Loading...
Search for:
khoei--a--r---amir-reza
0.201 seconds
Total 20399 records
Temperature-dependent multi-scale modeling of surface effects on nano-materials
, Article Mechanics of Materials ; Volume 46 , March , 2012 , PP. 94–112 ; Ghahremani, P ; Sharif University of Technology
2012
Abstract
In this paper, a novel temperature-dependent multi-scale method is developed to investigate the role of temperature on surface effects in the analysis of nano-scale materials. In order to evaluate the temperature effect in the micro-scale (atomic) level, the temperature related Cauchy–Born hypothesis is implemented by employing the Helmholtz free energy, as the energy density of equivalent continua relating to the inter-atomic potential. The multi-scale technique is applied in atomistic level (nano-scale) to exhibit the temperature related characteristics. The first Piola–Kirchhoff stress and tangential stiffness tensor are computed, as the first and second derivatives of the free energy...
Multi-scale modeling of edge effect on band gap offset in polygonal cross-section silicon nanowires
, Article Computational Materials Science ; Volume 79 , November , 2013 , PP. 262–275 ; DorMohammadi, H ; Aramoon, A ; Sharif University of Technology
2013
Abstract
The band gap offset is an effect of coordination numbers (CNs) of atom reduction at the edge of transversal cross-section of Silicon nanowires (SiNWs). In this paper, a hierarchical multi-scale technique is developed to model the edge effect on the band gap shift of SiNWs since the geometric effect is dominant in the energy gap due to the appearance of strain in the self-equilibrium state. The multi-scale model is performed based on the molecular dynamics approach and finite element method for the micro- (atomistic) and macro-scale levels, respectively. The Cauchy–Born (CB) hypothesis is used to relate the atomic positions to the continuum field through the deformation gradient. Finally, the...
Hydro-mechanical modeling of cohesive crack propagation in multiphase porous media using the extended finite element method
, Article International Journal for Numerical and Analytical Methods in Geomechanics ; Volume 37, Issue 10 , July , 2013 , PP. 1247–1279 ; Khoei, A. R. (Amir Reza) ; Sharif University of Technology
2013
Abstract
In this paper, a numerical model is developed for the fully coupled hydro-mechanical analysis of deformable, progressively fracturing porous media interacting with the flow of two immiscible, compressible wetting and non-wetting pore fluids, in which the coupling between various processes is taken into account. The governing equations involving the coupled solid skeleton deformation and two-phase fluid flow in partially saturated porous media including cohesive cracks are derived within the framework of the generalized Biot theory. The fluid flow within the crack is simulated using the Darcy law in which the permeability variation with porosity because of the cracking of the solid skeleton...
Validity and size-dependency of cauchy–born hypothesis with Tersoff potential in silicon nano-structures
, Article Computational Materials Science ; Volume 63 , October , 2012 , PP. 168–177 ; Dormohammadi, H. (Hossein) ; Sharif University of Technology
2012
Abstract
One of the most popular constitutive rules that correlate the continuum and atomic properties in multi-scale models is the Cauchy–Born (CB) hypothesis. Based on this constitutive law of continuum media, it assumes that all atoms follow the deformation subjected to the boundary of crystal. In this paper, the validity and failure of CB hypothesis are investigated for the silicon nano-structure by comparison of the continuum and atomic properties. In the atomistic level, the stresses and position of atoms are calculated using the molecular dynamics (MD) simulation based on the Tersoff inter-atomic potential. The stresses and strains are compared between the atomistic and continuous media to...
