Sharif Digital Repository

The magnesium alloy AZ31 has emerged as a suitable alternative to stainless steels and titanium for the production of medical stents due to its superior biocompatibility and biodegradability compared to other biodegradable metals such as zinc and iron. However, magnesium and its alloys exhibit high reactivity, with a standard potential of -2.36 V relative to the standard hydrogen electrode, making them the most active metals in human blood plasma. As a result, the high corrosion rate of magnesium and its alloys presents one of the greatest challenges in the production of medical materials from this metal. One approach to reducing the corrosion rate of AZ31 magnesium alloy is surface... 

Reducing the size of transistors has dramatically increased the impacts of NBTI, HCI, and EM phenomena in comparison with decade ago. These phenomena are able to have influence on properties of different parts of a chip to make it changed gradually; including threshold voltage of a transistor and electrical conductivity of interconnections. These changes are known as the aging of a transistors that diminish the performance and reliability of a chip. A common reason that plays a significant role in all these phenomena is temperature. The temperature becomes more important when the power density increases per unit area due to decreased size of transistors, which is a trend in multi-core... 

Deposition of solid particles present in the process air on the walls of furnaces is one of the challenges in industries. This phenomenon has various causes. For example, in a furnace, the temperature of particles increases as they pass in front of the furnace burners, and due to the softening of the particles, their chances of deposition increase when they collide with the walls. Over time, with the growth of deposited particle layers on the combustion chamber walls, the performance of the combustion system gradually becomes impaired. present research aims to design air jets intersecting with the process air path, with the purpose of reducing the temperature of particles prone to deposition... 

In many papers, GDP is used as a measure of economic growth, which is shown to possess a nonlinear structure. The study focuses on the nonlinear Granger causality between liquidity, government spending, oil revenues and economic growth by markov-switching vector autoregressive and markov- switching Granger causality approach for the period of 1369-1397 for Iran. The empirical results from MSAI(2)-VAR(4) suggest that Oil revenues and government spending have a significant impact on economic growth in both regimes. Also, markov- switching Granger causality results are quite different from the traditional Granger causality results, that suggests the nonlinear Granger causality has more... 

A sliding mode nonlinear controller has been developed to control a monohull hard-chine planing vessel. Planing vessels has been used for shoreward marine missions. On the other hand, according to vast shores of our country, offshore applications could be of extra importance for us. Autopilot of a vessel consists of two stages: trajectory design and trajectory control. In this project, trajectory control is being done using sliding-mode nonlinear control method, considering the fact that according to nonlinear dynamics of planing vessels, linear control would not show desirable results. In this dissertation, after description of main information about planing vessels and prior researches,... 

The use of microelectrode arrays in neural networks has offered a wonderful opportunity for evaluating numerous properties and interactions with excitable tissues such as the brain and spinal cord in the laboratory. In neural network applications, microelectrode arrays can be utilized to analyze nerve impulses or interact with neurons. They are either used to record neural data or to stimulate spinal cord nerves. Previous research has revealed that stimulating the spinal epidural region can help restore after spinal cord injuries (SCIs), which is the key to using them in the rehabilitation of living beings, including humans. According to the international campaign for the treatment of SCI... 

In this study, at first the V2O5 spent catalysts were characterized. The aim was to identifying whether these catalyst may be regenerated. Then, for feasibility study of them, three methods were applied: washing, diffusion and milling the spent catalysts for their subsequent reproduction. A variety of techniques such as XRD, XRF, AAS, TPR and TPO were applied in order to identify factors contributing in vanadium pentoxide catalysts deactivation. The results showed that approximately all of pollutants caused their performance deterioration, while they still have some active phases. Then, the polluted catalysts were washed (their yield increased from 43% to 72.2%). In the underside of the... 

The first project ,paid to review and analysis types of crack and agents Also, was referred methods to control them in rail industry .
The wheel of Alestom train and UIC60 rail were simulated and stress analysis by software FEM (ANSYS and ABAQUS) , and effect of various parameters such as applied forces , train wheel diameter , contact position and rail inclination on stress values was studied . For comparision of results has been used analytic Hertz theory .
By using experimental data on the subsurface crack , those cracks were created in the wheel . For load on the modeling wheel and rail, ,assuming contact on the wheel rolling point , were used vertical load of wagon weight ( with... 

In this study, Interfacial crack is modeled by the extended finite element method (XFRM) in order to evaluate the stress intensity factors for interface crack problems. These problems have crack edge discontinuity, material interfaces and singularity at the crack tip, which is inappropriate to finite element (FEM) mesh. Furthermore, around crack tips and interface edge, extremely fine discretization is required to achieve reasonable accuracy. All these factors increase the time and cost of the FEM solution. Recently the extended finite element method is developed and applied to interface fracture problems. In the XFEM, special enrichment functions are added to the finite element displacement... 

First stage blades tempreture in the turbine rotor section for the advanced turbines has increased up to 1100oc. The blades should be under mentioned condition for the long times up to 70000 hours and accordingly creep happens. Fatigue–creep interaction is the main reason for the failures of many engineering components under high temperature and cyclic loading. In this thesis, a model for the life prediction of fatigue–creep interaction which was developed by other investigators has been modified to predict fatigue–creep interaction on the nickel-based superalloys. In this model, the law of energy conservation and the momentum conservation principle are used to describe the process of... 

