular assemblies of brittle hollow tubular particles: investigation through a 3D discrete element model

High compression of granular assemblies of brittle hollow tubular particles: investigation through a 3D discrete element model

Effect of Bentonite Content and Hydration Time on Mechanical Properties of Sand–Bentonite Mixture

The bentonite is commonly used mixed with soils for groundwater retention and waste contaminant facilities. The incorporation of bentonite could significantly reduce hydraulic conductivity. In this study, the effects of bentonite content, hydration time and effective confining pressure on the static properties of a sand–bentonite mixture were studied using experimental and numerical methods. Firstly, a large number of drainage static triaxial tests on the sand–bentonite mixture with various bentonite contents were conducted. The test results show that the increase in bentonite content and hydration time leads to a slight decrease in shear strength and initial tangent modulus of the sand–bentonite mixture. The presence of bentonite reduces the shear shrinkage and dilatancy trend of the mixture. The cohesion of the mixture increases with the increase in bentonite content and hydration time, but the internal friction angle decreases correspondingly. The hydration of bentonite on the surface of sand particles changes the contact form between particles. The bentonite slurry between pores of the sand skeleton also affects the mechanical behavior of the sand–bentonite mixture. Then, a series of 3D discrete element models were established for numerical simulations of drainage static triaxial tests. The numerical model parameters were calibrated by experimental results. The meso-mechanism of bentonite content affecting the mechanical behavior was revealed according to the contact force distribution between particles. The research results are helpful to understand further the mechanism of bentonite on the mechanical properties of the sand–bentonite mixture.

Experimental and numerical investigation on flexural and fracture performance of steel fiber reinforced shotcrete

This paper presents an experimental and numerical study on the flexural behaviour and fracture performance of steel fiber reinforced shotcrete (SFRS). Firstly, a series of full-scale tests were carried out under four-point bending test procedure to investigate the ultimate strength and failure behaviour of SFRS. Then, parameter calibration was conducted against several lab tests to obtain the accurate meso-scale parameters which could be employed in the 3D discrete element model of SFRS. Finally, a 3D discrete element model for the SFRS specimen was established based on the bond size theory in the parallel bonding model. The numerical results obtained from the proposed 3D discrete element model of SFRS showed a good agreement with the experimental results in terms of the flexural behaviour and the failure performance. By analyzing the bond failure micrograph and contact force chain distribution diagram between particles, the flexural resistance characteristics of SFRS beams were revealed, and the results showed that the random distribution of steel fibers can be well simulated by using the random function. The proposed 3D particle discrete element model and the meso-parameters of bonded granules obtained from the parameter calibration could provide good results to investigate the flexural behaviour and failure characteristics of SFRS.

DEM analysis of the effect of interface transition zone on dynamic splitting tensile behavior of high-strength concrete based on multi-phase model

The tensile strength is one of the main parameters in the design of concrete structures and has an important influence on the cracking performance and durability of concrete materials. As the weak region in concrete, the strength of the interface transition zone (ITZ) directly affects the dynamic mechanical response of concrete. Meanwhile, considering that the tensile performance of concrete is far weaker than its compressive performance, the study of the strength of ITZ is of great significance to the dynamic tensile fracture characteristics and mechanical properties of concrete. In this paper, a method combining 3D scanning technology and Clump-Cluster was proposed to generate a real geometry shape model of crushable aggregate. On this basis, the 3D discrete element model (DEM) of high-strength concrete was constructed using the Flat-Joint Model (FJM). The effects of the ITZ strength on the failure mode, number of microcracks, tensile strength and strain rate effect of high-strength concrete under strain rates from 0.01/s to 100/s were studied by splitting tensile test. The simulation results indicate that the number of microcracks generated decreases with the increase of ITZ strength, and the proportion of ITZ cracks decreases obviously. When the ITZ strength exceeds the critical value of 0.8 times that of mortar, the tensile strength is almost not affected by ITZ strength. In addition, based on the numerical simulation results, a modified formula that can reflect the functional relationship between the ITZ strength and the dynamic increasing factor of the concrete tensile strength (TDIF) is proposed, and the rationality of the TDIF modified formula is verified.

