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Residual Stress Determination of Multi-pass Welding Process, Stress Relieving Procedures and Its Effect on Crack Propagation
, M.Sc. Thesis Sharif University of Technology ; Farahi, Gholamhossein (Supervisor)
Abstract
One of the most important issues in welding is residual stresses. It is desirable to produce less residual stresses when possible and to reduce them which remain in the structure. At the present, various methods have been developed to reduce the residual stresses. The most important methods are heat treatment, shot peening, and grinding. The main problem in using these methods is cost, and for large structures these methods are often impractical. In recent decades, a technique called vibratory stress relief has been developed. It is used for large structures without limitation of size and weight; besides, the cost of this process is much less in comparison with other conventional methods...
On the quantification of seismic performance factors of Chevron Knee Bracings, in steel structures
, Article Engineering Structures ; Volume 46 , 2013 , Pages 155-164 ; 01410296 (ISSN) ; Mofid, M ; Sharif University of Technology
2013
Abstract
As a matter of fact, it is necessary to have the values of Response Modification Factor R, Over-strength Factor Ω0, and Deflection Amplification Factor Cd in order to design seismic-force-resisting systems according to design and loading codes. This study is intended to evaluate these factors for a structural lateral bracing system called Chevron Knee Bracing (CKB). In this type of bracing, the knee elements assist the system to dissipate energy through the formation of plastic flexural and/or shear hinges within the presented bracing system. The approach utilized in this study is according to FEMA P695 based on low probability of structural collapse and involves nonlinear static and dynamic...
Quantification of Seismic Performance Factors of Chevron Knee Bracing Systems
, M.Sc. Thesis Sharif University of Technology ; Mofid, Masoud (Supervisor)
Abstract
On the ground, it is necessary to have the values of Response Modification Factor R, Over-strength Factor Ω0, and Deflection Amplification Factor Cd to design seismic-force-resisting systems according to design and loading codes. This study is intended to evaluate these factors for a structural lateral bracing system called Chevron Knee Bracing (CKB). In this type of bracing, the knee elements help the system to dissipate energy through the formation of plastic flexural and/or shear hinges within the presented bracing system. The ap- proach used in this study is according to FEMA P695 based on low probabili- ty of structural collapse and involves nonlinear static and dynamic analyses....
Rolling Contact Fatigue under Influence of Residual Stresses
, M.Sc. Thesis Sharif University of Technology ; Farahi, Gholamhossin (Supervisor)
Abstract
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...
Permanence and Global Attractivity of a Periodic Predator–prey System with Mutual Interference and Impulses
, M.Sc. Thesis Sharif University of Technology ; Fotoohi Firoozabadi, Morteza (Supervisor)
Abstract
In this thesis,we consider aperiodic predator–ysystem with mutualinter ference andimpulses. By constructing a suitable Lyapunov function and using the compar-ison the orem of impulsive differential equation, sufficient conditions which ensure the permanence and global attractivity of the system are obtained. We also present some examples to verify our main results
Invesigating the Damage Effect of Earthquake Induced landslides on Buried Pipelines by Physical Modeling
, Ph.D. Dissertation Sharif University of Technology ; Jafarzadeh, Fardin (Supervisor)
Abstract
This thesis intends to evaluate the effect of landslides on dynamic response of soil slopes and buried pipelines. The core findings of the research are itemized as below: According to the experiments, dynamic slope response which moderately develops at deeper depths at toe and lower section, displays a clear downtrend at upper section and crest. Also, the pipe response depends on pipe route and slope displacement pattern. The pipe deformations have positive correlation with depth at upper section and crest, but show opposite behavior at toe and lower section. Moreover, the horizontal strains impact on total strains in buried pipes reduces at greater depths.The pipe strain analysis shows that...
Application of base-line correction methods to obtain permanent displacements of a near-source ground motion: The 2003 bam earthquake
, Article Journal of Earthquake Engineering ; Volume 13, Issue 3 , 2009 , Pages 313-327 ; 13632469 (ISSN) ; Khatam, H ; Farahi Jahromi, H ; Sharif University of Technology
2009
Abstract
In this article, two base-line correction methods for correcting the near-source strong motions were studied on the near-source recording of the 2003, Bam earthquake. To obtain permanent ground displacements at Bam station, two methods were applied with some modifications and without using any kind of filtering. Even though it is difficult to calculate true permanent displacements from data attained by present accelerograms, the results of this study show that the rational limit of permanent ground displacements can be successfully achieved by rational choosing of effective parameters. In addition, it is observed that some pre-processing steps suggested in literature have no considerable...