Modeling of hot isostatic pressing of metal powder with temperature–dependent cap plasticity model
, Article International Journal of Material Forming ; Volume 6, Issue 3 , September , 2013 , PP. 363-376 ; Molaeinia, Z ; Keshavarz, Sh ; Sharif University of Technology
2013
Abstract
In this paper, the coupled thermo–mechanical simulation of hot isostatic pressing (HIPing) is presented for metal powders during densification process. The densification of powder is assumed to occur due to plastic hardening of metal particles. The constitutive model developed is used to describe the nonlinear behavior of metal powder. The numerical modeling of hot powder compaction simulation is performed based on the large deformation formulation, powder plasticity behavior, and frictional contact algorithm. A Lagrangian finite element formulation is employed for the large powder deformations. A modified cap plasticity model considering temperature effects is used in numerical simulation...
Numerical simulation of ductile crack growth under cyclic and dynamic loading with a damage–viscoplasticity model
, Article Engineering Fracture Mechanics ; Volume 99 , February , 2013 , PP. 169–190 ; Eghbalian, M ; Azadi, H ; Saffar, H ; Sharif University of Technology
2013
Abstract
In this paper, the crack propagation in ductile materials is simulated under cyclic and dynamic loading. The adaptive finite element method is used to model the discontinuity due to crack propagation. The ductile fracture assumptions and continuum damage mechanics are utilized to model the material rupture behavior. Moreover both the rate-independent and rate-dependent constitutive equations are elaborated and the crack closure effect and combined hardening model are discussed in addition to some aspects of finite element implementation. Finally, a comparison is performed between the numerical simulation results and those of experiments to illustrate the robustness of proposed computational...
XFEM Modeling of Dynamic Cohesive Crack Propagation in Saturated Porous Media
, M.Sc. Thesis Sharif University of Technology ; Khoei, Amir Reza (Supervisor)
Abstract
In this thesis, a fully coupled numerical model is developed for the modeling of dynamic cohesive crack propagation and hydraulic fracture in saturated porous media using extended finite element method. Many engineering structures like concrete or soil dams and buildings foundation are built with porous materials like concrete, rock and soil. Behavior of these materials in which void among the solid particles are filled with one or more fluids are so complicated rather than single solid phase. Dynamic analysis of porous mediums containing a discontinuity has many applications in various civil engineering fields including structure, earthquake, hydraulic structures, etc. For instance...
Polygonal Finite Element Modeling of Fracture Mechanism and Crack Propagation
, M.Sc. Thesis Sharif University of Technology ; Khoei, Amir Reza (Supervisor)
Abstract
Fracture is one of the most important engineering problems, and the lack of knowledge about this phenomenon will result in loss of life and property. Before the computer age, fracture mechanics has been studied by many analytical mechanics researchers; and after that, lots of attempts have been done to accurately model this phenomenon.
Finite element method, one of the best methods in Computational Mechanics, is common in computational fracture mechanics. Polygonal finite element is a new concept which has been recently applied in finite element analysis. This research utilized this concept in com-putational fracture mechanics. In another word, the crack discontinuity and crack tip...
Finite element method, one of the best methods in Computational Mechanics, is common in computational fracture mechanics. Polygonal finite element is a new concept which has been recently applied in finite element analysis. This research utilized this concept in com-putational fracture mechanics. In another word, the crack discontinuity and crack tip...
Multi-sclae Modeling for Determination of Thermal Properties of Silicon Nanostructures Via Molecular Dynamics (MD) and Finite Element Method (FEM)
,
Ph.D. Dissertation
Sharif University of Technology
;
Khoei, Amir Reza
(Supervisor)
Abstract
The band gap offset is an effect of coordination numbers (CNs) of atoms reduction at the edge of transversal cross-section Si nanowires (SiNWs) which would be of increasingly important for greater shell-core ratio sections. In this paper, a hierarchical multi-scale modeling has been developed to simulate edge effect on the band gap shift of SiNWs due to geometry effect induced strain in the self-equilibrium state. Classical Molecular Dynamics (MD) approach and Finite Element Method (FEM) are used in the micro (atomic) and macro scale levels, respectively. Using the Cauchy-Born (CB) hypothesis as a correlator of continuum and atomic properties, the atomic positions are related to the...