Modeling the self-penetration process of a bio-inspired probe in granular soils

Soil penetration is an energy-intensive process that is common in both nature and civil infrastructure applications. Many human construction activities involve soil penetration that is typically accomplished through impact-driving, pushing against a reaction mass, excavating, or vibrating using large equipment. This paper presents a numerical investigation into the self-penetration process of a probe that uses an ‘anchor–tip’ burrowing strategy with the goal of extending the mechanics-based understanding of burrower–soil interactions at the physical dimensions and stress levels relevant for civil infrastructure applications. Self-penetration is defined here as the ability of a probe to generate enough anchorage forces to overcome the soil penetration resistance and advance the probe tip to greater depths. 3D Discrete element modeling simulations are employed to understand the self-penetration process of an idealized probe in noncohesive soil along with the interactions between the probe’s anchor and tip. The results indicate that self-penetration conditions improve with simulated soil depth, and favorable probe configurations for self-penetration include shorter anchor–tip distances, anchors with greater length and expansion magnitudes, and anchors with a greater friction coefficient. The results shed light on the scaling of burrowing forces across a range of soil depths relevant to civil infrastructure applications and provide design guidance for future self-penetrating probes.

Discrete element modeling of interface debonding behavior in composite material: Application to a fragmentation test

This paper presents a 3D simulation of interface debonding in composite material using 3D Discrete Element Model (DEM). The simulation is based on the experimental results obtained by Guillebaud-Bonnafous and al. Bonnafous2012. A single fragmentation testing of an singer impregnated hemp yarn embedded into a epoxy matrix was investigated. In DEM, matrix and yarn are supposed to be brittle materials and follow a linear fracture model. The discrete elements of matrix are connected by the cohesive beams whereas the one of yarn are connected by the spring links. The cohesive contact laws are implemented to model interface debonding between yarn and matrix (yarn/matrix). Piecewise linear elastic laws usually used in Cohesive Zone Models are retained in this work. The numerical results obtained by DEM are compared with experiment data and finite element modeling on the stress–strain curve and the fragmentation process in yarn during the test. This comparison allows to validate the models used in DEM. To reduce the discrete elements number and save computational time, the bi-disperse medium in DEM for matrix and yarn is specifically elaborated in this study.

Rock fragmentation induced by a TBM disc-cutter considering the effects of joints: A numerical simulation by DEM

The study of rock fragmentation mechanism significantly contributes to the advancing speed of tunnel boring machines (TBMs). In this sense, numerous factors are shaping the rock fragmentation mechanism of their disc-cutters. Among them, rock structural parameters, such as joint inclination, orientation, spacing, etc., considerably impact cutting force as well as rock-cutting process. However, detailed relationships between rock-cutting parameters and rock mass ones have not been to date proven, and rock fragmentation mechanism still needs to be investigated in depth. In this paper, a three-dimensional (3D) discrete element model of rock-cutting using a disc-cutter was established via the high-performance MatDEM software. To simulate the fragmentation performance of the rock undergoing the impact load, an elastic–plastic contact model was firstly proposed based on the linear elastic contact model, verified by several mechanical experiments. Then, the influence of different joint parameters (e.g., joint inclination, orientation, spacing, etc.) on cutting force and rock fragmentation volume was examined. At the same time, the effect of joint parameters on specific energy (SE) was calculated to explore the joint influence on rock-cutting efficiency. The simulation results show that the elastic–plastic contact model can better simulate the plastic deformation characteristics of the brittle rock, and the improved model can imitate the generation of the rock fragment structure. As well, the influence of the joint orientation and spacing on the mean value of the cutting force and the rock fragmentation volume is obvious, and the propagation mode of the internal cracks in the rock varies with reference to the joint orientation. The existence of the joints also decreases the rock-cutting SE, and consequently the rock-cutting efficiency can reach the highest level when the joint spacing is equal to 50 mm and the joint orientation is about 30°. The opposite inclination of the joint correspondingly leads to the opposite change in the rock-cutting parameters, which can be eliminated through rotating cutting in practice. This study explores the influence of joint parameters on rock-cutting ones and reveals crack distribution mechanism in jointed rock mass.

