Modeling Fracturing in Rock Using a Modified Discrete Element Method with Plasticity

2010 ◽  
Vol 452-453 ◽  
pp. 861-864 ◽  
Author(s):  
Haitham Alassi ◽  
Rune Holt
Keyword(s):  
Shear Bands ◽  
Discrete Element ◽  
Initial Crack ◽  
Reservoir Rock ◽  
Biaxial Compression ◽  
Compression Tests ◽  
Direct Tension ◽  
Direct Tension Test ◽  
New Development ◽  
Element Approach

In this contribution we present more developments in the modified discrete element approach (MDEM) which was proposed to model fractures propagation in reservoir rock during production and fluid injection. The new development in this paper includes adding plasticity which allows the material to go into plastic deformation before initiating fractures. Several numerical tests are performed which mimic the real lab tests usually perform on rock samples, this includes direct tension test with initial crack to model crack propagation, and several biaxial compression tests to model the development of shear bands.

scholarly journals A Constitutive Model for Locally Drained Shear Bands in Globally Undrained Dense Sand

2019 ◽  
Vol 92 ◽  
pp. 16005
Author(s):  
Hansini Mallikarachchi ◽  
Kenichi Soga
Keyword(s):  
Shear Band ◽  
Constitutive Model ◽  
Bifurcation Analysis ◽  
Shear Bands ◽  
Constitutive Relationship ◽  
Excess Pore Pressure ◽  
Biaxial Compression ◽  
Compression Tests ◽  
Equilibrium Equations ◽  
Dense Sand

When saturated granular materials which are dilative in nature are subjected to the undrained deformation, their strength increases due to the generation of negative excess pore pressure. This phenomenon is known as dilative hardening and can be witnessed in saturated dense sand or rocks during very fast loading. However, experimental evidence of undrained biaxial compression tests of dense sand shows a limit to this dilative hardening due to the formation of shear bands. There is no consensus in the literature about the mechanism which triggers these shear bands in the dense dilative sand under isochoric constraint. The possible theoretical reasoning is the local drainage inside the specimen under the globally undrained condition, which is challenging to be monitored experimentally. Hence, both incept of localisation and post-bifurcation of the saturated undrained dense sand demand further numerical investigation. Pathological mesh dependency hinders the ability of the finite element method to represent the localisation without advanced regularisation methods. This paper attempt to provide a macroscopic constitutive behaviour of the undrained deformation of the saturated dense sand in the presence of a locally drained shear band. Discontinuation of dilatant hardening due to partial drainage between the shear band and the adjacent material is integrated into the constitutive model without changing governing equilibrium equations. Initially, a classical bifurcation analysis is conducted to detect the inception and inclination of the shear band based on the underlying drained deformation. Then a post-bifurcation analysis is carried out assuming an embedded drained or partially drained shear band at gauss points which satisfy bifurcation criterion. The smeared shear band approach is utilised to homogenise the constitutive relationship. It is observed that the dilatant hardening in the saturated undrained dense sand is reduced considerably due to the formation of shear bands.

Effects of Rolling Resistance on Shear Behavior and Anisotropy of Granular Matters

2013 ◽  
Vol 631-632 ◽  
pp. 198-204
Author(s):  
Yi Ming Liu ◽  
Hai Jun Mao ◽  
Chun He Yang
Keyword(s):  
Shear Strength ◽  
Discrete Element Method ◽  
Numerical Simulations ◽  
Rolling Resistance ◽  
Discrete Element ◽  
Shear Behavior ◽  
Contact Model ◽  
Biaxial Compression ◽  
Compression Tests ◽  
Dramatic Effect

Standard discrete element method does not take the effect of rolling resistance into account. To overcome this shortcoming, a contact model considering rolling resistance is developed and implemented into PFC2D. Using this contact model, a series of numerical biaxial compression tests are carried out. The results of these numerical simulations show that rolling resistance has remarkable effects on shear strength and shear dilatancy of granular matters, and these trends are agreed with previous studies, which proves that this model works well. Then the effect of rolling resistance on anisotropy of granular matters is studied in this paper. It can be seen that rolling resistance has dramatic effect on the anisotropy of granular matters. The anisotropy of granular matters increases with rolling resistance.

scholarly journals Microstructure-based modeling of snow mechanics: a discrete element approach

2015 ◽  
Vol 9 (2) ◽  
pp. 1425-1460 ◽  
Author(s):  
P. Hagenmuller ◽  
G. Chambon ◽  
M. Naaim
Keyword(s):  
Three Dimensional ◽  
Element Model ◽  
Discrete Element ◽  
Compression Tests ◽  
Confined Compression ◽  
Compression Behavior ◽  
Bonding System ◽  
Element Approach ◽  
Snow Mechanics ◽  
Snow Samples

