Investigation of strain localization in sheared granular material using 3D numerical discrete element model

2021 ◽  
Author(s):  
Chien-Cheng Hung ◽  
Andre Niemeijer ◽  
Amir Raoof ◽  
Thomas Swijen
Keyword(s):  
Grain Size ◽  
Particle Motion ◽  
Contrast Ratio ◽  
Strain Tensor ◽  
Discrete Element ◽  
Contact Forces ◽  
Spherical Particles ◽  
Frictional Heat ◽  
Granular System ◽  
Seismic Velocities

<p>We used three-dimensional numerical simulations of the discrete element method (DEM) to investigate slip localization in sheared granular faults under seismic velocities. An aggregate of non-destructive spherical particles with assigned contact properties is subjected to direct shear with periodic boundary in horizontal directions. To investigate whether particle size distribution (PSD) influences slip accommodation, three distinct PSDs, namely Gaussian, log-normal, and power-law with fractal dimension D ranging from 0.8 to 2.6, are employed. In additional simulations, we impose a thin layer of particles with smaller grain size along the boundary as well as in the middle of the granular assemblages to simulate boundary and Y shears occurring in both natural and laboratory fault gouges. Transient microscopic properties, such as particle motion and contact forces, as well as macroscopic properties, such as friction, of the granular layer, are continuously monitored during numerical shearing. Results show that no visible slip localization is observed for all different PSDs based on the current particle motion analysis. On the other hand, we find that much more strain (i.e., displacement) is accommodated in the finer-grained layer even with a small contrast in grain size. Up to 90 % of the displacement is localized in a finer-grained layer when the contrast ratio of the grain size is 50 %. Since more frictional heat will be generated in the localized slip zone, the results provide crucial information on the heat generation and associated slip accommodation in sheared gouge zones. A possible mechanism of slip localization in the simulations is the transfer of the momentum across the granular system. We conclude that the occurrence of a weaker, fine-grained layer within a dense fault zone is likely to result in self-enhanced weakening of the fault planes.  Ongoing work includes (1) varying the thickness, grain size, and internal friction of the thinner layer; (2) applying triangulation methods to further analyze the microscale stress and strain tensor between particles; (3) changing the rolling friction of particles.</p>

Experimental analysis of segregation in granular flows

2020 ◽  
Author(s):  
Silvia D'Agostino
Keyword(s):  
Grain Size ◽  
Free Surface ◽  
Numerical Models ◽  
Granular Flows ◽  
Contact Forces ◽  
Spherical Particles ◽  
Granular Temperature ◽  
Bulk Solid ◽  
Large Particles ◽  
Mean Diameter

<p>Natural granular flows have a widely dispersed grain size distribution. The majority of the numerical models and laboratory investigations of granular flows are developed assuming a single grain size. However, the geophysical massive flows involve several classes of particles and the bulk solid evolves spatially in a non-uniform state [1]. Segregation causes a different spatial distribution of the particles and influences the kinematic of the bulk solid, like the concentration, the run-out, the velocity and the granular temperature. During the flow motion, the largest particles are found at the surface due to the imbalances in the contact forces, and the smallest at the bottom as they percolate due to gravity [2].</p><p>To investigate the physical processes responsible of the particles transfer, we conducted a series of laboratory experiments, using two different grain size classes to reproduce the binary mixture. The measured data are required to calibrate the mathematical model and to set the coefficients that describe the percolation and the kinetic sieving mechanism. The experiments to study the free surface flow started considering the dry condition. Two different type of classes of particles flow over a loose deposit in homogenous and steady conditions. We used spherical particles of non-expanded polystyrene with a density of 1035 kg/m<sup>3</sup>. The small beads are black with a mean diameter of 0.00075 m and the large beads are white with a mean diameter of 0.0014 m. At the end of the flume there is a weir with two openings. The material is manually inserted and flow in the flume, it is then recirculated by an auger and finally conveyed in a hopper, from where it falls down in the chute again. The system works for at least 30 minutes, after reaching the steady condition.</p><p>The measurements were taken through a high speed camera in a section lateral to the flume. The flow field was measured with an optical method, that gives the velocity, the concentration and the granular temperature for both the small and the large particles, from the sidewalls.</p><p>Analyzing the experimental data, as regards the longitudinal velocity, it is possible to observe that the velocities of the two classes are similar and the large particles flow a bit faster. In contrast, there is a strong segregation in the concentration rates. After the running time, segregation causes the separation of the two classes: the largest classes are in the upper part and the smallest fraction at the bottom.</p><p> </p><p>References</p><p>1 Drahun J.A., Bridgwater J. The mechanisms of free surface segregation, Powder Technology, 36, 39-53, 1988.</p><p>2 Savage S., Lun K.K. Particle size segregation in inclined chute of dry cohesionless granular solids, Journal of Fluid Mechanics, 189, 311-335, 1988.</p><p> </p>

