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    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... 

    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... 

    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... 

    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... 

    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.... 

    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 Fracture Problems with X-FEM

    , M.Sc. Thesis Sharif University of Technology Broumand, Pooyan (Author) ; 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 Haghighat, Ehsan (Author) ; 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 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... 

    A None-Associated Cap Plasticity with Isotropic-Kinematic Hardening-Softening Rule

    , M.Sc. Thesis Sharif University of Technology Seyedein, Soheil (Author) ; 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 Vafa, Alireza (Author) ; 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 Moslemi, Hamid (Author) ; 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 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... 

    Finite Element Modeling of Impact Problems with Friction and Large Deformations

    , M.Sc. Thesis Sharif University of Technology Zeinali, Mostafa (Author) ; 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... 

    Modeling of Uplift and Impact with Finite Element and Generalized Finite Element Methods

    , M.Sc. Thesis Sharif University of Technology Eslahi, Reza (Author) ; Khoei, Amir Reza (Supervisor)
    Abstract
    One of the most important issues in modeling natural phenomena that always takes into consideration is the problem of contact and collision between objects. Finite element method is known as the most basic and commonly used in numerical modeling techniques to simulate the above phenomenon.This method has specific problems such as dependency of this approach to the domain mesh especially near the contact region. Due to the dynamic and complex nature of impact problems in which drastic changes occur in short duration that often deals with large deformations, one of the perennial concerns, is the weaknesses of methods applied to enforce contact constrains, and so it is essential to use an... 

    Temperature-Dependent Hierarchical Multi-Scale Modeling of Nano-Materials Considering Surface Effect

    , M.Sc. Thesis Sharif University of Technology Ghahremani, Pegah (Author) ; Khoei, Amir Reza (Supervisor)
    Abstract
    In continuum mechanics, the constitutive models are usually based on the Cauchy-Born (CB) hypothesis which seeks the intrinsic characteristics of the material via the atomistic information and it is valid in small deformation. The main purpose of this thesis is to investigate the temperature effect on the stability and size dependency of Cauchy-Born hypothesis and 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. Three-dimensional temperature-related Cauchy-Born formulation are developed for crystalline structure and the stability and size dependency of temperature-related... 

    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 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... 

    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... 

    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...