Abstract. Rapid and large deformations of snow are mainly controlled by grain rearrangements, which occur through the failure of cohesive bonds and the creation of new contacts. We exploit a granular description of snow to develop a discrete element model based on the full three-dimensional microstructure captured by microtomography. The model assumes that snow is composed of rigid grains interacting through localized contacts accounting for cohesion and friction. The geometry of the grains and of the intergranular bonding system are explicitly defined from microtomographic data using geometrical criteria based on curvature and contiguity. Single grains are represented as rigid clumps of spheres. The model is applied to different snow samples subjected to confined compression tests. A detailed sensitivity analysis shows that artifacts introduced by the modeling approach and the influence of numerical parameters are limited compared to variations due to the geometry of the microstructure. The model shows that the compression behavior of snow is mainly controlled by the density of the samples, but that deviations from a pure density parameterization are not insignificant during the first phase of deformation. In particular, the model correctly predicts that, for a given density, faceted crystals are less resistant to compression than rounded grains or decomposed snow. For larger compression strains, no clear differences between snow types are observed.

scholarly journals Prediction of Shear Localisation in Granular Materials based on a Critical State Non-coaxial Model

2019 ◽  
Vol 92 ◽  
pp. 16006 ◽  
Author(s):  
Hansini Mallikarachchi ◽  
Kenichi Soga
Keyword(s):  
Granular Materials ◽  
Plane Strain ◽  
Critical State ◽  
Progressive Failure ◽  
Shear Bands ◽  
State Parameter ◽  
Biaxial Compression ◽  
Compression Tests ◽  
Principal Axes ◽  
Shear Localisation

Experimental evidence indicates that the shear localisation acts as a precursor to the failure in biaxial compression tests of granular materials. Once formed they are persistent and lead to progressive failure of most geotechnical structures. It is generally accepted that the primary mode of deformation within these shear bands is simple shear which is accompanied by rotation of principal axes. Hence, the conventional plasticity theories based on the assumption of coaxility is not sufficient to describe the behaviour within those shear bands. This paper highlights the influence of the non-coaxility on the initiation and orientation of shear bands in both drained and undrained sand. The con-coaxial plasticity theory is integrated into a critical state constitutive model enriched with the state parameter concept. The model is capable of taking account of the variation of lode angle under plane strain condition. Numerical plane strain biaxial compression tests are conducted to observe the effect of non-coaxility on shear localisation. Bifurcation criteria based on the acoustic tensor are checked to predict the onset and inclination of the shear band. Predictions from the non-coaxial model are compared with those of coaxial model. The influence of the initial void ratio for the formation of shear bands is explored. Results are compared qualitatively with experimental observations.

scholarly journals Microstructure-based modeling of snow mechanics: a discrete element approach

2015 ◽  
Vol 9 (5) ◽  
pp. 1969-1982 ◽  
Author(s):  
P. Hagenmuller ◽  
G. Chambon ◽  
M. Naaim
Keyword(s):  
Large Deformations ◽  
Element Model ◽  
Discrete Element ◽  
Compression Tests ◽  
Confined Compression ◽  
Compression Behavior ◽  
Bonding System ◽  
Element Approach ◽  
Snow Mechanics ◽  
Snow Samples

Abstract. Rapid and large deformations of snow are mainly controlled by grain rearrangements, which occur through the failure of cohesive bonds and the creation of new contacts. We exploit a granular description of snow to develop a discrete element model based on the full 3-D microstructure captured by microtomography. The model assumes that snow is composed of rigid grains interacting through localized contacts accounting for cohesion and friction. The geometry of the grains and of the intergranular bonding system are explicitly defined from microtomographic data using geometrical criteria based on curvature and contiguity. Single grains are represented as rigid clumps of spheres. The model is applied to different snow samples subjected to confined compression tests. A detailed sensitivity analysis shows that artifacts introduced by the modeling approach and the influence of numerical parameters are limited compared to variations due to the geometry of the microstructure. The model shows that the compression behavior of snow is mainly controlled by the density of the samples, but that deviations from a pure density parameterization are not insignificant during the first phase of deformation. In particular, the model correctly predicts that, for a given density, faceted crystals are less resistant to compression than rounded grains or decomposed snow. For larger compression strains, no clear differences between snow types are observed.