Most of the current application and potential FGM included in the areas related to contact mechanic .These issues essentially are related to the transfer of force between two objects, that in general is with friction, so in this study the contact under the loading of horizontal and vertical with its different aspect such as the effect of contact region, the effect of round and also the effect of getting harden and soften of FGM.
The used method is a numerical method ¬(finite element method) that will resulted in contact equation discretisce by use of Penalty method. In this thesis is also presented the needed algorithms for solving the problem and the finite element code for solving the... 

Modeling of contact is a important problem in word science, so a lot of method presented by researchers .
In practical industry, if members of contact have complex shape or large deformation, analytical method can not solve the problem and use numerical method.
This method by using ansys , first analyze one-area contact and compare between numerical result and analitical result, then analyze two-area contact between two body with vertical force, and compare result with considering effect of slip and target surface.
Finaly to analyze a practical multiple contact problem, use modeling of tyre and road surface partial slip, and compare between ansys result with experimental result ,... 

In this thesis failure analysis of a critical part in a landing gear of a commercial aircraft was conducted.Fatigue phenomenon was determined as the reason for the failure and fracture of the mentionedcomponent. Fractography investigation of the fracture surface and charasteristicswas conducted.Scanning Electron Microscope was utilized for the study of the fracture surface. In the fracture surface, fatigue striations were observed. An experimental model of the crack growth is presented by using the fracture surface characteristics. By counting the fatigue striations and measuring their distance betwwn them on the fracture surface, the rate of the crack growth and crack growth life were... 

The problem of fretting fatigue due to the inherent complexity and high probability of its occurrence in many engineering systems, is very important for engineering design. Fretting Fatigue is a significant failure mode in many contacting mechanical components, in which two contact surfaces experience low amplitude oscillatory movement due to cyclic loading. There are many significant contact fatigue problems in structural design which fretting fatigue at lap joints in aging aircraft and fretting fatigue of dovetail joints in gas turbine engine are the most important of them. Numerical simulations provide an appropriate and powerful tool for parametric study of fretting fatigue behavior of... 

Surface roughness has an enormous influence on many important physical phenomena such as contact mechanics, sealing, adhesion and friction. Contact mechanics was studied as an important topic for long years in order to realize the friction phenomena, contact fatigue and fluid leaking between rough surfaces. Therefore, the contact of rough surfaces has been widely invistageted by engineers for different purposes like rejouvination, prevention of environmental pollution and stress analysis. Failure modes have been studied in this research for a specific type of injector by experimental methods. After that, injector's rough surfaces were modeled. Firstly the proper point cloud was modeled by... 

The reliability of integrated circuits (IC) is related to material imperfections such as holes, cavities, cracks and etc. which occur in the manufacturing process. The operational conditions of IC create thermal stresses. These thermal stresses increase the chance of failure by making the imperfections critical in IC’s components such as VIA that is used as connection between interconnects in the multilevel integrated circuits. As a result, thermal fatigue needs to be investigated in VIA thoroughly.
The thermal stresses in VIA increase the possibility of hole forming due to hydrostatic stresses during manufacturing process and the amplitude of applied stresses under the operational... 

Nowadays, the use of finite element Software applications in enigineering departments has become Inevitable and commonplace. Acceptance of the necessity of using such software applications for reducing the cost of research and development does not require any evidences of financial instruments to be presented. Reducing design and manufacture time together with pseudo-empirical approach has cuased the finite element science to become a conventional designing tools.
Since contact is the main method to apply a load on a ductile object,and stress concentration caused thereby is often a major cause of failure in mechanical parts, the issue of contact is a fundamental topic in mechanics of... 

Mode I and II stress intensity factors are derived for an inclined crack breaking the surface of a half-plane which is subjected to an un-symmetrical singular contact loading due to the contact of a tilted wedge with the half-plane. The method used is that of replacing the crack by a continuous distribution of edge dislocations and assume the crack to be traction-free over its entire length. A traction free crack is achieved by cancelling the tractions along the crack site that would be present if the half-plane was un-cracked. The stress distribution for an elastic uncracked half-plane subject to a tilted wedge shaped indenter is derived analytically using the Muskhelishvili’s potential... 

In this study composite is built based on epoxy which is reinforced with 3D graphen foam using the RTM (resin transfer molding) method in which the molding is done by hand, through injecting epoxy to graphen foam.Good scattering and neat meshing of 3d graphen planes in composite have resulted in its improved mechanical quilities.The subject of the present study is the synthesis of graphen foam from graphen oxide and making graphen foam/epoxy composite in order to improve its mechanical properties. In order to do that, firstly graphen foam is forged from graphen oxide in a 3d mesh, in a way that single graphen planes in polymer matrixes are eliminated.Having very little density, very high... 

Purpose of these investigations is modeling and prediction of foam graphene like linear elastic modules and density of that material. In this modeling with two numerical method molecular dynamics and finite element method is used in that with multiscale method various numerical method match together. At the beginning of the modeling various image of foam graphene in Nano abd meso scale in the case of size and scale survaied till with this checking, scale of the work and varios of modeling was created and with this modeling linear elastic modules and density were estimation