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    Multi-scale modeling of edge effect on band gap offset in polygonal cross-section Silicon nanowires

    , Article Computational Materials Science ; Volume 79 , 2013 , Pages 262-275 ; 09270256 (ISSN) Khoei, A. R ; 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... 

    Prediction of particle deposition in the respiratory track using 3D-1D modeling

    , Article Scientia Iranica ; Volume 19, Issue 6 , December , 2012 , Pages 1479-1486 ; 10263098 (ISSN) Monjezi, M ; Dastanpour, R ; Saidi, M. S ; Pishevar, A. R ; Sharif University of Technology
    2012
    Abstract
    Airflow simulation of the whole respiratory system is still unfeasible due to the geometrical complexity of the lung airways and the diversity of the length scales involved in the problem. Even the new CT imaging system is not capable of providing accurate 3D geometries for smaller tubes, and a complete 3D simulation is impeded by the limited computational resources available. The aim of this study is to develop a fully coupled 3D-1D model to make accurate prediction of airflow and particle deposition in the whole respiratory track, with reasonable computational cost and efficiency. In the new proposed method, the respiratory tree is divided into three parts to be dealt with using different... 

    Validity and size-dependency of Cauchy-Born hypothesis with Tersoff potential in silicon nano-structures

    , Article Computational Materials Science ; Volume 63 , 2012 , Pages 168-177 ; 09270256 (ISSN) Khoei, A. R ; Dormohammadi, H ; Sharif University of Technology
    Elsevier  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... 

    A multi-scale modeling of surface effect via the modified boundary Cauchy-Born model

    , Article Materials Science and Engineering C ; Volume 32, Issue 7 , 2012 , Pages 1993-2000 ; 09284931 (ISSN) Khoei, A. R ; Aramoon, A ; Sharif University of Technology
    Elsevier  2012
    Abstract
    In this paper, a new multi-scale approach is presented based on the modified boundary Cauchy-Born (MBCB) technique to model the surface effects of nano-structures. The salient point of the MBCB model is the definition of radial quadrature used in the surface elements which is an indicator of material behavior. The characteristics of quadrature are derived by interpolating data from atoms laid in a circular support around the quadrature, in a least-square scene. The total-Lagrangian formulation is derived for the equivalent continua by employing the Cauchy-Born hypothesis for calculating the strain energy density function of the continua. The numerical results of the proposed method are... 

    Multiscale modeling of the effect of carbon nanotube orientation on the shear deformation properties of reinforced polymer-based composites

    , Article Physics Letters, Section A: General, Atomic and Solid State Physics ; Volume 375, Issue 14 , 2011 , Pages 1588-1597 ; 03759601 (ISSN) Montazeri, A ; Sadeghi, M ; Naghdabadi, R ; Rafii Tabar, H ; Sharif University of Technology
    Abstract
    A combination of molecular dynamics (MD), continuum elasticity and FEM is used to predict the effect of CNT orientation on the shear modulus of SWCNT-polymer nanocomposites. We first develop a transverse-isotropic elastic model of SWCNTs based on the continuum elasticity and MD to compute the transverse-isotropic elastic constants of SWCNTs. These constants are then used in an FEM-based simulation to investigate the effect of SWCNT alignment on the shear modulus of nanocomposites. Furthermore, shear stress distributions along the nanotube axis and over its cross-sectional area are investigated to study the effect of CNT orientation on the shear load transfer  

    Mechanical Behavior Analysis of Carbon Nanotube-Based Polymer Composites using Multiscale Modeling

    , Ph.D. Dissertation Sharif University of Technology Montazeri Hedesh , Abbas (Author) ; Naghdabadi, Reza (Supervisor) ; Rafii Tabar, Hashem (Supervisor) ; Bagheri, Reza (Supervisor)
    Abstract
    In this project, two multiscale modeling procedures have been implemented to study the mechanical behavior of SWCNT/polymer composites. First, a new three-phase molecular structural mechanics/ finite element (MSM/FE) multiscale model has been introduced which consists of three components, i.e. a carbon nanotube, an interphase layer and outer polymer matrix. The nanotube is modeled at the atomistic scale using MSM, whereas the interphase layer and polymer matrix are analyzed by the FE method. Using this model, we have investigated the macroscopic material properties of nanocomposite with and without considering the interphase and compared the results with molecular dynamics (MD) simulations.... 

    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... 