scholarly journals Geological implication of grain-size segregation in dense granular matter

2018 ◽  
Vol 377 (2136) ◽  
pp. 20170390 ◽  
Author(s):  
Ryo Itoh ◽  
Takahiro Hatano
Keyword(s):  
Grain Size ◽  
Normal Stress ◽  
Volume Fraction ◽  
Granular Matter ◽  
Fluid Pressure ◽  
Fault Gouge ◽  
Frictional Heat ◽  
Granular System ◽  
Common Belief ◽  
Size Segregation

To the current common belief, grain size segregation in granular matter requires sufficient porosity. Therefore, grain size segregation found in a natural fault gouge could imply elevated fluid pressure and the reduced normal stress on fault, possibly caused by the frictional heat during an earthquake. To clarify whether fluidization is essential to grain size segregation, we conduct numerical simulation on a simple model of fault gouge in a plane shear geometry under constant volume condition: the volume fraction is fixed at 0.6, at which the granular system possesses yield stress. We observe apparent grain size segregation at this volume fraction, meaning that grain size segregation alone does not imply fluidization of granular matter. We also show that segregation is driven by the nonlinear velocity profile, and that the gravity is not essential to segregation. The physical condition tested here may be relevant to earthquake faults: the normal stress of 1 MPa, the sliding velocity of 1 m s −1 , and the duration of 0.1 s. This article is part of the theme issue ‘Statistical physics of fracture and earthquakes’.

scholarly journals Investigation of Damage Evolution in Heterogeneous Rock Based on the Grain-Based Finite-Discrete Element Model

Materials ◽  
2021 ◽  
Vol 14 (14) ◽  
pp. 3969
Author(s):  
Shirui Zhang ◽  
Shili Qiu ◽  
Pengfei Kou ◽  
Shaojun Li ◽  
Ping Li ◽  
...  
Keyword(s):  
Mechanical Properties ◽  
Grain Size ◽  
Tensile Strength ◽  
Grain Orientation ◽  
Damage Evolution ◽  
Discrete Element ◽  
Uniaxial Tensile ◽  
Tensile Cracks ◽  
Rock Heterogeneity ◽  
Heterogeneous Rock

Granite exhibits obvious meso-geometric heterogeneity. To study the influence of grain size and preferred grain orientation on the damage evolution and mechanical properties of granite, as well as to reveal the inner link between grain size‚ preferred orientation, uniaxial tensile strength (UTS) and damage evolution, a series of Brazilian splitting tests were carried out based on the combined finite-discrete element method (FDEM), grain-based model (GBM) and inverse Monte Carlo (IMC) algorithm. The main conclusions are as follows: (1) Mineral grain significantly influences the crack propagation paths, and the GBM can capture the location of fracture section more accurately than the conventional model. (2) Shear cracks occur near the loading area, while tensile and tensile-shear mixed cracks occur far from the loading area. The applied stress must overcome the tensile strength of the grain interface contacts. (3) The UTS and the ratio of the number of intergrain tensile cracks to the number of intragrain tensile cracks are negatively related to the grain size. (4) With the increase of the preferred grain orientation, the UTS presents a “V-shaped” characteristic distribution. (5) During the whole process of splitting simulation, shear microcracks play the dominant role in energy release; particularly, they occur in later stage. This novel framework, which can reveal the control mechanism of brittle rock heterogeneity on continuous-discontinuous trans-scale fracture process and microscopic rock behaviour, provides an effective technology and numerical analysis method for characterizing rock meso-structure. Accordingly, the research results can provide a useful reference for the prediction of heterogeneous rock mechanical properties and the stability control of engineering rock masses.