The Effect of Reverse Loading on Ductile Fracture by Damage Plasticity
, Ph.D. Dissertation Sharif University of Technology ; Farrahi, Gholamhossein (Supervisor)
Abstract
Recent research studies on ductile fracture of metals have shown that the ductile fracture initiation is significantly affected by the stress state. In this study, the effects of the stress invariants as well as the effect of the reverse loading on ductile fracture are considered. To estimate the reduction of load carrying capacity and ductile fracture initiation, a scalar damage expression is proposed. This scalar damage is a function of the accumulated plastic strain, the first stress invariant and the Lode angle. To incorporate the effect of the reverse loading, the accumulated plastic strain is divided into the tension and compression components and each component has a different weight...
A Thermo-Mechanical Fatigue Life Prediction Model for Aluminum Alloy (A356.0) with Thermal Barrier Coating
, Ph.D. Dissertation Sharif University of Technology ; Farrahi, Gholamhossein (Supervisor)
Abstract
Thermal barrier coating (TBC) systems have been widely used in gas turbines and diesel engines. Their advantages include reducing the fuel consumption, improving the performance and increasing the fatigue lifetime at high temperatures. In the present thesis, the fatigue behavior of an aluminum alloy (the A356.0 alloy), with and without a typical TBC system is studied. The application of this research can be introduced as coating the cylinder head in diesel engines. Initially, coating process parameters (including the feed rate and the nozzle distance) of the plasma spraying, with the objective of bending strength are optimized. The optimum value of the TBC thickness is obtained based on...
Fatigue Analysis of Welded Joint Considering Residual Stresses and Stress Relaxation Using Different Models of Material Hardening
, M.Sc. Thesis Sharif University of Technology ; Farrahi, Gholamhossein (Supervisor)
Abstract
Welded joints are the most used joints in industrial applications. Fatigue analysis of welded joints is of great importance. Residual stresses produced during welding process play an important role in fatigue failure of welded joints. Residual stresses’ effect on fatigue life of the structures is similar to mean stresses and can be accounted for like mean stress effect. However residual stresses are not constant and do change in service. In this thesis, welding process has been modeled using FE software ABAQUS in order to predict residual stress distribution due to welding. The predictions show good agreement with measurements especially for longitudinal residual stresses. Then cyclic load...
Test Methodology and Wear Characteristics of Material at Subzero Temperatures
, M.Sc. Thesis Sharif University of Technology ; Farrahi, Gholamhossein (Supervisor)
Abstract
Components in cryogenic milieu with relative motion, like seals and pumps cannot be lubricated by using oils or other lubricants. Therefore materials wear in cryogenic condition is a great concern. To earn valid data about the wear properties of common and super useful material in cryogenic tribosystems, reconstruction wear test apparatus at cryogenic temperature is needed. Investigations on metal-metal couple show that austenitic stainless steels have favorable and auspicious sliding properties at cryogenic temperature. This persuades us to survey the wear parameter of these prolific materials like austenitic stainless steels specifically. This research provides the wear parameters of...
Cyclic Behavior Modeling of Magnesium Alloy (AZ91) Under Thermo-Mechanical and Low Cycle Fatigue Loadings
, M.Sc. Thesis Sharif University of Technology ; Farrahi, Gholamhossein (Supervisor)
Abstract
Magnesium alloys have been used in the production of light weight components in automobile industries (for example: the ladder frame and the valve cover) due to a good combination of low density (1740 kg/m3), the tensile strength (160-365 MPa) and the elastic modulus (45 GPa). For the design and the optimization of these components under various loadings and different conditions, it is necessary to determine fatigue properties of the material by using thermo-mechanical and low cycle fatigue tests at different temperatures. In thermo-mechanical fatigue tests, the temperature varies in a specific range with the mechanical strain. The goal of this research is to investigate low cycle fatigue...
Determination of Fatigue Behavior of Nanostructure of Severe Shot-Peened A356.0
, M.Sc. Thesis Sharif University of Technology ; Farrahi, Gholamhossein (Supervisor)
Abstract
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...
