Sharif Digital Repository

Electromagnetic wave scattering from objects is one of the most important problems in applied electromagnetics. Usually the electromagnetic scattering characteristic from an object is analyzed with the assumption that the object is in free space. If the object is not near other objects or surfaces this is acceptable. However if the object is near other objects or surfaces then the interactions between the scattered wave from the object and surroundings can significantly alter the scattering characteristic from the object.In this thesis we have proposed a method for the analysis of electromagnetic scattering from an object in the presence of rough surfaces. This method is based on the... 

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... 

Non-destructive testing method is for detect specimen flaws such as (crack, porosity, impure, welding defects, corrosion etc.) without destruction the specimen under inspection. Particular characteristic of radiography is an image product from specimen inside under testing where help to identity and better comprehend at flaw nature. Photon scattering is one of important factor to reduce image quality and signal to noise ratio. Scattered photons can due toexistent ingredient in the radiography environment and curvature of material transmission pipe where cause the disorder creation in the radiography images. In this research, pipe radiography simulated using the TIC and TIR radiography... 

This research introduces a novel thermo-mechanical multiscale technique, utilizing machine learning, for simulating the compaction process of aluminum nanopowders with surface oxidation at various temperatures. The methodology employed involves the utilization of nonlinear thermo-mechanical Finite Element Method (FEM) for macro scale analysis, while employing the Molecular Dynamics (MD) method to calculate the mechanical and thermal characteristics of aluminum nanopowders at the nano-scale. The first part of the research presents a comprehensive study on the thermal conductivity of alumina-coated aluminum nanopowders, which is a crucial property for their application in powder metallurgy,... 

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... 

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... 

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 present research forming process of nanopowders, which is a part of powder metallurgy was investigated by molecular dynamics method. Powder metallurgy is a relatively new method for production of industrial parts by pouring powder into die and compaction to desired density. One can reach parts with higher quality and strength by decreasing size of powder’s particles and entering the nano scale. Particle with smaller size have higher specific surface and due more intensity to react. Classic methods for investigation of this process don’t cover the atomic scale effects, so using newer procedures such as molecular dynamics is highly recommended. In present research, at first compaction of... 

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.... 

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... 

Modeling is one of the most important steps in analysis and design of structures. Most of the time behavior of floor diaphragms is assumed completely rigid in their plane in order to simplicity in practical design. Although this assumption leads to acceptable results in analysis and design of conventional buildings, erroneous results may assess in some particular buildings—for instance narrow buildings with stiff end walls. Accordingly, behavior of these buildings in future earthquakes opposes those were predicted in design phase, and they are so vulnerable since in-plane deflection of floor diaphragms are striking which cannot be ignored. This thesis investigates the significance of... 

This thesis deals with the financial risk management and coordination of supply chain members. To investigate this issue, two echelon supply chain consisting of one manufacturer and one retailer with a newsvendor inventory policy for a period is considered. The mathematical modeling has been used to find the exact answer. MATLAB software for solving and scenario generation is used for the analysis. In the proposed method, financial risk management, partial trade credit policy, coordination of supply chain members and cash discount for retailers, have been considered. According to the proposed method, the manufacturer can determine immediate payment at the start of the trade credit period... 

The acidizing operations have significant impact on productivity and production of oil and gas wells, so that wells having shell or regional damage are good candidates for well stimulation operations which could result in significant increase for efficiency or injectivity. Generally, the neural networks are nonlinear learning mathematical systems. In this project consists, At first, wellhead data of acidizing operation are collected in two Gachsaran oil field (38 wells) and Nar and Kangan gas field (35 wells). Generally, the neural networks are nonlinear learning mathematical systems. In the neural Network by using two algorithms, back propagation algorithm and Quick Propagation algorithm... 

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... 

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... 

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... 

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... 

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... 

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...