A fully coupled 3D hydro-mechanical model with real porous seepage is presented for simulating hydraulic fracturing. In this model, fluid flow in a fracture is expressed by 2D fracture seepage in the ...broken joint elements based on the Cubic law, while fluid flow in the rock matrix is represented by 3D porous seepage in the tetrahedral elements based on Darcy's law. Several problems that have closed-form solutions and a 3D fracturing problem are given to verify the model. The simulation results show that the model can capture crack initiation and propagation, and the fluid pressure evolution during hydraulic fracturing.
•The maximum mesh size and maximum loading rate that can be adopted for different specimens are proposed.•The maximum mesh size and calculation method for tunnel excavation simulation of different ...diameters are proposed.•The method for determining the critical kinetic energy of the system during excavation simulation is proposed.•The reduction function of the Young's modulus of the core material is proposed.
The combined finite-discrete element method (FDEM) is extensively employed to model rocks and rock-like materials, in which calibration against the results from uniaxial/triaxial compression tests, Brazilian tests and shear tests have been widely carried out. However, since different element sizes and loading rates were used, it is difficult to assess the numerical results of these studies if the effects of the element size and loading rate are ignored. This paper discusses the effect of the element size, loading/unloading rate and unloading mode on the cracking processes in laboratory-scale (uniaxial compression tests and Brazilian tests) and field-scale (circle tunnel excavations). The results indicate that the element size and loading rate should not be less than 27–28 meshes in a diameter zone and larger than 0.5 m/s, respectively, in laboratory-scale. In field-scale models, four different tunnel diameters (3, 4, 5 and 6 m) are used in tunnel excavation simulations. The results reveal that the mesh number around the tunnel wall should not be less than 120, meanwhile the element size should not be longer than the length of fracture process zone (lFPZ). The unloading rate for the field-scale model is influenced by the model size and rock density. It cannot be determined directly but a new method is proposed to determine the unloading rate based on crack development curve. To compare the fracture patterns well to those observed from the tunnel simulations, the unloading mode of an exponential formula is apposite.
Two discrete element models, super-ellipsoid model and multi-sphere model, are employed in this paper to describe the ellipsoidal particles. And the packing and flow behavior of ellipsoidal particles ...are investigated by the discrete element method (DEM) simulation and experiment. To compare the two models for ellipsoid, three tests are conducted: (i) the packing of ellipsoidal particles in a rectangular container, (ii) the flow of ellipsoidal particles in a horizontal rotating drum, and (iii) the discharge of ellipsoidal particles from a flat bottom hopper. Simulation results show that the super-ellipsoid model can accurately reproduce the packing and flow behavior of ellipsoidal particles. In the simulations using multi-spheres, when the spheres are more in a multi-sphere particle, the accuracy of simulation is acceptable while the computational time is much longer than the super-ellipsoid model. When fewer spheres are used to approximate the ellipsoidal particle, the computational time can be saved while the accuracy of the simulation decreases.
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•The packing and flow behavior of ellipsoidal particles are investigated by DEM.•The ellipsoidal particles are modeled by super-ellipsoids and multi-spheres.•Two models are compared in a series of simulations.•Super-ellipsoids are faster and more accurate than multi-spheres to model ellipsoids.•Multi-spheres with more sub-spheres can also model the ellipsoids accurately.
This paper presents the application of the Discrete Element Method (DEM) to describe the physical properties of plant-origin grains with a low elasticity modulus for the needs of numerical simulation ...of processing processes. The study proposes the modelling of maize grains based on 3D scanning of real grains and further use of the multi-sphere method to fill the numerical model with a conglomerate of elementary spheres. The main aim of the paper is to develop a calibration method based on exploring parameter spaces at points selected using Sobol's grids. As criteria and functional limitations for the calibration, the following were proposed: the slope angle of repose (AoR), the radius of the heaped cone's vertex (Rad), the number of grains, and the slope height. The study results show that for the calibration of the DEM model describing maize grains, test points with a set of the following nine parameters should be used: Poisson's Ratio for grain, Density of grain, Shear Modulus, Coefficient of Restitution for grain-grain, Coefficient of Restitution for grain-material, Coefficient of static friction for grain- grain, Coefficient of static friction for grain-material, Coefficient of rolling friction for grain-grain, and Coefficient of rolling friction for grain-material.
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•Simple gravity bulk method used for calibration.•Optimization method of determination properties of maize grains.•Formalization exploration of the parameters maize in Sobol grids and Pareto curves.•The multi-sphere method of filling the numerical model cereal maize.•Dissipative component is used for model verification.