Experimental Investigation of the Effect of Almen Intensity and Coverage on Microstructure and Fatigue Behavior of High Carbon Steel Subjected to Severe Shot Peening
, M.Sc. Thesis Sharif University of Technology ; Farrahi, Gholamhossein (Supervisor)
Abstract
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...
Finite Element Analysis of Repair Welding Residual Stress in Pressure Vessels and Piping System
, M.Sc. Thesis Sharif University of Technology ; Farrahi, Gholamhossein (Supervisor)
Abstract
Repair welding is one of the most prevalent processes in repair and maintenance part of all industries. Amongst the issues of welding processes are residual stresses and distortions. Thus prediction of residual stresses and their distribution are important for designing welding procedure specifications. Circumferential butt weld is often used for joining steam and hot water lines in plants. Due to the intense concentration of heat in the welding, the regions near the weld line undergo severe thermal cycles. The thermal cycles cause non-uniform heating and cooling in the material, thus generating inhomogeneous plastic deformation and residual stresses in the weldment. The presence of residual...
Molecular Dynamics Simulation of Crack Propagation in Nanocrystalline Materials
, M.Sc. Thesis Sharif University of Technology ; Farrahi, Gholamhossein (Supervisor)
Abstract
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...
Effect of Cryogenic Treatment on Distribution of Residual Stress and Wear Resistance of AISI 316 and AISI 304 Stainless Steel
, M.Sc. Thesis Sharif University of Technology ; Farrahi, Gholamhossein (Supervisor)
Abstract
Various methods are used to improve the behavior of steel parts. Cryogenic treatment is one of the prevalent methods used for this purpose. It is not a substitute process but rather a supplement to conventional heat treatment process. In this Study the effect of the minimum temperature and the time of cryogenic treatment on wear resistance and residual stress of stainless steel AISI 316 and AISI 304 were studied. For this purpose conventional heat treatment, deep cryogenic treatment (24, 48 and 72 hours) and shallow cryogenic treatment (5, 10 and 15 hours) were performed on the samples. The wear resistance were measured by Pin-on-disk test according to the standard G99-95 (under the load of...
Wear Behavior of the Nanostructured A356 Aluminum Alloy Induced by Severe Shot Peening
,
M.Sc. Thesis
Sharif University of Technology
;
Farrahi, Gholamhossein
(Supervisor)
Abstract
Wear is the most important cause of surface damage occurs by direct contact of surfaces. Increasing the quality and strength of surface against different kinds of destructive phenomena is significant in manufacturing of mechanical parts. Surface nanocrystallisation can improve the surface protection against wear and can be done by lazer beam and shot peening. In this investigation by sever shot peening process the surface of A356 Aluminium alloy transforms to nanocrystal structure. The dry sliding wear and friction behaviors of A356 Aluminum were evaluated using a pin-on-disk apparatus at ambient conditions. The stationary diameter of 5mm stainless steel pin produced a wear track (scar) on...
Determination of Mechanical and Fracture Properties of Nanostructure of Severe Shot Peened A356.0 at Room Temperature
,
M.Sc. Thesis
Sharif University of Technology
;
Farrahi, Gholamhossein
(Supervisor)
Abstract
The most failures caused by wear, fatigue and corrosion are very sensitive to surface of material. Since the most failures starts or occurs at the surface of material, so control and improvement of the surface properties can have a significant effect on increasing the service life and improve the overall behavior of the material. Reforming of the structure and grain size is one of the methods to improve elastic properties, hardness, softness, toughness, etc. Also the macroscopic residual stresses in fracture toughness test specimens, can have tremendous effect on fracture toughness measurements. Many methods are used to modify the surface structure and to create a compressive residual stress...
Prediction of Stable Fatigue Crack Growth in Laser Peened 316L SS with Variable Beam Diameter and Pre-crack Length Using Artificial Neural Network
, M.Sc. Thesis Sharif University of Technology ; Farrahi, Gholamhossein (Supervisor)
Abstract
Processing an alloy can enhance its mechanical properties. Based on repeated radiation of high-power pulsed laser beam, laser shock peening (LSP) is one way to impart favorable residual stress in an alloy and to enhance its damage tolerance property. Study of overlapping LSP is possible through the simulation of limited laser shots. Still, its computational cost is higher than that of a single shot. This research, primarily, is aimed at improving a methodology for reduction of computations in the study of an overlapping LSP and its impacts on fatigue crack growth. Addressing research gaps by utilizing the methodology is the secondary goal. The methodology consists of plastic strain...