Modeling the Dynamic Contact with Large Deformations Using the G-ALE-FEM Method
, M.Sc. Thesis Sharif University of Technology ; Khoei, Amir Reza (Supervisor)
Abstract
Contact between different parts of a system and their interactions on each other is one of the most important phenomena that we face in modeling a variety of mechanical issues which should be carefully considered. Sometimes, this phenomenon occurs between different components in a phase and some other times between several phases, which, causes changes in the performance and response of the system. Therefore, in order to investigate its effect in particular on dynamic problems that are subject to severe changes over a short period of time, and to provide more effective methods for dealing with it, the subject of this research has been devoted to dynamic contact modeling with large...
Coarse-gained Multi-scale Modeling for Numerical Simulation of Nonlinear Behavior of Materials in Nano-scale
, M.Sc. Thesis Sharif University of Technology ; Khoei, Amir Reza (Supervisor)
Abstract
In this thesis, a coarse-grained multi-scale method for 2D crystallyn solids based-on finite element consepts has presented. In this method, both scales are atomic scale and similar to what we see in non-local QC method, the entire atomic structure will be intact. Accordingly, calculations of potential functions and forces in the domain will have the atomic accuracy. In the presented method to reduce the domain’s degrees of freedom, the classical finite-element meshing concept to mesh the elastic linear areas in the domain is used and the MD calculations will done on the mesh nodes. Therefore, degrees of freedom in the system will reduce and consequently, the computational cost will reduce....
Crack Propagation Modeling in Arched Concrete Structures Reinforced by FRP Using XFEM and Damage Model
, M.Sc. Thesis Sharif University of Technology ; Khoei, Amir Reza (Supervisor)
Abstract
In practice, structures made of concrete are full of cracks. The strength of concrete is mainly determined by the tensile strength, which is about 10% of the compressive strength. As long as cracking in concrete is unavoidable, we have to try to minimize their detrimental effects. This objective can be achieved by resisting (or limiting) propagation of existing cracks. Because of this, reinforcement (mostly steel) is used to increase the carrying capacity of the material and to control the development of cracks. Concrete structures that fail, already shows a large number of large and small cracks before their maximum carrying capacity is reached. The failure of concrete can be characterized...
Modeling Fracture Problems with X-FEM
, M.Sc. Thesis Sharif University of Technology ; Khoei, Amir Reza (Supervisor)
Abstract
Every year, fracture imposes high economic costs and casualties to all societies. Since the beginning of the twentieth century, scientific approach to this issue has lead to invention of a new branch in mechanics, called fracture mechanics. In general, fracture problems fall into two categories. Brittle fracture, like what happens in glass, in which, few plastic deformations and energy absorption occurs and ductile fracture, which is preceded by large plastic deformations and energy absorption. This kind of fracture is usual in ductile metals like low carbon steel. Finite Element which is considered as the most important numerical method in the mechanics of materials, is also, widely used in...
Modeling Saturated Porous Media Using Extended Finite Element Method
, M.Sc. Thesis Sharif University of Technology ; Khoei, Amir Reza (Supervisor)
Abstract
Soil structures have important roles in civil engineering applications. The most important types of these structures are soil dams and foundations which their damages may cause huge loss. Thus, appropriate analysis of these structures under possible loading conditions is unavoidable. The real behavior of this media can be achieved from the solution of its coupled governing differential equations. In order to solve this set of equations in practical applications, appropriate numerical solutions should be used. The finite element method can be called as one of the most important numerical solutions of differential equations which have been used in analyzing different types of engineering...
Modeling of Incompressible Materials Using the Extended Finite Element Method (XFEM)
,
M.Sc. Thesis
Sharif University of Technology
;
Khoei, Amir Reza
(Supervisor)
Abstract
In the limit case of incompressibility, the displacement-based finite element methods are not capable of finding the solutions with adequate accuracy. Moreover, discontinuities in displacement field or strain field which exist in the interior of the elements should be dealt with appropriately. The u/p mixed formulation provides a suitable context for modeling the incompressible problems. It is capable of solving general problems in which there exist geometrical or material nonlinearities. In the case of employing the eXtended Finite Element Method (XFEM), uniform meshes can be used for problems with discontinuities and in fact the discontinuities can be decoupled from the mesh. In this...