A Discrete Element Approach to Model Breakable Railway Ballast

2012 ◽  
Vol 7 (4) ◽  
Author(s):  
Christian Ergenzinger ◽  
Robert Seifried ◽  
Peter Eberhard
Keyword(s):  
Experimental Data ◽  
Element Model ◽  
Discrete Element ◽  
Quantitative Agreement ◽  
Discrete Element Model ◽  
Compression Tests ◽  
Railway Ballast ◽  
Element Approach ◽  
Strong Rock ◽  
Granular Solid

A discrete element approach to assess degradation processes in ballast beds is presented. Firstly, a discrete element model describing strength and failure of strong rock is introduced. For this purpose a granular solid is created by bonding of adjacent particles. A method to define angular ballast stones made from the granular solid is proposed. The strength of these stones is evaluated by compression between parallel platens. Comparing these results to published experimental data yields very good qualitative and reasonable quantitative agreement. Finally, the failure of aggregates of breakable stones is investigated by simulation of oedometric compression tests and indentation of a sleeper into a ballast bed.

scholarly journals EVOLUTION OF THE CARBON NANOTUBE BUNDLE STRUCTURE UNDER BIAXIAL AND SHEAR STRAINS

2020 ◽  
pp. 525
Author(s):  
Leysan Kh. Rysaeva ◽  
Dmitry V. Bachurin ◽  
Ramil T. Murzaev ◽  
Dina U. Abdullina ◽  
Elena A. Korznikova ◽  
...  
Keyword(s):  
Carbon Nanotube ◽  
Shear Strain ◽  
Cross Sections ◽  
Degrees Of Freedom ◽  
Shear Bands ◽  
Volumetric Strain ◽  
Structural Evolution ◽  
Biaxial Compression ◽  
Chain Model ◽  
Nanotube Bundle

Close packed carbon nanotube bundles are materials with highly deformable elements, for which unusual deformation mechanisms are expected. Structural evolution of the zigzag carbon nanotube bundle subjected to biaxial lateral compression with the subsequent shear straining is studied under plane strain conditions using the chain model with a reduced number of degrees of freedom. Biaxial compression results in bending of carbon nanotubes walls and formation of the characteristic pattern, when nanotube cross-sections are inclined in the opposite directions alternatively in the parallel close-packed rows. Subsequent shearing up to a certain shear strain leads to an appearance of shear bands and vortex-like displacements. Stress components and potential energy as the functions of shear strain for different values of the biaxial volumetric strain are analyzed in detail. A new mechanism of carbon nanotube bundle shear deformation through cooperative, vortex-like displacements of nanotube cross sections is reported.

Hot working of SP-700 titanium alloy with lamellar α + β starting structure using a processing map

2020 ◽  
Vol 111 (4) ◽  
pp. 297-306
Author(s):  
Amir Hosein Sheikhali ◽  
Maryam Morakkabati
Keyword(s):  
Titanium Alloy ◽  
Strain Rate ◽  
Hot Deformation ◽  
Processing Map ◽  
Microstructural Analysis ◽  
Shear Bands ◽  
Alpha Phase ◽  
Strain Rates ◽  
Compression Tests ◽  
Temperature Range

Abstract In this study, hot deformation behavior of SP-700 titanium alloy was investigated by hot compression tests in the temperature range of 700-9508C and at strain rates of 0.001, 0.1, and 1 s-1. Final mechanical properties of the alloy (hot compressed at different strain rates and temperatures) were investigated using a shear punch testing method at room temperature. The flow curves of the alloy indicated that the yield point phenomenon occurs in the temperature range of 800- 9508C and strain rates of 0.1 and 1 s-1. The microstructural analysis showed that dynamic globularization of the lamellar α phase starts at 7008C and completes at 8008C. The alpha phase was completely eliminated from b matrix due to deformation- induced transformation at 8508C. The microstructure of specimens compressed at 8508C and strain rates of 0.001 and 0.1 s-1showed the serration of beta grain boundaries, whereas partial dynamic recrystallization caused a necklace structure by increasing strain rate up to 1 s-1. The specimen deformed at 7008C and strain rate of 1 s-1was located in the instability region and localized shear bands formed due to the low thermal conductivity of the alloy. The processing map of the alloy exhibited a peak efficiency domain of 54% in the temperature range of 780-8108C and strain rates of 0.001- 0.008 s-1. The hot deformation activation energy of the alloy in the α/β region (305.5 kJ mol-1) was higher than that in the single-phase β region (165.2 kJ mol-1) due to the dynamic globularization of the lamellar a phase.

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