Distinct Element Method Simulations Research Articles

Distinct element method simulation of mechanical properties of material layer of pellet belt roasting machine

Distinct element method simulation of mechanical properties of material layer of pellet belt roasting machine

Distinct element modeling of the in-plane and out-of-plane response of ordinary and innovative masonry infill walls

Masonry infill walls (MIWs) are essential partition components that are often damaged during earthquakes because of their brittle nature. As a result, innovative solutions are attractive to improve performance of MIWs by reducing damage during in-plane drift or increasing damping. Examples of innovative solutions have been investigated experimentally, but mesoscale or microscale numerical modelling techniques could also be useful to evaluate performance and investigate the principal failure mechanisms of both ordinary and innovative MIWs, and subsequently optimize their design. This paper employs the Distinct Element Method (DEM) to predict the in-plane (IP) and out-of-plane (OOP) response of ordinary and innovative MIWs within RC frames. The DEM methodology used in this study account for the crushing of concrete and masonry, the shear-compression behavior of mortar and the shear behavior of a novel damping layer joint (DLJ) inserted in the MIW (named damped masonry infilled wall (DMIW)). DEM results are compared against experimental results for both MIWs and DMIWs. Two IP and four OOP experimental tests were simulated in total. The DEM simulation methodology is capable of predicting the IP and OOP load–displacement response as well as the damage patterns of the ordinary MIWs and DMIWs, although the local displacement mechanism at the DLJ locations of the DMIW varied for the OOP tests. Local stress distribution differences, as well as modeling modifications for simulating the damage pattern of DMIWs, are further discussed. Overall, the effectiveness of the DEM simulation provides confidence that it could be used to improve upon the DMIW design in the simulation environment.

Investigation of the mechanical and shrinkage properties of plastic-rubber compound modified cement mortar with recycled tire steel fiber

To facilitate the recycling of waste solids in cement-based materials, plastic-rubber (PR) compound modified mortar (PRM) samples and recycled steel fiber (RSF)-reinforced plastic-rubber modified mortar (PRSRM) samples were prepared and tested to evaluate their mechanical and drying shrinkage properties. PR compound particles, with a mesh size of #8, replaced the natural aggregates with four volume percentages: 2%, 5%, 7.5%, and 10%. The fiber was added with a constant volume fraction of 0.2%. Accordingly, control mortar, PRM mortar with different PR replacement ratios, and PRSRM mortar with RSF were prepared and tested. A numerical splitting tension test model was also established based on the two-dimensional Distinct Element Method (DEM) to investigate the crack initiation and propagation mechanism of the PRM samples. The mechanical test results indicated that the addition of RSF enlarged the compressive strength by 14–27% PRM samples. Meanwhile, both PR and RSF improved mortar flexural behavior by increasing the fracture energy. The SEM image of a fracture surface and DEM simulation showed the crack propagation path was changed by PR particles due to its lower stiffness. Meanwhile, the 7-days drying shrinkage length change was reduced by about 25% due to the PR’s porous structure and water-retaining ability. Overall, the sustainable material design with the combination of PR and RSF would promote the full-recycling of waste tires in cement-based materials.

3D DEM investigation of the resistance of ice and frozen granular soils

Frozen soils are mixtures of ice and solid grains. Since the mechanical properties of ice and dry granular soils are completely different, the behaviour of the ice-soil mixture is highly dependent on the ice content. In this paper, a 3D model based on the distinct element method (DEM) is introduced and used for evaluating the resistance of frozen soil. In the DEM model, two groups of elements are used for representing soil particles and ice. The modelling of granular soils by DEM has been widely treated in the literature. However, few researches are available regarding DEM modelling of ice, and no DEM simulations have been published regarding soil-ice mixtures. For these reasons, preliminary simulations are performed to calibrate the DEM ice model and assess the relationship between the micromechanical parameters and the macroscopic behaviour. Then, several specimens are built by mixing different proportions of the two constituents to reproduce different ice contents, and appropriate parameters for the ice-soil interface are introduced. A series of triaxial compression and shear simulations are performed and analysed. The obtained macroscopic behaviour of the ice-soil mixtures is qualitatively and quantitatively described, and compared to existing experimental data in terms of resistance parameters.

