Temperature-dependent multi-scale modeling of surface effects on nano-materials

, Article Mechanics of Materials ; Volume 46 , March , 2012 , PP. 94–112 Khoei, A. R. (Amir Reza) ; 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 Khoei, A. R. (Amir Reza) ; 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 Mohammadnejad, T. (Toktam) ; 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 Khoei, A. R. (Amir Reza) ; 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 Khoei, A. R. (Amir Reza) ; 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 Khoei, A. R. (Amir Reza) ; 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...

Polygonal Finite Element Modeling of Fracture Mechanism and Crack Propagation

, M.Sc. Thesis Sharif University of Technology Yasbolaghi, Reza (Author) ; 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...

XFEM Modeling of Dynamic Cohesive Crack Propagation in Saturated Porous Media

, M.Sc. Thesis Sharif University of Technology Babazadeh, Mohsen (Author) ; 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...

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 DorMohammadi, Hossein (Author) ; 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...

Coarse-gained Multi-scale Modeling for Numerical Simulation of Nonlinear Behavior of Materials in Nano-scale

, M.Sc. Thesis Sharif University of Technology Mohammadi, Khashayar (Author) ; 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....

Modeling the Dynamic Contact with Large Deformations Using the G-ALE-FEM Method

, M.Sc. Thesis Sharif University of Technology Mohajeri, Sina (Author) ; 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...

Crack Propagation Modeling in Arched Concrete Structures Reinforced by FRP Using XFEM and Damage Model

, M.Sc. Thesis Sharif University of Technology Mohammadi, Amir Hossein (Author) ; 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 of Incompressible Materials Using the Extended Finite Element Method (XFEM)

, M.Sc. Thesis Sharif University of Technology Mirkhosravi, Poorya (Author) ; 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...

Application of Isogeometric Method in Modeling and Analyzing Crack Growth Problems

, M.Sc. Thesis Sharif University of Technology Esmaeili, Mir Sardar (Author) ; Khoei, Amir Reza (Supervisor)

Abstract

Isogeometric Analysis method is a newly introduced method for the analysis of problems governed by partial differential equations. The method has some features in common with the finite element method and some in common with the mesh-less methods. This method uses the Non-Uniform Rational B-Splines (NURBS) functions as basis function for analysis. With this basis functions, the refinement procedure is much easier than the classical finite element method by eliminating the need to communicate with the CAD model. Modeling cracks in classical finite element method requires very fine mesh near the crack tip. One can model crack propagation by means of classical finite element, using an updating...

Modeling of Crack Propagation in Non-isothermalsaturatedPorous Media using XFEM

, M.Sc. Thesis Sharif University of Technology Moallemi, Sina (Author) ; Khoei, Amir Reza (Supervisor)

Abstract

The probability of crack appearance in soil structures and porous media is not avoidable, which could be the reason of structures collapse. According to the important affects, which they play in the vulnerability of the structures, they should be taking into account. The cracks have different effects on various materials. The most properties that cracks have, is their ability of conveying the fluid flow. For the most accurate analysis of discontinues domains, their governing equations should be taken and solved. Finite Element Method is one of the best solutions of differential governing equations. However, the appearance of some problems in the modeling of discontinues domain, was the...

Simulation of Crack Propagation in Ductile Metals Under Dynamic Cyclic Loading by Adaptive Finite Element Method and Continuum Damage Mechanics Model

, M.Sc. Thesis Sharif University of Technology Eghbalian, Mahdad (Author) ; Khoei, Amir Reza (Supervisor)

Abstract

Crack nucleation and growth is unfavorable in many industrial and every day-life cases. designers’ effort is to prevent or delay it by taking into account safety and maintenance considerations; but in some industrial operations, the main target is to form a crack in a part to achieve a particular shape; and designers’ duty is to control the way it happens. so numerical modeling of this phenomena has many useful applications in preventing the structures’ failure and designing the production processes for industrial goods; and because of this, a great attention has been paid to it in the last two decades. a situation usually encountered in every day-life is the earthquake excitation which...

Three-Dimensional Cohesive Modeling of Curved Crack Growth in Quasi-brittle Material Using Adaptive Technique

, M.Sc. Thesis Sharif University of Technology Sharifi, Mahdi (Author) ; Khoei, Amir Reza (Supervisor)

Abstract

Prediction of crack growth is one of the greatest achievements of continuum mechanics in 20th century. However, in spite of Griffith’s achievements, nowadays lots of subjects remain unchallenged in the field of Fracture Mechanics. Concrete and asphalt concrete are two of the most popular material in civil engineering and crack growth prediction in these materials are very important. Cohesive crack model is one of the models which is used for prediction of crack growth in quasi-brittle material such as concrete and it has been used widely in recent years because of simplicity and good agreement with experiment.The aim of this thesis is three-dimensional static and dynamic cohesive modeling of...

Modeling of Crack Propagation in Saturated Two Phase Porous Media Using X-FEM

, M.Sc. Thesis Sharif University of Technology Vahhab, Mohammad (Author) ; 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...

Numerical Modeling of Contact-Impact Problems Using Polygonal Finite Element Method

, Ph.D. Dissertation Sharif University of Technology Biabanaki, Omid Reza (Author) ; Khoei, Amir Reza (Supervisor)

Abstract

A Polygonal finite element method without conforming mesh was introduced by the authors for modeling large deformation elastoplastic problems. In this method, the geometry and interfaces of the problem are modeled on a uniform mesh. The boundaries are defined on the uniform background mesh using the level set method. Different polygonal elements will be created at the intersection of the interface and the uniform mesh. Polygonal element shape functions are used for the interpolation. In this paper, the capability of this polygonal finite element approach for modeling large deformational frictionless dynamic contact-impact problems is investigated. Contact interfaces are modeled independent...

Concurrent Multi-Scale Approach for Modeling Mechanical Behavior of Crystalline Nano-Structures

, M.Sc. Thesis Sharif University of Technology Aramoon, Amin (Author) ; Khoei, Amir Reza (Supervisor)

Abstract

Mindboggling advances in nanotechnology have urged researchers to develop state-of-the-art numerical methods to enable them to simulate and to interpret phenomena at this scale. Unfortunately, Classical models have numerous shortcomings which hinder their applications in new contexts. For instance, classical Continuum Mechanics fails to appropriately depict material behavior at small scales, and, on the other hand, Molecular Dynamics simulations are computationally prohibitive. As a consequence, researchers have devised multi-scale methods during the past decade to overcome these obstacles. In fact, in multi-scale methods information is passed from one mathematical description to the other....