Sharif Digital Repository

This research studies behavior and implementing of telescopic lead shear damper in structures. Telescopic lead shear damper (TLSD) is a type of metallic yielding dampers that can dissipated energy with usage of shear yielding of lead. TLSD is made from behind act of two lead shear damper (LSD) like a telescope. Lead shear damper composed of two coaxial steel pipes where their interfacial gap is filled with pure lead. The interface of lead and steel pipes are roughened in a threaded form to guarantee the no-slip condition across the interface. Due to stable and fat force displacement hysteresis loops of TLSD, this device can be used in structures subjected to wind, seismic, and other dynamic... 

In this thesis, the process of laser assisted machining (LAM) of metal matrix composites (MMC) is experimentally studied. The effects of process parameters (cutting speed, feed rate and depth of cut), laser parameters (laser power, laser frequency and laser beam angle), and the percentage of reinforcing particles on the tool wear and the surface roughness are investigated. Furthermore, the effects of these parameters on the built-up edge (BUE), chip shape, and workpiece temperature are explored.The experiments were performed using uncoated tungsten carbide and PCD tools under dry conditions. In order to analyze the sensitivity of the process parameters, the Plackett-Burman method was used... 

One of the requirements of designing structures is the properties of their materials. The behavior of the material should be studied under the same conditions (loading regime) as it would be in practice. Regarding the structures which are subjected to cyclic loading, usually the results of monotonic tests on their material are used for their analysis under cyclic loads. Taking this fact into account that the behavior of materials under cyclic loading is normally much different from that under monotonic loading, one should doubt the results of the analyses of structures subjected to cyclic loading for which monotonic test results are used. In this project, the properties of steel rebars under... 

The strength of welded joints is determined by the lower value of the weld metal and base metal strength. In welded bracing connections, the latter includes the connecting element (i.e., the gusset plate) and the bracing member. According to the American steel design standard AISC 360, the base metal strength is determined based on the limit states of tensile rupture, shear yielding, shear rupture, and block shear. While extensive research has been conducted on the limit state of tensile rupture, limited attention has been given to the limit states of shear yielding, shear rupture, and block shear in welded bracing connections. Furthermore, the design strength equations adopted by AISC 360... 

Leachate migration from solid waste landfills into the subsurface environment is one of the significant problems of landfills. This can cause lots of problems for local soil and water to sources according to the contaminants appear in the leachate. Usually, landfill sites include at least one compact clay liner to prevent contamination of groundwater by produced leachate. Over the last decades investigators have been attracted to the structure and removal capacity of this liner. In this study, the effect of the addition of Zeolite to Kaolinite soil for the removal of heavy metal (manganese and zinc) from landfill leachate and the effect of decreasing the thickness of the soil layer has been... 

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.... 

In this study, an equivalent homogenous material model is proposed for Al6061-T6/Al2O3 metal matrix composite using the hierarchical multi-scale modeling, and the capability of this model in simulation of machining process of this composite is investigated. Molecular dynamic study of the Al/Al2O3 interface is used to obtain the effect of crystallographic orientations and terminations on the tensile and shear strengths at the interface. The Reaxff potential function is employed to estimate the traction-separation relations at Al/Al_2 O_3 interface at temperatures of 300, 400, and 500 K. Next, a cohesive zone model is used at the micro-scale to simulate the interface between the matrix and... 

Relying on current fuels and technologies in vehicle industry in recent years has led to an enormous challenge such as air pollution, climate change and energy security. In the other hand, fossil fuel reservoirs are being depleted with an intensive rate. Nowadays, hydrogen, as a substitute for current fuels and hybrid, fuel cell and electric vehicles, as substitutes for current vehicular technologies are under research and development. Because of abundant advantages such as high efficiency and long cycle life, no depreciation for parts and scalability, fuel cell vehicles is of interest for many car manufacturers throughout the world and lots of them have demonstrated their prototype fuel... 

Nowadays, regarding the wide achievements in micro scale technology, it is possible to manufacture a micro robot. Micro swimmer have extensive applications in different fields especially in medical sciences. One of the most important problems of the micro robot is their propulsion system. The locomotion of microorganisms in fluids is ubiquitous and plays an important role in numerous biological processes. The physics of swimming governing life under the microscope is very different from the one we experience in the macroscopic world. This project is aimed at designing a propulsion system for a micro swimmer based on the swimming mechanism of the eukaryotic Spermatozoa. Simplicity to build is... 

Arc welding is one of the key processes in the industry. Its vocational status in manufacturing industry comes to third place. From applicable standpoint gas metal arc welding (GMAW) has especial place between various electrode arcs. Base on the mode of transferring welding drop from electrode into welding pool the GMAW process could be categorized. The physics of metal transfer is not yet well understood, due to the facts that the arcs area is too small, the temperatures is too high and the metal transferring is at too high rate. However, physical behavior of process of each model is different from another. So equations that express each of them, need to reform toward others. The conditions... 