Burger’s Bonded Model for Distinct Element Simulation of the Multi-Factor Full Creep Process of Soft Rock

Pervasive, unavoidable and uncontrollable creep failure generated in soft-rock engineering occasionally happens and therefore attracts extensive attention recently. However, due to soft rock’s multi-factor creep mechanism, it is still difficult to simulate the full-stage creep with the Distinct Element Method (DEM). In this study, we proposed an improved simulation method based on the classical Burger’s model and the Parallel Bonded model in Particle Flow Code (PFC). We apply the abovementioned models together to simulate the full-stage creep process in soft rock. The proposed process has considered the mesoscopic mechanical characteristics of DEM carefully and finally resulted in a parallel physical model, which is called Burger’s Bonded model in this paper. The DEM simulation test using Burger’s Bonded model was designed to compare with experiments. The experiments include a normal creep test and a uniaxial loading test with prefabricated cracks. In contrast to experimental results, the numerical results show that the average error during the whole creep process is less than 3%; the stress–strain curves and crack development process show great agreement. It is also found that the wing crack coalescence in soft rock is independent of the prefabricated crack angle, propagating with a fixed dip angle. The results show that the numerical method proposed in this paper can simulate the multi-factor-caused full stage (attenuated, steady, accelerated) creep process of soft rock in DEM, which provides new insights for theoretical research and engineering design.

Numerical simulation of realistic top coal caving intervals under different top coal thicknesses in longwall top coal caving working face

In the process of longwall top coal caving, the selection of the top coal caving interval along the advancing direction of the working face has an important effect on the top coal recovery. To explore a realistic top coal caving interval of the longwall top coal caving working face, longwall top coal caving panel 8202 in the Tongxin Coal Mine is used as an example, and 30 numerical simulation models are established by using Continuum-based Distinct Element Method simulation software to study the top coal recovery with 4.0 m, 8.0 m, 12.0 m, 16.0 m, 20.0 m and 24.0 m top coal thicknesses and 0.8 m, 1.0 m, 1.2 m, 1.6 m and 2.4 m top coal caving intervals. The results show that with an increase in the top coal caving interval, the single top coal caving amount increases. The top coal recovery is the highest with a 0.8 m top coal caving interval when the thickness of the top coal is 4.0 m, and it is the highest with a 1.2 m top coal caving interval when the coal seam thickness is greater than 4.0 m. These results provide a reference for the selection of a realistic top coal caving interval in thick coal seam caving mining.

Discrete modelling of debris flows for evaluating impacts on structures

Lahars (volcanic debris flows) are natural phenomena that can generate severe damage and wreak havoc in densely populated urban areas. The evaluation of the forces and pressures generated by these mass flows on constructions (e.g., buildings, bridges and other infrastructure) is crucial for civil protection, assessment of physical vulnerability and risk management. The current tools developed to model the spread of flows at large scale in densely populated urban areas remain inaccurate in the evaluation of mechanical efforts. Here, we developed a discrete numerical model for evaluating debris-flow (DF) impact forces at the local scale of one structure (pillar or column) like a building, a bridge and other infrastructure. In this model, the large-sized solid particles that damage infrastructures and edifices are explicitly modelled using Distinct Element Method (DEM). We considered the fluid and fine-grained solid particles not only in the frame of the pressure exerted on structures, but also through their effects on the movement of particles, i.e. buoyancy and drag. The fluid velocity field and the fluid free surface obtained from Computational Fluid Dynamics (CFD) calculation based on Navier–Stokes equations are imported in the DEM simulation. This model is able to reproduce a range of magnitudes of DFs in terms of volumes, velocities and flow heights. Finally, the model provides insights on impact forces generated by particles on structures and on hydrostatic and/or dynamic pressure due to the combined effect of fluid and solid phases. The model provides results consistent with existing empirical models.

Establishing an opening size criterion in direct shear test using DEM Simulation

Direct shear test has been widely used to examine the shear strength of geomaterials because of the simplicity of the testing method and apparatus. Three factors significantly affect the accuracy of the experimental results of direct shear tests, namely (1) the type of direct shear apparatus, (2) the specimen size (scale effect), and (3) the opening size between shear boxes. This study focused on the Threshold Line (TL), which is obtained based on experimental tests, as a guideline for setting the opening size between the shear boxes. The validity of the TL was examined using distinct element method (DEM) 3D simulations from the following four perspectives: the first and second perspectives investigated the influence of the mean particle size and particle size distribution for mean particle sizes larger than 0.8 mm. In the third perspective, the scale effect of the specimens for fixed and varying D:H ratios of the shear box to reduce the shear box size was examined. Lastly, in the fourth perspective, the validity of using the TL to determine the appropriate opening size for the samples with a mean particle size smaller than 0.8 mm was also examined based on the Threshold Point (TP). For each case, the results of the TPs obtained from the DEM simulations agreed well with those of the TL. These findings suggest that the TL is valid and the TL relational equation can be used for setting the opening size between the shear boxes in the direct shear test regardless of saturated and unsaturated soils.