A None-Associated Cap Plasticity with Isotropic-Kinematic Hardening-Softening Rule
, M.Sc. Thesis Sharif University of Technology ; Khoei, Amir Reza (Supervisor)
Abstract
The constitutive modeling of material is clearly a keystone of successful quantitative solution possibilities. No finite element code will provide results of better quality than that of the constitutive equation implemented in it. In this study, a nonassociated three-invariant cap plasticity model is presented for material under compressive stress. Generalized single-cap plasticity is developed which is based on the hardening-softening material functions. The hardening functions are consisted of isotropic and kinematic parts. Using the definition of yield surface, material functions and nonlinear elastic behavior, as function of hardening-softening parameters, the constitutive elasto-plastic...
Contact Friction Modeling Using a new Node-to-Surface Algorithm
, M.Sc. Thesis Sharif University of Technology ; Khoei, Amir Reza (Supervisor)
Abstract
The present research illustrate the finite element modeling of contact between solid bodies, with a special emphasis on the imposing the contact constraints and modification of contact properties on surface in the case of frictional slip. A new approach for both two-dimensional and three-dimensional formulation of contact constraint that allows for a simple and unified treatment of all potential contact scenarios in the presence of large deformations in static case, is presented. The most important outstanding issue in this approach is symmetrical contact stiffness matrix which reduces computational efforts. Based on the observation of numerical results and comparison by experimental models,...
Modelling of Elastic and Plastic Deformation Fracture and Crack Propagation in 3D Problems Using Adaptive Finite Element Method
, Ph.D. Dissertation Sharif University of Technology ; Khoei, Amir Reza (Supervisor)
Abstract
Numerical methods in fracture and crack propagation problems usually involve high computational costs. Adaptive finite element method is one of the techniques which can be used to simulate the crack propagation with an acceptable accuracy. In this thesis, various constitutive models are implemented for simulation of fracture, including the linear elastic fracture mechanics, cohesive zone model and continuum damage mechanics. The fracture mechanical evaluation is performed on a general integral technique for non-planar curved cracks in LEFM. In the second model, a bilinear cohesive zone model is applied to implement the traction-separation law. The Lemaitre damage model is employed and the...
Modeling of Crack Propagation in Saturated Two Phase Porous Media Using X-FEM
, M.Sc. Thesis Sharif University of Technology ; Khoei, Amir Reza (Supervisor)
Abstract
Twophase medias are one of the most complicated medias in engineering and because of its importance, its been considered by a lot of researchers ever since. Varaioty of the problems in these medias, has ended in lots of methods for studing them. The primariative efforts in modeling deformable pouros medias was done by Terzaghi and others have improved the primary consepts and have suggested different methods. One of the most common and applicable methods in these medias is u-p formulation. This form is applicable in low frequencies (such as earthquakes) with great accuracy. In this thises, this form is used as primery formulation. Because deformation in multiphase problems can be large, in...
Finite Element Modeling of Impact Problems with Friction and Large Deformations
, M.Sc. Thesis Sharif University of Technology ; Khoei, Amir Reza (Supervisor)
Abstract
One of the most important problems that challenge the researchers in modeling physical phenomena is the Impact Problem. The complexity of impact problems is due to the fact that this problem consists of various sub-problems, each of which of high complexity and in order to have a clear understanding and correct modeling of the problem, one needs to address each and every one of these problems with great effort, severity and punctuality.Impact problems often involve one or more dynamic systems that can also be subjected to dynamic loading. This loading could be a result of external agents or dynamic interaction of the two bodies with each other. In cases where the problem involves interaction...