    Numerical Multiscale Modelingof Cardiovascular System

    , M.Sc. Thesis Sharif University of Technology Yousefi, Amir (Author) ; Firoozabadi, Bahar (Supervisor) ; Saeedi, Mohammad Saeed (Supervisor)
    Abstract
    One of the main reasons of brain attack is stenosis in Carotid bifurcation. As a result, it's a major field of interest for a large group of scientists including medical doctors and engineers. In the engineering fields a large number of studies have been done on this topic based on Numerical Simulation. As blood boundary conditions in this region strictly depend on the rest of the body's vascular system, the problem is the definition of the boundary condition in this approach. In addition, because of lack of enough hardware sources simulation of the whole blood system is impossible; thus the rest of the system should be simulated based on radical methods with less details. In this project a... 

    Multi-scale Modeling of Crack Using Nano-XFEM

    , M.Sc. Thesis Sharif University of Technology Ghaffari, Reza (Author) ; Haddadpour, Hassan (Supervisor)
    Abstract
    In this thesis a mutliScale model based on the Cauchy-Born hypothesis and via usage of XFEM is proposed for crack modeling. By solving an example, the important of surface effects in the surface stresses region is shown. Considering not being able to model the surface effects with the Cauchy-Born method, the boundary Cauchy-Born method for modeling crack effects is used. Moreover, three Molecular Dynamics method for modeling crack will be proposed. According to the obtained results from these methods, it was deduced that for calculating the correct surface stresses in Molecular Dynamics the mutual interaction of upper and lower atoms of crack should be omitted. Finally, the validation of... 

    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.... 

    Multiscale Modeling of Carbon Nano Structures Using Tersoff Potential Function

    , M.Sc. Thesis Sharif University of Technology Najjari, Alireza (Author) ; Khoei, Amir Reza (Supervisor)
    Abstract
    During the last decade, thanks to a combination of exploding computational power and improved physical insight into material behavior, continuum and atomistic simulations improved greatly. Both classes of methods are now used to solve problems, which are more complicated than ever with greater accuracy than ever before. Nevertheless, there still exist problems for which neither method alone is sufficient. In general, atomistic simulations cannot be used for such length scales due to the restrictions on the number of atoms that can be simulated, along with the time scales, which they can be simulated for. In contrast, continuum simulations tend to fail at the atomic scale, for example due to... 

    Multi Scale Modeling of Carbon Nano Structures Using Brenner Potential Function

    , M.Sc. Thesis Sharif University of Technology Ziapour, Rouzbeh (Author) ; Khoei, Amir Reza (Supervisor)
    Abstract
    Due to high cost and ineffectiveness of molecular models a new method for coupling continuum models with molecular models is used. In this method, the continuum and molecular domains are overlapped. Comparing the results obtained from the concurrent simulations and molecular dynamic simulations proves the accuracy of the method used. The method is used for modeling single layered graphene sheets, stress contours are presented for multiscale and both static and dynamic simulations of concurrent. For multiscale simulations two different carbon nano tubes are investigated and strees-bond angle and strees-bond length are also presented  

    Modelling and Simulation of Melanoma Cancer, Based on Cellular Automata Approaches

    , M.Sc. Thesis Sharif University of Technology Rad, Jaber (Author) ; Habibi, Jafar (Supervisor)
    Abstract
    Nowadays, M&S is critical as a powerful tool for human to fight against cancer. Skin cancer is one of the most widespread cancers and melanoma would be the most dangerous kind of it. In cancerous micro-environment, cancer cells interact with vasculature, and compete with normal cells over nutrients. This plays a major role in tumor progression pattern and speed. In recent years, a few multiscale models have been developed considering these phenomena. Such a model provides a platform for future researches, especially in drug effects prediction. A reliable simulation must satisfy the constraints and facts in the real world as much as possible. M&S credibility assessment is a major concern to... 

    Continuum Analysis of Defects Based on Atomistic Simulat

    , M.Sc. Thesis Sharif University of Technology Heidarzadeh, Narges (Author) ; Khoei, Amir Reza (Supervisor) ; Jahanshahi, Mohsen (Co-Advisor)
    Abstract
    In this study, a new multi-scale hierarchical technique has been employed to investigate the role of temperature on nano-plates with hex atomic structure. Different number of primary edge dislocations is considered and the temperature varies from 0 up to 800 K. Primary edge dislocations are created by proper adjustment of atomic positions to resemble discrete dislocations (DD’s) and then the application of equations of motion to the relaxed configuration of this adjustment. The interatomic potential used for atomistic simulation is Finnis-Sinclair Embedded-Atom-Method (FS-EAM) as many-body interatomic potential and the Nose-Hoover thermostat has been implemented to adjust the modulation of... 