Failure in granular materials based on acoustic tensor: a numerical analysis

We investigate localization in granular material with the support of numerical simulations based upon DEM (Distinct Element Method). Localization is associated with a discontinuity in a component of the incremental strain over a plane surface through the condition of the determinant of the acoustic tensor to be zero. DEM simulations are carried out on an aggregate of elastic frictional spheres, initially isotropically compressed and then sheared at constant pressure p0. The components of the stiffness tensor are evaluated numerically in stressed states along the triaxial test and employed to evaluate the acoustic tensor in order to predict localization. This occurs in the pre-peak region, where the aggregate hardens under the circumstance to be incrementally frictionless: it is a regime in which the tangential force does not change as the deformation proceedes and, consequently, the deviatoric stress varies only with the normal component of the contact force.

Influence of the particle shape on the impact force of lahar on an obstacle

Lahars represent natural phenomena that can generate severe damage in densely populated urban areas. The evaluation of pressures generated by these mass flows on constructions (buildings, infrastructure…) is crucial for civil protection and assessment of physical vulnerability. The existing tools to model the spread of flows at large scale in densely populated urban areas remain inaccurate in the estimation of mechanical efforts. A discrete numerical model is developed for evaluating debris flow (DF) impact pressures at the local scale of one structure. The large-sized solid particles are modelled explicitly using Distinct Element Method (DEM) and the fine-grained solid particles are integrated in a fluid phase which generates two effects on the movement of particles, i.e. buoyancy and drag. Fluid velocity field and the fluid free surface are obtained from Computational Fluid Dynamics (CFD) code then imported in the DEM simulation in a one way coupling scheme. In this paper, the influence of particle shape on the impact forces generated on the obstacle is investigated: spherical particles and polygonal rigid blocks (r-blocks) are considered. The shape of the particle influences the contact surface and therefore the impact pressure. With an angular shape and several facets like r-blocks, the impact pressure on an obstacle is more important for a flow with the same characteristics.

Densification of single-walled carbon nanotube films: Mesoscopic distinct element method simulations and experimental validation

Nanometer-thin single-walled carbon nanotube (CNT) films collected from the aerosol chemical deposition reactors have gathered attention for their promising applications. Densification of these pristine films provides an important way to manipulate mechanical, electronic, and optical properties. To elucidate the underlying microstructural level restructuring, which is ultimately responsible for the change in properties, we perform large scale vector-based mesoscopic distinct element method simulations in conjunction with electron microscopy and spectroscopic ellipsometry characterization of pristine and densified films by drop-cast volatile liquid processing. Matching with the microscopy observations, pristine CNT films with a finite thickness are modeled as self-assembled CNT networks comprising entangled dendritic bundles with branches extending down to individual CNTs. Simulations of these films under uniaxial compression uncover a soft deformation regime extending up to an ∼75% strain. When removing the loads, the pre-compressed samples evolve into homogeneously densified films with thickness values depending on both the pre-compression level and the sample microstructure. The significant reduction in thickness is attributed to the underlying structural changes occurring at the 100 nm scale, including the zipping of the thinnest dendritic branches.

An empirical strength criterion for deep rock incorporating the effect of fracture intensity using distinct element method

Deep rock strength is one of the most important research areas in rock mechanics and engineering which is closely related to bearing capacity and stability of underground structures. Due to high ground stress, the mechanical behaviours of deep rock are different from those of shallow rock. In order to characterize deep rock strength precisely, a great number of rock strength criterions have been proposed over the past few decades to quantitatively describe the relationship between complex stress state and rock strength parameters. Some microscopic characteristics like fracture intensity(P32 ) have received some attention. They are thought to have significant influences on the strength of deep rock. Currently, most of the constitutive models and strength criterions focused on the shallow rock and macro mechanical parameters, and they can not directly explain the relationship between the microscopic fracture characteristics and the strength of deep rock. In this research, the effects of fracture characteristics and confining pressure on deep rock strength were investigated using distinct element method(DEM). Firstly, an improved bond contact model incorporating rolling and twisting resistances was implemented in the rock numerical specimens, and the Smooth-Joint contact model was used for simulating fractures of rocks. The fracture length distributions in specimens were characterised by an exponent parameter a. Then numerical simulation of triaxial compression tests were carried out on numerical rock specimens under different confining pressures and P32 . The results show an approximate linear relationship between the maximum principal stress and P32 . Rock strength decreases with the increase of P32 , and it decreases rapidly at a higher confining pressure. Based on the results, a new empirical strength criterion is proposed. The criterion considers the nonlinear relationship between maximum principal stress and confining pressure of deep rock, additionally reveals the correlation between microscopic fracture characteristics and rock strength. In principle stress space, the corresponding failure envelopes projected onto the π-Plane showed a good agreement with DEM simulation results, which indicates a good applicability for the prediction of deep rock strength.