Investigation of the Effect of Block Size, Shape, and Freeze Bond Strength on Flexural Failure of Freshwater Ice Rubble Using the Discrete Element Method

Abstract As part of the Mechanics of Ice Rubble project, recent experiments have been carried out to study the strength and failure behavior of ice rubble beams and the freeze bonds that form between individual ice blocks. In this study, we present results obtained from a newly developed model for the three-dimensional (3D) discrete element modeling (DEM) open-source code LIGGGHTS. The ice model contains normal and shear springs that operate between neighboring particles which are bonded or that overlap due to compressional stresses. Energy dissipation is accounted for by using a viscous damping model. Using this DEM model, medium-scale freshwater ice rubble punch tests have been simulated. Rubble specimens were generated by “raining” individual DEM ice pieces into a rectangular form and compacting the rubble mass to achieve the target porosity. Before the compacting pressure was removed, bonds between contacting blocks were introduced with parameter values based on representative freeze bond experiments. The rubble beam was then deformed by pushing a platen vertically downward through the center of the beam until failure occurred. Two types of block size and shapes have been simulated: cuboid blocks generated based on the size distribution of the actual rubble, and rubble blocks generated by image processing of actual blocks of broken ice used in the comparison experiments. The mechanism of flexural rubble failure for both cuboid block [s4.2] simulations and empirical block [s4.3] simulations is in line with experimental results; however, the empirical block simulations provide a significantly better estimation of the failure force.

Numerical Study of Stability and Connectivity of Vertical Goaf Drainage Holes

During underground longwall coal extraction, overburden strata deformation may result in vertical goaf drainage holes, which are drilled in advance of mining for tail gate gas management, to fail. The performance of these vertical goaf drainage holes is controlled by mine design parameters and local geomechanical properties. This paper investigates the use of advanced 3D finite element modelling and 2D discrete element modelling simulation techniques to understand the fundamentals of vertical goaf drainage hole failure mechanism due to strata shear at a currently operating gassy Australian mine site. Finite element modelling is used to investigate the location of high shear in the overburden strata at the vertical goaf gas drainage hole region during longwall mining and the discrete element modelling is used to examine connectivity from the goaf region to the goaf-gas drainage system.

Early prediction of macrocrack location in concrete, rocks and other granular composite materials

Heterogeneous quasibrittle composites like concrete, ceramics and rocks comprise grains held together by bonds. The question on whether or not the path of the crack that leads to failure can be predicted from known microstructural features, viz. bond connectivity, size, fracture surface energy and strength, remains open. Many fracture criteria exist. The most widely used are based on a postulated stress and/or energy extremal. Since force and energy share common transmission paths, their flow bottleneck may be the precursory failure mechanism to reconcile these optimality criteria in one unified framework. We explore this in the framework of network flow theory, using microstructural data from 3D discrete element models of concrete under uniaxial tension. We find the force and energy bottlenecks emerge in the same path and provide an early and accurate prediction of the ultimate macrocrack path {mathcal {C}}. Relative to all feasible crack paths, the Griffith’s fracture surface energy and the Francfort–Marigo energy functional are minimum in {mathcal {C}}; likewise for the critical strain energy density if bonds are uniformly sized. Redundancies in transmission paths govern prefailure dynamics, and predispose {mathcal {C}} to cascading failure during which the concomitant energy release rate and normal (Rankine) stress become maximum along {mathcal {C}}.

Effects of Needle Flake Particles on Splitting Experiment of Asphalt Mixture: An Experimental and Numerical Investigation

The purpose of this work is to study the influence of the content and shape of needle flake particles on the meso-scale splitting test performance of asphalt mixture, a three-dimensional laser scanning method is used to build the needle and flake particle model, and then the particle model is put into the numerical simulation software EDEM to establish the splitting experimental model. The influence of loading speed and model parameters on the experimental results is revealed by assigning macro and micro parameters to different particles. With calibrating the parameters by Marshall experiment, the simulation experiment results after parameter adjustment are in good agreement with the experimental results in laboratory. The results show that the splitting experimental model can be established intuitively by means of using 3D discrete element modeling technology, and the stress of each particle size in the loading process can be observed directly. The loading speed, the content of needle flake particles, the size of needle flake particles and the distance from the loading area all have a great influence on the experimental results.