Structuralized cementing technology takes full advantage of non-pressure controlled grouting technology to fill pores in coarse aggregates using self-compacting material with no need for vibration or ...mixing. By cementing the aggregates in a structured manner, structurally cemented material has properties between those of a discrete granular material and a continuous medium. The interface between different component structures exerts an important effect on the physical properties of the whole material. In this paper, the monotonic and cyclic mechanical properties of the contact interface properties of a structurally cemented material under different normal stresses are studied by performing experiments and numerical simulations. Through the study, the deformation displacement time history and volume change with the shear process are provided, the deformation process of the sample near the contact interface during the shear process is discussed, and the shear mechanics law with the strength mechanism under the simple shear condition is summarized.
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•A novel device was designed to analyze structurally cemented contact interface.•The cementation strengthens the contact interface within the influence zone.•The full-scale numerical simulation was conducted to match the experiment.
A numerical study is conducted using the improved continuum‐based discrete element method (CDEM) to investigate the effect of holes on the dynamic fracturing of multi‐flawed rocks. The specimen ...geometries contain a perpendicular crack‐like flaw, a hole‐like flaw, and an inclined crack‐like flaw. A fracture model is implemented into the improved CDEM that combines the nonlinear pressure‐dependent shear strength and tensile strength of rocks. The digital image correlation method combined with ultra‐high‐speed photography is applied in a split Hopkinson pressure bar system to verify the accuracy of the proposed model. The experimental results show that the improved CDEM accurately reproduces dynamic crack behavior in rocks. The simulation results show that hole‐like flaws significantly affect crack behavior compared with the two investigated crack‐like flaws. However, this effect gradually weakens with increasing loading stresses. This study provides important insight into the dynamic fracturing of multi‐flawed rocks.
Aggregates are the main carrier to resist the external loads for asphalt pavement, and the particle contact characteristics in aggregate blend are closely related to the load transfer properties of ...asphalt mixture. This study analyzed the internal contact characteristics of aggregate blend to provide a reference for revealing load transfer mechanism. Nine types of gradations were selected to perform the discrete element method (DEM) simulation tests of aggregate blends. In the blends, aggregates were divided into main skeleton aggregates and disruption aggregates based on particle function, and the contacts were classified into strong contacts and weak contacts according to the average contact force, and the contact types were divided into the contacts only between main skeleton aggregates (S-S), that between main skeleton aggregates and disruption aggregates (S-D), and that only between disruption aggregates (D-D). On this basis, the indicators for evaluating the contact characteristics between particles were developed. The internal contact characteristics and load transfer properties of aggregate blend were analyzed. The results showed that the proportion of multiple contacts in S-S is higher than that in S-D and D-D, indicating that the interlocking ability of S-S is better than that of S-D and D-D. S-D has the highest number of contacts in stone matrix asphalt (SMA) and open graded friction course (OGFC) gradations, while D-D has the highest number of contacts in dense asphalt concrete (AC) gradations. Most of the contacts in S-S are strong contacts, while that in S-D and D-D are weak contacts. For skeleton asphalt mixtures, most of the external load is transferred in S-S. For AC asphalt mixtures, the external load is mainly transferred in S-D. The average contact force in S-S is much greater than that in S-D and D-D, indicating that S-S has the strongest load transfer ability. The contact length in S-S of AC gradations is longer than that of SMA and OGFC gradations, illustrating that S-S of suspend-dense structures has lower load transfer efficiency compared with skeleton structures.
•A series of indicators characterizing the contacts of particles were proposed.•The interlocking ability of S-S is better than that of S-D and D-D.•For skeleton asphalt mixtures, most of the external load is transferred in S-S.•For AC asphalt mixtures, the external load is mainly transferred in S-D.•S-S has better load transfer ability compared with S-D and D-D.
A new erosion model, which is referred to as SIEM (shear impact energy model), is used to investigate elbow erosion under different working conditions using numerical simulations. The fluid motions ...are predicted by CFD (computational fluid dynamics), and the particle movements are calculated using DEM (discrete element method) in the simulations. Both a one-way coupling method and a two-way coupling method in CFD-DEM are adopted to calculate the gas-solid interaction. The prediction results of elbow erosion subject to a condition of dilute gas-particle flow are validated against corresponding experimental data. Because DEM and SIEM can be easily applied to dense gas-particle flows, the effect of the particle concentration on the erosion is also investigated. The simulation results show that the coupling methods have little influence on the erosion prediction when the particle concentration is low, whereas erosion in the elbow is significantly sensitive to the particle concentration. Finally, the effects of the friction coefficient, restitution coefficient and spring stiffness coefficient on elbow erosion are also investigated using simulations, and the effects were not as remarkable as the concentration.