Granular soils: from DEM simulation to constitutive modeling

In this study, the distinct element method (DEM) was employed to numerically explore the mechanical responses of granular soils and to extract key components for an elastoplastic constitutive model for them. Considering the irregular shapes of real particles, a contact model considering rolling and twisting resistances was used in the DEM simulations. It is shown that this contact model can successfully capture the density-dependent, pressure-dependent behavior of granular soils as well as stress–dilatancy behavior. A critical state-based, double yielding surface constitutive model was used to describe the mechanical behavior of the DEM numerical material. A marked feature of the proposed constitutive model is the incorporation of a flow rule that defines the relationship between the stress Lode angle and strain Lode angle in the deviatoric plane. The model parameters were completely determined from DEM simulation results. The agreement between the DEM simulation results and the constitutive model prediction indicates that realistic complex mechanical behavior of granular soils in generalized loading paths can be reproduced well in both discrete and continuum ways. This will allow effective coupling of discrete–continuum modeling (such as DEM–FEM) in solving geotechnical boundary value problems with large deformation, where a consistent representative of soil is required for the near-field DEM modeling and far-field FEM modeling.

Distinct Element Modelling of Masonry Walls under Out-Of-Plane Seismic Loading

ABSTRACTThe out-of-plane vulnerability of masonry walls plays a crucial role in the seismic response of existing structures. Depending upon mechanical properties and section morphology, collapse may occur by the onset of a mechanism or, as historic constructions often exhibit, leaf separation, disaggregation or sliding. In these latter cases, structural analyses based on rigid-body mechanics may overestimate the seismic capacity, thus resulting unconservative. The distinct element method (DEM), which represents masonry as an assembly of discrete blocks and nonlinear interfaces, could instead be used. Nevertheless, it is more complex and requires more input parameters, so it is still barely applied in engineering practice. In this paper, the seismic out-of-plane response of masonry walls was modelled with DEM. A shake table test on a two-leaf rubble stone masonry wall and a single-leaf wall in tuff blocks was simulated through nonlinear dynamic analyses. The mechanical properties of joints were calibrated on the basis of dynamic identification under low-intensity white noise input, leading to a good prediction of the seismic response. Then, they were further tuned based on the surveyed crack pattern for an improved matching between experimental results and numerical postdictions. Finally, the results provided by the limit analysis were discussed in the light of DEM simulations.

Do the normal compression lines of cemented and uncemented geomaterials run parallel or converge to each other after yielding?

Whether the normal compression lines (NCLs) of cemented and uncemented materials run parallel or converge to each other after yielding is a fundamental yet controversial issue in the study of compression behaviour of geomaterials. An effort is made here to clarify the question by means of the distinct element method (DEM) combined with available experimental observations. In the DEM simulations, the effects of cement content, void ratio, bond elastic modulus, bond strength and bond distribution on the compression curves are considered. In addition, the microscopic mechanical responses, including bond breakage events, deviatoric fabric and stress shared by bonded part or unbonded part, are investigated. The DEM simulations show that the NCLs for all the cemented specimens run parallel with the NCL for the uncemented specimen within the range of stresses adopted in the study, which is not influenced by the aforementioned factors. Through comparing the DEM results in this study, previous 3D DEM data and experimental observations, it can be preliminarily concluded that the interactions between cement breakage and particle crushing determine the convergence or non-convergence of NCLs. The NCLs of the cemented and uncemented sands run parallel for the strong-particle case, while those tend to converge for the fragile-particle case.