Prediction and validation of ash sticking probability under fouling conditions in pulverized coal combustion

Under the fouling conditions in stationary coal combustion systems, the sticking/rebound behavior of solid incident particles is a key issue in determining the ash deposition rate. From a dynamic point of view, the bulk fly ash, which dominants the deposited mass, successively interacts with the clean tube, the inner fine deposited layer and the bulk deposited layer during ash deposition. In this paper, we experimentally investigate the time-resolved evolution of ash fouling in a 25 kW coal combustor. The deposited mass flux rapidly reaches a stable state that fluctuates around a mean value of ∼3 g/(m2·s) for two kind of probe materials. The rapid initial stage only allows the formation of 1–2 layers of bulk deposited ash, revealing the dominant role of bulk deposit in capturing large incident particles. Inspired by the observation, we apply a 3D adhesive discrete element model (DEM) to fully describe the many-body evolving process subject to the incident events of a 30-µm particle. The simulation agrees well with the experiments when using a higher particle surface energy of 200 mJ/m2. The rapidly growing feature of ash sticking probability with increasing the bulk deposit layers can be reproduced in this case, and an empirical formula is proposed. It is also validated that, at the deposit growth stage, the newly-deposited particles stay just where they impact. The effectiveness of the DEM tool shall benefit a fully-validated sticking/rebound model under the fouling condition that is convenient for CFD use.

Comparative modelling of indirect tensile strength of asphalt mixtures with and without considering air void characteristics

ABSTRACT This paper aims at comparing the 3D discrete element modelling of the indirect tensile (IDT) strength of asphalt mixtures with and without considering the air void size and distribution. Micromechanical models for predicting the IDT strength of asphalt mixtures, with and without considering the air void size and distribution, were established using X-ray computed tomography and 3D discrete element method (DEM) incorporated with a cohesive zone model (CZM). The IDT strength of three asphalt mixtures were simulated at three different temperatures on the basis of these models. Simulation results were verified against known experimental IDT tests. The results show that air void characteristics affected greatly the IDT strength of asphalt mixtures, and with the same material parameters, the IDT strength with considering the real air void size and distribution was lower and closer to the experimental IDT strength than that without considering the real air void size and distribution. Furthermore, aggregate size and temperature also significantly affected the IDT strength. IDT strength increased with increasing aggregate size or decreasing temperature. Aggregate size had an increased impact on the IDT strength with increasing temperature, whereas air void characteristics had a decreased influence on the IDT strength with increasing temperature.

Effects of Aggregate Mesostructure on Permanent Deformation of Asphalt Mixture Using Three-Dimensional Discrete Element Modeling.

This paper investigated the effects of aggregate mesostructures on permanent deformation behavior of an asphalt mixture using the three-dimensional (3D) discrete element method (DEM). A 3D discrete element (DE) model of an asphalt mixture composed of coarse aggregates, asphalt mastic, and air voids was developed. Mesomechanical models representing the interactions among the components of asphalt mixture were assigned. Based on the mesomechanical modeling, the uniaxial static load creep tests were simulated using the prepared models, and effects of aggregate angularity, orientation, surface texture, and distribution on the permanent deformation behavior of the asphalt mixtures were analyzed. It was proven that good aggregate angularity had a positive effect on the permanent deformation performance of the asphalt mixtures, especially when approximate cubic aggregates were used. Aggregate packing was more stable when the aggregate orientations tended to be horizontal, which improved the permanent deformation performance of the asphalt mixture. The influence of orientations of 4.75 mm size aggregates on the permanent deformation behavior of the asphalt mixture was significant. Use of aggregates with good surface texture benefitted the permanent deformation performance of the asphalt mixture. Additionally, the non-uniform distribution of aggregates had a negative impact on the permanent deformation performance of the asphalt mixtures, especially when aggregates were distributed non-uniformly in the vertical direction.

