Sharif Digital Repository

The swelling phenomenon in the porous media causes many problems in various engineering issues, including foundation construction and oil and gas extraction. For this reason, in the last few decades, flow modeling in reactive porous media and investigation of coupled hydro-chemo-mechanical problems have attracted a lot of attention. On the other hand, despite the simplifications, porous media have a heterogeneous structure, and the numerical modeling of these heterogeneities directly increases the computational costs. A suitable method for modeling heterogeneous problems is the computational homogenization method. In this method, the problem is solved in two scales in a correlated manner and... 

Nowadays, single-crystal nickel-based superalloys are widely used in the manufacture of gas turbine blades in the aerospace industry due to their high resistance to creep, fatigue and corrosion at high temperatures. The superior behavior of these materials at elevated temperatures is a result of their two-phase microstructure, which includes the matrix phase (γ) of nickel and the precipitate phase (γ') of Ni3Al intermetallic compounds with a high volume fraction. The aim of this thesis is to develop computational modeling tools to study the creep deformation of single-crystal Ni-based superalloys. At high temperatures, the creep deformation of Ni-based superalloys is determined by the atomic... 

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  

In this research, multi-scale modeling of mixed-mode failure mechanism of quassi-brittle materials is presented. For modeling a realistic crack growth in heterogeneious media, crack initiation criterion, crack growth orientation and the macroscopic cohesive law are derived from a microscopic sample. As a microscopic crack initiation criterion, acoustic tensor is investigated and scaled acoustic tensor has been proposed by comparison its results with maximum principal tensile stress. For crack growth direction based on micro-scale, acoustic tensor and multiscale aggregating discontinuities has been investigated and multiscale aggregating discontinuities by comparison results of these methods... 

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... 

In this study, static softening kinetics and microstructural evolution during isothermal and non-isothermal annealing of aluminum-magnesium alloy and low carbon steel after cold plastic deformation was investigated. It is known that in industrial cold forming processes, the material properties and microstructure changes during the annealing depends on the inhomogeneous initial microstructure and the deformation history of the cold worked material. To predict the microstructural changes during the annealing of cold worked alloys, a multiscale model based on probabilistic cellular automata (at mesoscale), and the finite element method (at macroscale), was used. To do so, a two-dimensional... 

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... 

Cancer is a class of diseases characterized by out-of-control cell growth. Cancer is among the leading causes of death worldwide.Cancer modeling is increasingly being recognized as a powerful tool to refine hypotheses, focus experiments, and enable predictions that are more accurate.We investigate a three-dimensional multiscale model of vascular tumour growth, which couples blood flow, angiogenesis, vascular remodelling, nutrient/growth factor transport, movement of, and interactions between, normal and tumour cells. We constructed a hybrid multi- scale agent-based model that combines continuous and discrete methods.Each cell is represented as an agent. The agents have rules that they must... 

The use of conventional multi-scale models for materials that exhibit softening behavior is not possible due to the phenomenon of localization, because the fundamental assumptions of homogenization are no longer satisfying around the localization region, and therefore the macro-scale results depend on the size of the representative volume element and the macro-scale mesh discretization. Also, if the finite elements mesh be fine enough, the solution does not converge to acceptable physical values. This research concerns the multi-scale study of failure and proposes an appropriate method for computational homogenization of that. The main idea is to apply the enhanced homogenization method to... 

The hierarchical multi-scale approach is one of the most powerful techniques that takes the advantage of different scales and succeeds the limitations of each method in a way that the large systems in coarse-scale can be simulated with atomic precision. In this thesis, the hierarchical atomistic-continuum multi-scale method is developed for modeling the phenomena with non-homogenous deformation, large deformation and plastic behavior. In this regard at first, an atomistic-based higher-order continuum model is formulated in the framework of nonlinear finite element method to present the geometrically nonlinear behavior of nano-structures. The efficiency of higher-order Cauchy-Born hypothesis... 

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... 

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... 

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... 

In this study, a novel multiscale hierarchical molecular dynamics (MD) – finite element (FE) coupling method is proposed to illustrate the influence of temperature on mechanical properties of heterogeneous nano-crystalline structures. The embedded-atom method (EAM) many-body interatomic potential is implemented to consider pairwise interactions between atoms in the metallic alloys with face-centered-cubic (FCC) lattice structure at different temperatures. In addition, the Nose-Hoover thermostat is employed to adjust the fluctuation of temperature. In order to calculate the equivalent lattice parameter, a weight average between the lattice parameters of atomic structures is utilized. The... 

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... 

In this paper, a multiscale coarse graining method is developed on the basis of the force matching theorem in order to model the nonlinear behavior of crystalline material. In this framework, two scales are involved, i.e. the nano- and meso-scales. In each of scales, molecular dynamic simulations are employed with this difference that the fine scale is modeled via embedded atom method many body potential and at the coarse scale the simulation is based on pairwise potential. In addition, the linkage between fine and coarse scales is achieved by data transfer between two scales; in a way that required information for coarse grain analyses, i.e. inter-particle potential and coarse scale initial... 

The fact that materials and crack behave differently under different temperatures, brings a need for further investigation in this field. Recent studies regarding this behavior are mainly based on molecular dynamic method. While this method garuntee a highpercicion, the computational costs of this method can be high when dealing with crack propagation problem. Taking advantage of multiscale methods allows us to overcome this challenge by reducing the calculation time while providing acceptable results. In this research a multi-scale method capable of considering thermal effects has been developed. A concurrent model is created by using theory of elasticity for continuum part and taking... 

Porous materials, with diverse applications in engineering branches, are categorized as multi-scale. A multi-scale material is one which shows different structure and/or behavior in two or more different length scales. There are physical models which can calculate the macroscopic properties of such materials by using both the properties and volume fractions of the ingredients. However, the number of such theories which can handle problems in the fields of elasticity and hydrodynamics is much less; the fields in which the tensor orders of the properties are more than one. Fortunately, in recent years, a new method named "Computational Multi-scale Homogenization" has been offered to homogenize... 

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....