Distinct element simulations of earthquake fault rupture through materials of varying density

Surface manifestations of earthquake fault rupture are strongly affected by the dilatant response of the soil deposit overlying the bedrock fault displacement. The granular material’s in-situ void ratio and effective confining stress affect its dilatancy, and hence, its stress-strain response and ductility. Distinct element method (DEM) assemblages of 3D, non-spherical particles are prepared with different void ratio distributions, and their dilatancy is characterized using direct shear test simulations. DEM simulations capture the response of sand in centrifuge experiments of earthquake fault rupture propagation. Macro-scale mechanisms of ground deformation and micro-scale mechanisms of shear band formation during dip-slip fault rupture propagation are analyzed through particle rotations, homogenized strains, frictional dissipation, and particle displacements. The brittle and ductile responses of granular media undergoing fault rupture are related to changes in the coordination numbers in each particle assemblage. The deformational characteristics of a metastable fabric in the loosest particle assemblages and a stable fabric in the densest particle assemblages are revealed through the accumulation of energy dissipated through friction. The normalized strong contact forces are also greater in magnitude in the loosest particle assemblages and greater in number in the densest assemblages.

Distinct element method simulations of rock-concrete interfaces under different boundary conditions

The shear behaviour of concrete-rock interfaces has been the aim of extensive research in geotechnical engineering applications such as rock socketed piles, rock bolts and concrete dam arch bridge foundations. Several experimental studies through direct shear tests have been conducted to evaluate the shear behaviour of rock-concrete interfaces under CNL (Constant Normal Load) and CNS (Constant Normal Stiffness) conditions. In this paper, PFC2D numerical simulations of unbonded rock-concrete planar and saw-tooth triangular joints under CNL and CNS boundary conditions are conducted using the Shear Box Genesis (SBG) approach proposed by Bahaaddini et al. (2013b). The numerical simulation results are compared with experimental data published by Gutiérrez (2013) and Gu et al. (2003). Results indicate that the SBG approach reproduces suitably the shear behaviour, failure mode and asperity damage of unbonded (planar and triangular) rock-concrete interfaces, specially under CNL conditions.

Experimental and DEM analyses on wheel-soil interaction

In this paper, the wheel-soil interaction for a future lunar exploration mission is investigated by physical model tests and numerical simulations. Firstly, a series of physical model tests was conducted using the TJ-1 lunar soil simulant with various driving conditions, wheel configurations and ground void ratios. Then the corresponding numerical simulations were performed in a terrestrial environment using the Distinct Element Method (DEM) with a new contact model for lunar soil, where the rolling resistance and van der Waals force were implemented. In addition, DEM simulations in an extraterrestrial (lunar) environment were performed. The results indicate that tractive efficiency does not depend on wheel rotational velocity, but decreases with increasing extra vertical load on the wheel and ground void ratio. Rover performance improves when wheels are equipped with lugs. The DEM simulations in terrestrial environment can qualitatively reproduce the soil deformation pattern as observed in the physical model tests. The variations of traction efficiency against the driving condition, wheel configuration and ground void ratio attained in the DEM simulations match the experimental observations qualitatively. Moreover, the wheel track is found to be less evident and the tractive efficiency is higher in the extraterrestrial environment compared to the performance on Earth.

Size-dependent mechanical behavior of an intergranular bond revealed by an analytical model

The size of intergranular bonds significantly affects the macroscopic mechanical properties of geomaterials. A size-dependent bond contact model is desired in the distinct element method (DEM) for geomaterials formed by aggregates of bonded particles. This paper proposes an analytical solution of highly-precise stress fields of a biconcave bond between two identical disc-shaped particles under different loading paths based on Dvorkin’s solution. The Unified Strength theory is then introduced to obtain the initial failure domain in the bond. The proposed solution is consistent with results predicted by finite element simulations and experimental observations. The functions of bond stiffness with respect to all influencing parameters, i.e. bond width/thickness, particle radius and elastic parameters of bond material, are provided by the solution and empirically formulated by fitting a large number of analytical results. Additionally, the failure criterion or envelope under different combined loads is formulated for typical brittle bonds. The resulting failure criterion, approximated as an ellipsoid, depends on the size and material properties of the bonds. The proposed solution and equation can be implemented into a bond contact model used in DEM simulations of a geomaterial, where variation of bond sizes is significant and size-dependent contact model is important.

Excluded Volume Approach for Ultrathin Carbon Nanotube Network Stabilization: A Mesoscopic Distinct Element Method Study.

Ultrathin carbon nanotube films have gathered attention for flexible electronics applications. Unfortunately, their network structure changes significantly even under small applied strains. We perform mesoscopic distinct element method simulations and develop an atomic-scale picture of the network stress relaxation. On this basis, we put forward the concept of mesoscale design by the addition of excluded-volume interactions. We integrate silicon nanoparticles into our model and show that the nanoparticle-filled networks present superior stability and mechanical response relative to those of pure films. The approach opens new possibilities for tuning the network microstructure in a manner that is compatible with flexible electronics applications.