The parametric study of a new introduced metallic yielding damper called telescopic damper is addressed. The telescopic damper comprises of three concentric steel pipes connected together by lead shear keys. The inner and outer steel pipes are subjected to axial loading which results in the shear yielding of the embedded lead cylinders. The inner steel pipe has a lock which limits its movement to some certain extent and beyond this limit the outer tube activates. Employing ABAQUS software, the behavior of damper under cyclic loading is studied numerically. The finite element model is verified by available results in the literature. The effects of inner and outer steel pipes thickness,... 

Nanocrystalline metals and alloys have some appealing characteristics with significance potential compared to their microcrystalline counterparts for engineering applications. These include ultra-high yield and fracture strengths, decreased elongation and toughness, superior wear resistance, and the promise of enhanced superplastic formability at lower temperatures and faster strain rates. This leads us to study the effects of different nanocrystalline parameters on crack propagation process in these materials. In the present study, the behavior of a crack in a columnar nanocrystalline structure is examined. One of the methods of modelling nanocrystals primary structures is the Voronoi... 

In this thesis, a simplified method was introduced to model the wave scattering by the metallic objects in a microcell, in order to determine the coverage pattern. First, the equivalent images method was introduced, which models the building walls effect. This method is fast, memory saving and accurate enough to estimate the scattered fields. Two-ray incidence model accuracy was investigated and confirmed in modeling the scattering from the metallic objects. This method also enhances saving in time and memory. The accuracy of single scattering assumption was investigated and confirmed in a scenario of multiple metallic scatterers. Based on this assumption, we can superimpose the scattered... 

In this thesis, we have studied systems called Ruthenates, which are common in possessing Ru element in their chemical compounds. Ruthenates are a group of transition metal oxides which have attracted so attention, especially in last years duo to their rich phase diagram which shows different orbital, charge and spin orderings.They cover a vast range of materials properties, from a Superconductor to Mott Hubbard insulator, from a paramagnetic metal to an antiferromanetic insulator. In these systems, substituting a covalent atom with structure base, leads to a vast variety changing, not only in electronic properties but also in magnetic and structural ones: Although Sr"RuO£ is a... 

Duplex stainless steels and advanced high strength steels are both known as new materials in automobile industry applications. Process of resistance spot welding is the main process in welding of auto-body structures. Hence investigating the welding metallurgy and weldability of similar and dissimilar joints of duplex stainless steels and advanced high strength steel are crucial. The purpose of this study is to analyse the relationship between welding process and geometric characteristic of weld nugget, phase transformations of Weld nugget and heat affected zone, mechanical properties (i.e force bearing capacity and energy absorption capability) and failure mode (interfatial or pullout). in... 

In metal matrix composites, the metal is reinforced with ceramic particles to get appropriate properties for hardness, resistance to fatigue, wear resistance and strength. Aluminum matrix composites have lower density than steels. These kinds of materials are used in military, automotive and aerospace industries due to their high strength to weight ratio and resistance to corrosion and wear. Application development of these materials are limited due to high machining and production costs. High ductility and strength of these materials and also hardness of ceramic particles cause rapid wear of tools and higher roughness of the surface. In this project the effect of ultrasonic vibration of a... 

Commercial solid phase microextraction fibers posses a few drawbacks such as relatively low recommended operating temperature, instability and swelling in organic solvents, breakage of the fiber, stripping of coatings, and bending of the needle. Some of these problems have been obviated by covalent bonding of the polymer phase to the fused-silica substrate by sol-gel technology, but the easy breakage of the fiber remains a problem. In this study a new technique for preparation of unbreakable, thin and very porous fiber coatings for solid phase microextraction was proposed. In this technique firstly an intermediate film consisting ... 

Metal foams are considered as a group of new materials and are applied widely in various areas due to their particular properties such as low density, high strength-to-weight ratio and appropriate mechanical, thermal, physical, electrical and magnetic properties. The aim of the present study is acquiring the technical knowledge of Zinc foam production by melting route and also optimizing the effective parameters on the foam stability. For this reason, 4 Zinc foams were produced at different times and temperatures. The amount of Calcium as the melt fluidity reducing agent and Titanium hydride and Zinc carbonate as the blowing agent were the same in all the specimens. The effect of heat... 

Voltage Controlled Oscillator (VCO) is one of the essential blocks in communication transceivers. The important specifications of the VCOs like Phase Noise, always is a motivation for researchers in order to introduce novel approaches to conquer these challenges. By development of technology and appearance of new fabrication technologies, this issue take more attention that the past. Also the out of control parameters like process variation and temperature, affect that. So, according to the challenges ahead and the demand of transceiver systems for low phase noise and reasonable power consumption, solutions for overcome these challenges. Finally, a class-C VCO with the tuning range of 4.25-5...