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•The erosion in the elbow is studied by CFD-DEM simulation.•A new erosion model is employed and the simulation results are verified.•Effect of particle concentration on erosion in elbow is obtained.•Effects of coefficients of friction and restitution on erosion are also obtained.
Earthquake ruptures dynamically activate coseismic off‐fault damage around fault cores. Systematic field observation efforts have shown the distribution of off‐fault damage around main faults, while ...numerical modeling using elastic‐plastic off‐fault material models have demonstrated the evolution of coseismic off‐fault damage during earthquake ruptures. Laboratory‐scale microearthquake experiments have pointed out the enhanced high‐frequency radiation due to the coseismic off‐fault damage. However, the detailed off‐fault fracturing mechanisms, subsequent radiation, and its contribution to the overall energy budget remain to be fully understood because of limitations of current observational techniques and model formulations. Here, we constructed a new physics‐based dynamic earthquake rupture modeling framework, based on the combined finite‐discrete element method, to investigate the fundamental mechanisms of coseismic off‐fault damage, and its effect on the rupture dynamics, the radiation and the overall energy budget. We conducted a 2‐D systematic case study with depth and showed the mechanisms of dynamic activation of the coseismic off‐fault damage. We found the decrease in rupture velocity and the enhanced high‐frequency radiation in near field due to the coseismic off‐fault damage. We then evaluated the overall energy budget, which shows a significant contribution of the coseismic off‐fault damage to the overall energy budget even at depth, where the damage zone width becomes narrower. The present numerical framework for the dynamic earthquake rupture modeling thus provides new insights into earthquake rupture dynamics with the coseismic off‐fault damage.
Plain Language Summary
The medium surrounding fault cores can be damaged due to stress concentration caused by the dynamic earthquake ruptures propagating on the faults, which is called coseismic off‐fault damage. Systematic field observation efforts have shown the distribution of off‐fault damage around main faults, while numerical modeling has demonstrated the evolution of off‐fault damage during earthquake ruptures. Laboratory‐scale microearthquake experiments have pointed out the enhanced high‐frequency radiation due to the off‐fault damage. However, the detailed off‐fault fracturing mechanisms, subsequent seismic wave radiation and its contribution to the overall energy budget remain to be fully understood. Here, we constructed a new physics‐based dynamic earthquake rupture modeling framework to investigate the fundamental mechanisms of coseismic off‐fault damage and its effect on the rupture dynamics, the radiation, and the overall energy budget. We found the enhanced high‐frequency radiation in near field due to the coseismic off‐fault damage. We then evaluated the overall energy budget, which shows a significant contribution of the coseismic off‐fault damage to the overall energy budget even at depth. The present numerical framework for the dynamic earthquake rupture modeling thus provides the insight into the earthquake rupture dynamics with the coseismic off‐fault damage.
Key Points
Earthquake ruptures dynamically activate coseismic off‐fault damage, whose feedback plays an important role in rupture dynamics
We show the mechanism of dynamically activated off‐fault fractures and its effect on rupture velocity and enhanced high‐frequency radiation
The contribution of off‐fault damage to the overall energy budget associated with earthquakes is nonnegligible even at depth
•Structural response of dry-stone masonry structures is analysed by a 3D combined finite-discrete element method (FDEM).•Proposed approach includes modelling of discontinuities across dry joints and ...fracturing and fragmentation of the blocks, in order to correctly simulate the mechanical behaviour of dry-stone masonry.•Model is capable of predicting structural damage, failure mechanisms and collapse of these structures under static and seismic loads.•The influence of the numerical parameters, such as penalty terms and damping coefficient, on the accuracy of the solution was analysed through various examples.
The evaluation of damage mechanism and the failure of dry-stone masonry structures subjected to seismic load is one of the most important aspects of seismic assessment of existing masonry buildings. Discontinuities between the blocks, their mechanical interaction, large displacement and rotation are the main causes of degradation and collapse of these structures. This paper presents the application of the combined finite-discrete element method (FDEM) in the analysis of 3D dry-stone masonry structures which is able to capture such mechanisms and structural damage both at joint and at block level. Moreover, the proposed approach includes friction forces based on Coulomb-type of law accounting for friction effects across dry joints and open cracks, in order to correctly simulate the mechanical behaviour of dry-stone masonry. The capabilities of presented approach are demonstrated in a series of numerical tests, starting with sliding and rocking of single blocks, examination of dry joints shear behaviour, walls subjected to in-plane and out-of-plane load, and finally the simulation of the simple spatial stone masonry structure.
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