The Internal Structural Evolution of Calderas: Results from 3D Discrete Element Simulations

The structural evolution of calderas is a key issue in volcanology and has profound implications for hazard analysis and the exploitation of geothermal energy and hydrothermal ores. However, their internal geometry at depth and the detailed fault and fracture distribution are unclear and debated. In order to better constrain the internal structural evolution of calderas, I have developed a 3D discrete element model of a frictional cover undergoing piston-like subsidence at its base, simulating magma chamber deflation and cover collapse. I examine two piston geometries, simulating magma chambers with roofs that are circular and rectangular in plan view, to investigate patterns of faulting and subsidence in three dimensions. In both models a complex arrangement of normal and reverse faults accommodates deeper subsidence at higher structural levels. Bell- to cone-shaped, outward-dipping ring faults are consistently the first structures to develop; these faults propagate upwards from the piston edges towards the surface. Later caldera growth is mainly the result of movement on vertical, or steeply inward-dipping, normal ring faults which enclose the earlier reverse faults. As a result, all calderas widen, in terms of their surface expression, with time. The final stage of caldera development includes significant collapse of the caldera walls and transport of this material towards the caldera center. The results confirm that the evolutionary patterns/stages proposed from 2D numerical and analogue models can be generalized to three dimensions, although significant differences between long- and short-axis geometries do occur when the piston is elongate. Compared to 2D simulations, however, 3D results show the geometric complexity of ring faulting, with variations in strain and fault activity at various stages of development demonstrating that often a simple, continuous ring fault structure is not developed.

Macro and micro investigation of gravel content on simple shear behavior of sand-gravel mixture

The gravel content (GC) may influence the mechanical properties of sand-gravel mixture (SGM) used in earth structures and may compromise the stability of such structures. The purpose of this paper is to present a macro and micro study on the effect of GC on simple shear behavior of SGM using experimental and numerical approaches. Firstly a series of large-scale simple shear tests on SGM with different gravel contents under different normal stresses were conducted. The test results demonstrate that GC affects the shear strength and friction angle of SGM according to the change of void ratio at macro-scale. The variations of dilatancy and initial modulus with GC were also discussed. Then a 3D discrete element model was established for numerical simulations, in which the grading distributions of particles had similar shapes with the simple shear tests, but the particle size was 5 times scaled. The Herts-Mindlin model was used for contact, and the rolling resistance was taken into account. The numerical results showed a fairly good agreement with the experimental results. After that the micro mechanism of gravel content effect was investigated according to the analysis of micro force chain topology and contact force distributions. In summary, the experimental and numerical investigations provide valuable insights into the gravel content effect of SGM at both macro- and micro- scales.

INFLUENCE ANALYSIS OF ROCK MECHANICAL PARAMETERS ON THE TBM PENETRATING RATE

To study the influences of rock mechanical parameters on the penetrating rate of a tunnel boring machine (TBM), a 3D discrete element model, namely, 3D distinct element code (3DEC), was used to build simulated models on rock fragments using a TBM cutter based on the geological conditions of the west route of the South-to-North Water Transfer Project. The change rule of the TBM penetrating rate caused by different rock mechanical parameters was analyzed. The line graph of the relationship between the TBM penetrating rate and rock mechanical parameters was fitted using the least squares method. Results show that the TBM penetrating rate will decrease with an increase in uniaxial compressive strength and elastic modulus. By contrast, it will increase with an increase in rock Poisson’s ratio. This finding is in accordance with the result of the finished project. A numerical foundation was provided for the TBM tunneling of diversion tunnels in the west route of the South-to-North Water Transfer Project.

A 3D meso-scale solidification model for metallic alloy using a volume average approach

A 3D discrete-element model has been developed to simulate the solidification of steel at the meso-scale. The domain consists of a set of equiaxed grains along with the liquid channels, where the fully solid grain shape is given by a modified Voronoi diagram. The primary solidification and peritectic transformation within each grain is modeled using a volume average approach. Thus, phase evolution within the semisolid domain with either dendritic or globular microstructure can be predicted depending on different cooling rates. The coalescence phenomenon between grains is considered at the end of solidification using Bulatov's approach for estimating interfacial energy. It is seen that only 0.9% of the grains are attractive based on their orientations, significantly depressing final-stage solidification. The results demonstrate the ability of this modelling approach to investigate morphology transitions during the solidification of alloys having a range of composition from non-peritectic to hyper-peritectic. The influences of grain size, carbon content and cooling rate on the solidification behavior are also investigated. Further, it is shown that the semi-solid morphology of hypo-peritectic steel alloys at high solid fraction contains very thin liquid channels, in comparison to other compositions, when the peritectic transformation occurs thus increasing the hot tearing susceptibility. This meso-scale model will be used in conjunction with semi-solid fluid flow and deformation simulations for multi-physics modelling of solidification.