Sharif Digital Repository

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... 

This study considers blood flow in total cavopulmonary connection (TCPC) morphology, which is created in Fontan surgical procedure in patients with single ventricle heart disease. Ordinary process of TCPC operation reduces the pulmonary blood flow pulsatility; because of right ventricle being bypassed. This phenomenon causes a lot of side effects for patients. A cardiac surgeon has suggested that keeping main pulmonary artery (MPA) partially open, would increase pulmonary flow pulsations. MPA gets closed in ordinary TCPC operation. The purpose of current study is to verify the effects of keeping MPA partially open on pulmonary flow pulsations, by means of computational fluid dynamics (CFD).... 

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 thesis, a method has been exploited to couple the atomistic domain with the coarse-grained domain. Since molecular dynamics has a high computational cost when a large number of atoms exist, coarse-grained molecular dynamics was used in which a number of atoms are assumed as a bigger bid and interatomic potential is modified for bids so that the material’s mechanical properties remain constant. This method not only reduces the computational cost of calculating forces in molecular dynamics simulation but also, the time step used in Coarse-Grained Methods can be more than atomistic simulations as the frequency of occurring phenomena in CG scale is less than atomistic scale.The advantage... 

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, 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... 

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 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... 

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.... 

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... 

In this paper, a machine learning-based atomistic-continuum multi-scale scheme is introduced to model the materials' geometrically and materially nonlinear behavior. The kinematic and energetic consistency principles are employed to link the atomistic and continuum scales. In order to establish the kinematic consistency principle, the periodic boundary condition is implemented for the atomistic RVE. The Ni-based superalloy, including 0 to 3% porosity, is considered for the models. Several parameter analysis is done to distinguish the proper atomistic RVE to be used in multi-scale models. The data set, including the stress-strain samples, is generated through molecular dynamics analysis... 

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 present thesis aimed to develop a thermal-hydraulic model for simulation of multi-stream plate-fin heat exchangers. In this study, a multi-scale point of view was employed for three-dimensional simulation of multi-stream plate-fin heat exchangers. This is a cost-effective method that reflects the effects of complex phenomena at the micro-scale in the macro-scale model. In this model, fluid and solid matrix temperature distributions were obtained in the three-dimensional space, considering conduction heat transfer through separating plates, side plates, cap plates, and transverse conduction through fins. The modular form of the model has facilitated layer-by-layer simulation of parallel... 

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... 

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... 

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... 

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... 

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... 

One of the most widely used methods in the study of the mechanical behavior of fiber-reinforced polymers is modeling and simulation of a unit cell behavior. According to the arrangement of composite materials, the unit cell is selected in order to include and represent actual constructions of the material. In this study a numerical self-consistence method is proposed to estimate effective properties of Carbon-epoxy composite materials. In this method, in addition of two main phases i.e. matrix and fiber, a phase of composite properties is also considered surrounding the unit cell. First using analytical and semi-empirical methods, the properties are calculated and are converged after... 

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...