Sharif Digital Repository

Retrofitting of old mechanical structures by composite materials is one of the most important issues of the world.High reproducing and replacement cost of structures leads to, researchers try to retrofitting old structures and increase their capabilities and so increase their service age. Their research results leads to decreasing cost in one hand and introduce new application of composites in aviation industry in the other hand.Retrofitting mechanical structures show us the important of elements with more stress than the others. The probability of failure is more in these element than the other that we call critical element.These elements in fact are critical parts of an aerial structures... 

Metallurgical initial defects, stress concentration due to change in geometry and induced tensile residual stress during welding create an extraordinary high possibility of fatigue cracks nucleation and propagation in the weld toe region. Despite this, the ever increasing use of welding in offshore, aerospace and automotive industries, as the most applied mechanical joint, has occasioned a very extensive attempt to achieve an in-depth understanding of failure mechanisms and fatigue life improvement techniques of welded joints. Generally, these techniques could be classified into two broad categories. The first consists of methods that modify the geometry and the other is related to the... 

Rolling contact fatigue (RCF) phenomena is important in reducing the fatigue life and increasing the cost of maintenance. Accurate prediction of crack growth on wheels and the influence of residual stresses by FEM modeling can affect the maintenance planning. Therefore, investigation of rolling contact fatigue and its effect on rolling members life seem necessary. The first step to solve a rolling contact fatigue problem is the residual stress determination. In this research, wheel/rail contact is assumed. A bandage wheel with accurate geometry using finite element method is studied. For this purpose, a 3D elastic-plastic finite element model in ANSYS is conducted using model. The... 

In this study a layer with compressive residual stress was created by severe shot peening and hence surface grains was changed to nanostructure. Fatigue life will be increased as a result of compressive residual stress. Surface roughness increase, is one of the disadvantages of severe shot-peening process. Re-shot-peening with the smaller shots and lower intensity is proposed for reducing surface roughness. Numerical simulation as a powerful method, used to investigate the effects of shot-peening parameters on the residual stress distribution, surface roughness and grain size. Obtained results indicate that more energy in process can improve residual stress distribution and grain size unlike... 

In most cases, material failures occur on their surfaces, such as fatigue fracture, wear and friction, fretting fatigue and corrosion, so surface modification is one of the most effective techniques to improve the strength of materials. Correction of near surface grain size to nanoscale is one of the effective ways to improve the strength, fatigue life, toughness and etc. With increasing evidences of novel properties in nanostructured materials, it is reasonable to propose to achieve surface modification by the generation of nanostructured surface layer so that the overall properties and behaviors of the material are significantly improved.By means of surface severe plastic deformation, the... 

Gas and oil reservoirs on Caspian Sea made Engineers to choose suitable riser for conveying oil and gas. Steel catenary riser is one of the best choices for deep and ultra-deep water. Although these kinds of risers show very good resistance on buckling but cyclic loads made them to fail by fatigue damage. Two sensitive points regarding to fatigue damage are Hang off point and touch down point. Fatigue damage on TDP is related to soil properties. Because of the cyclic movement of the riser a trench would shape on seabed. In this research, based on Finite element analysis, the effect of trench on fatigue life of SCR would be analyzed  

Purpose of this research is an investigation on the effect of soil behavior on fatigue life of steel catenary risers in touch down zone. Steel catenary risers are a part of pipeline installed between the seabed and the offshore platform on the sea level, and its application is to transfer the gas or oil from the seabed to the sea level. Steel catenary risers for their resilience against buckling, easy installation, lower cost comparing other conductors are preferred to other means of conducting oil in recent years. The most challenging issue in the design of this kind of riser is the fatigue problem. The characteristic of the wave loadings we are facing in this environment is cyclic;... 

Fatigue life prediction of a mechanical part is one of issues which a group of engineers are engaged with it and always they try to design the parts with the maximum of lifetime. Although many researches have been done in this field but yet we can see that predicted life are different from that happens in the reality because there are some uncertainties in the phenomena. Our effort in this project is creating an algorithm design so that the parts are designed by it, have the maximum fatigue life and the minimum uncertainty in prediction. In this project we have considered geometrical, material and random loading uncertainties as error resources. Older methods those are presented in this... 

In this project, the structure of an aircraft and its sensitive areas has been studied for fatigue damages and then the design philosophies have been analyzed. Subsequently, relevant analysis and calculations have been conducted on a critically important part of the airframe (due to its location), which has suffered fatigue due to the age of the aircraft. This part belongs to the airframe of a Boeing 747, known as frame 360 in section 41 of the body of the aircraft, in which the numerous flight cycles have led to the creation and propagation of fatigue crack.
In the first stage of this project, the remaining life time of the frame has been estimated using theoretical relationships. In... 

In various industries such as aerospace, biomedicine and automobile manufacturing, they seek to manufacture light parts with complex geometry and acceptable mechanical properties. Additive manufacturing processes have gained a special place in the field of rapid prototyping, reverse engineering, mold production, etc. brought It is very important to know the mechanical properties, especially the fatigue behavior of the parts obtained by this method. In this research, the stress-fatigue life graph of parts made of Ti-6Al-4V (grade 5), which is one of the most widely used alloys in aerial structures, was extracted by selective laser melting. For this purpose, several standard fatigue test... 

Steering Knuckle is a mechanical part in an automobile which connects the break, suspension system, hub, and steering system to the chassis. In the recent years due to the variant imposing forces on the steering knuckle this part’s fatigue life has attracted engineers and designers’ attention. The considered steering knuckle in this project is a steering knuckle from an Iranian car. The goal of this project is to determine the fatigue life of a steering knuckle under street loads. The fatigue life of a steering knuckle has been investigated with both experimental and finite element methods. In order to perform the fatigue test a test fixture was designed. The finite element analysis consists... 

There are about 5,000 spot welds in the car body. Simulation of this number of spot welds to optimize the body of the car is practically impossible. In researches done in order to model and optimize the spot weld, standard spot weld sample with a nugget have been always used. Electrical current in spot welding process turns to heat according to the Ohm ratio, and causes the melting of the pieces and bonding. So amount of the electrical resistance, especially at the contact surface of the two pieces, is very important.In order to investigate the residual stresses induced in the connection of a three-sheet spot weld sample and its effect on fatigue life, first, a spot weld sample with... 

Modified binders have received a lot of attention in recent years because of their enhanced performance. Modification of binders, however, generally requires higher production temperatures that lead to increased energy consumption and more greenhouse gases. Warm Mix Asphalt additives such as Sasobit are applied to decrease the production temperatures. The objective of this study is to investigate the fatigue characteristics of modified binders utilized in WMA; therefore the influence of Sasobit as well as short term aging on fatigue characteristics of neat and modified binders were studied. The experimental design included one base binder and four modified binders. The base binder is a... 

The increasing growth of composite materials in various industries and the use of these materials in the production of the main structures, recognized study and simulation of accurate behavior of these materials at different scales. On the other hand, the appearance of Nano-scale materials with amazing physical and mechanical properties and the use of these nanoparticles as reinforcement in the structure of polymer composites, further highlights the importance of simulating the behavior of polymer nanoparticles at different scales. One of the issues that is considered in various industries, especially in the airspace industry, is the phenomenon of fatigue. The complexity of the fatigue...