scholarly journals A new time-delayed periodic boundary condition for discrete element modelling of railway track under moving wheel loads

2021 ◽  
Vol 23 (4) ◽  
Author(s):  
Huiqi Li ◽  
Glenn McDowell ◽  
John de Bono
Keyword(s):  
Boundary Condition ◽  
Periodic Boundary Condition ◽  
Periodic Boundary ◽  
Discrete Element ◽  
Contact Forces ◽  
Railway Track ◽  
Discrete Element Modelling ◽  
Fixed Boundary ◽  
Element Modelling ◽  
New Time

Abstract A new time-delayed periodic boundary condition (PBC) has been proposed for discrete element modelling (DEM) of periodic structures subject to moving loads such as railway track based on a box test which is normally used as an element testing model. The new proposed time-delayed PBC is approached by predicting forces acting on ghost particles with the consideration of different loading phases for adjacent sleepers whereas a normal PBC simply gives the ghost particles the same contact forces as the original particles. By comparing the sleeper in a single sleeper test with a fixed boundary, a normal periodic boundary and the newly proposed time-delayed PBC (TDPBC), the new TDPBC was found to produce the closest settlement to that of the middle sleeper in a three-sleeper test which was assumed to be free of boundary effects. It appears that the new TDPBC can eliminate the boundary effect more effectively than either a fixed boundary or a normal periodic cell. Graphic abstract

scholarly journals The effect of zirconia sintering temperature on flexural strength, grain size, and contrast ratio

2012 ◽  
Vol 17 (1) ◽  
pp. 269-274 ◽  
Author(s):  
Bogna Stawarczyk ◽  
Mutlu Özcan ◽  
Lubica Hallmann ◽  
Andreas Ender ◽  
Albert Mehl ◽  
...  
Keyword(s):  
Grain Size ◽  
Flexural Strength ◽  
Sintering Temperature ◽  
Contrast Ratio

Effect of Li on Mechanical Properties and Electrical Conductivity of the Al–Zn–Cu–Mg Based Alloys

2021 ◽  
Vol 21 (9) ◽  
pp. 4897-4901
Author(s):  
Hyo-Sang Yoo ◽  
Yong-Ho Kim ◽  
Hyeon-Taek Son
Keyword(s):  
Mechanical Properties ◽  
Electrical Conductivity ◽  
Grain Size ◽  
Tensile Strength ◽  
Spherical Particles ◽  
Al Alloy ◽  
High Angle ◽  
Average Grain Size ◽  
Strength Improvement ◽  
Li Content

In this study, changes in the microstructure, mechanical properties, and electrical conductivity of cast and extruded Al–Zn–Cu–Mg based alloys with the addition of Li (0, 0.5 and 1.0 wt.%) were investigated. The Al–Zn–Cu–Mg–xLi alloys were cast and homogenized at 570 °C for 4 hours. The billets were hot extruded into rod that were 12 mm in diameter with a reduction ratio of 38:1 at 550 °C. As the amount of Li added increased from 0 to 1.0 wt.%, the average grain size of the extruded Al alloy increased from 259.2 to 383.0 µm, and the high-angle grain boundaries (HGBs) fraction decreased from 64.0 to 52.1%. As the Li content increased from 0 to 1.0 wt.%, the elongation was not significantly different from 27.8 to 27.4% and the ultimate tensile strength (UTS) was improved from 146.7 to 160.6 MPa. As Li was added, spherical particles bonded to each other, forming an irregular particles. It is thought that these irregular particles contribute to the strength improvement.

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