    Investigation of the interphase effects on the mechanical behavior of carbon nanotube polymer composites by multiscale modeling

    , Article Journal of Applied Polymer Science ; Volume 117, Issue 1 , March , 2010 , Pages 361-367 ; 00218995 (ISSN) Montazeri, A ; Naghdabadi, R ; Sharif University of Technology
    2010
    Abstract
    In this article, a multiscale modeling procedure is implemented to study the effect of interphase on the Young's modulus of CNT/polymer composites. For this purpose, a three-phase RVE is introduced which consists of three components, i.e., a carbon nanotube, an interphase layer, and an outer polymer matrix. The nanotube is modeled at the atomistic scale using molecular structural mechanics. Moreover, three-dimensional elements are employed to model the interphase layer and polymer matrix. The nanotube and polymer matrix are assumed to be bonded by van der Waals interactions based on the Lennard-Jones potential at the interface. Using this Molecular Structural Mechanics/Finite Element... 

    Hierarchical Multiscale Modeling in Large and Plastic Deformations

    , M.Sc. Thesis Sharif University of Technology Sarkari Khorrami, Mohammad (Author) ; Khoei, Amir Reza (Supervisor)
    Abstract
    In this study, the hierarchical multiscale method is presented to model macro-scale materials with considering large and plastic deformations. Since the classic methods such as continuum mechanics were unable to model defects such as dislocations, voids, and etc. at nano scale. Also, the molecular dynamics methods have high computational costs; hence, recearchers try to develop multiscale methods in order to utilize them. Multiscale modeling in which two scales are considered and some information be transferred from fine scale to coarse scale. In this work, the coarse scale is modeled by finite element method (FEM) and the fine scale is analysed by molecular dynamics (MD). In fact, two... 

    Multiscale nonlinear constitutive modeling of carbon nanostructures based on interatomic potentials

    , Article Computers, Materials and Continua ; Volume 10, Issue 1 , 2009 , Pages 41-64 ; 15462218 (ISSN) Ghanbari, J ; Naghdabadi, R ; Sharif University of Technology
    Abstract
    Continuum-based modeling of nanostructures is an efficient and suitable method to study the behavior of these structures when the deformation can be considered homogeneous. This paper is concerned about multiscale nonlinear tensorial constitutive modeling of carbon nanostructures based on the interatomic potentials. The proposed constitutive model is a tensorial equation relating the second Piola-Kirchhoff stress tensor to Green-Lagrange strain tensor. For carbon nanotubes, some modifications are made on the planar representative volume element (RVE) to account for the curved atomic structure resulting a non-planar RVE. Using the proposed constitutive model, the elastic behavior of the... 

    Multi-scale Modeling of Heterogeneous Nano-materials Using Representative Volume Element

    , M.Sc. Thesis Sharif University of Technology Shafieyoon, Ali (Author) ; Khoei, Amir Reza (Supervisor)
    Abstract
    In this paper a new multi-scale method is developed for modeling heterogeneous materials, this method is based on homogenization and it is classified as hierarchical multi-scale method. For simulating problems in continues media, finding the elastic tensor is necessity, in homogeneous material this tensor come down from Young’s modulus and poison’s ratio, however in Nano-scale problems specially in heterogeneous material, this solution does not work and need to revise. To deal with heterogeneity in these problems homogenization by a representative volume element is a novel method. The properties of material is imported from RVE in each step of solving problem to larger scale, and by... 

    Calculation of Heterogenous Material Properties by Using of Eshelby based and BEM Methods

    , M.Sc. Thesis Sharif University of Technology Yazdanparast, Reza (Author) ; Hosseini Kordkheili, Ali (Supervisor)
    Abstract
    To days the heterogeneous material are used extensively in the engineering materials. Optimization ability is a key feature of these materials to reach desired properties. Heterogeneous materials are the materials that make up from the constituents of multiphase materials in lower length scale such as mesoscopic, microscopic or/and Nano scales. So the properties of these materials at each scale are depending on to several characteristics of heterogeneities such as geometry, material and packing. In these materials the effects of heterogeneities at the lower scales are very significant and the constitutive equations are different for each range of scale. The proper selection of this range... 

    A Multi-Scale Method for Modeling and Analysis of the Creep Behavior in Composite plates

    , M.Sc. Thesis Sharif University of Technology Barzegar, Mohsen (Author) ; Hosseini Kordkheili, Ali (Supervisor)
    Abstract
    Polymer matrix composites, which are composed of a wide variety of short or long fibers bound together by organic polymer matrix, have been widely utilized in many engineering aeras, particularly in aerospace engineering. Recently, studying and analyzing the mechanical behavior of composites was one of the major reaserch interests. Regarding the vast variety of data drived from experimental tests, a requirement of tools that could facilitate estimating creep properties of materials is an important concern for researchers. The present work at first, introduces some major creep models and then proposes a 3D creep Burgers model for implementing in abaqus which could be used in macro phase. This...