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crack-growth
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Total 53 records
Effect of Grinding and Shot Peening on Fatigue Life of Welded Joints
, M.Sc. Thesis Sharif University of Technology ; Farrahi, Gholamhossein (Supervisor)
Abstract
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...
Numerical and experimental study on the optimization of overload parameters for the increase of fatigue life
, Article Aerospace Science and Technology ; Vol. 35, issue. 1 , May , 2014 , p. 80-86 ; Foumani, M. S ; Sharif University of Technology
Abstract
Fatigue life and crack growth retardation due to periodic tensile overloads, which are superposed on constant amplitude cycles, have been investigated in the present study. In the numerical analyses, the Walker equation with the generalized Willenborg model has been used. The analyses and experiments have been performed on C(T) specimens made of 7075 aluminium. The periodic overloads have been induced at two different periodicities and the effect of overload ratio and overload periodicity on the fatigue life has been studied. The results of the experiments and analyses reveal that the normalized fatigue life versus overload ratio curve has a maximum point, which indicates that there are...
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...
Effects of Laser Peening with Different Coverage Areas on Residual Stress and Fatigue Crack Growth
, M.Sc. Thesis Sharif University of Technology ; Farrahi, Gholamhossein (Supervisor) ; Mehmanparast, Ali (Co-Advisor)
Abstract
Failure due to fatigue loading is one of the main reasons for the wear out of parts in the industry. Prevention of failure and improving the fatigue life of materials have been studied by many researchers. Desirable compressive residual stress have important effect on improving the lifetime of materials. Laser peenning leads to increase fatigue life of metals by creating compressive residual stress at the surface of the pieces.This study investigates the effect of different laser coverage areas on residual stresses using finite element method and also investigates the fatigue crack growth experimentally.Simulation results show that increasing the laser peening rows impart the residual...
Reliability of Components Operating under Creep- Fatigue Conditions
, Ph.D. Dissertation Sharif University of Technology ; Farrahi, Gholamhossein (Supervisor) ; Wang, Bin (Co-Supervisor) ; Mehmanparast, Ali (Co-Supervisor)
Abstract
The probabilistic behavior and the reliability of high-temperature defective components under creep-fatigue conditions have been assessed, considering different sources of uncertainty in “material behavior”, “geometric parameters” and “operation conditions”. For this purpose, deterministic calculations of creep-fatigue crack growth have been derived based on the R5 procedure, and then these relationships have been used at the heart of the probabilistic assessments. One of the aims of this dissertation was to present a procedure for assessing the reliability of defective components operating under creep-fatigue conditions. This procedure has been implemented on a benchmark problem, i.e....
Crack Propagation Monitoring of Gas Turbine Blades Using Modal Analysis
, M.Sc. Thesis Sharif University of Technology ; Zabihollah, Abolghasem (Supervisor) ; Behzad, Mehdi (Co-Advisor)
Abstract
Rotary equipment are being used in many industries over a wide range of applications, such as gas turbine. Due to the nature of the operating conditions, gas turbines are subjected to variety of damages especially crack propagation of a blade. The Cracks propagation may lead to catastrophic failure during operation. Therefore, detection of the crack presence and its propagation is very important and may reduce possible significant cost of the whole system failure. The thesis aims to investigate the cracks presence and their propagation effects on vibration of a turbine blade which is located at the National Iranian South Oilfields. First, theoretical study is calculated to validate an...
On Three-dimensional Modelling of Crack Growth Using Extended Isogeometric Method
, M.Sc. Thesis Sharif University of Technology ; Khoei, Amir Reza (Supervisor)
Abstract
Isogeometric Analysis method is a newly introduced method for the analysis of problems governed by partial differential equations.The major difference between classic finite element method and isogeometric method is that FEA uses piecewise polynomial shape functions for analysis while the Non-Uniform Rational B-Splines (NURBS) functions are applied as basis function in isogeometric method. To model strong discontinuities like cracks in classical finite element method , very fine mesh near the crack tip must be used. Application of eXtended Finite Element Method (X-FEM) avoid making very fine mesh near crack tip which is a time consuming task. In the extended finite element method , a...
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...
Surface Initiated Crack Growth Simulation of Normal Contact Using X-FEM
, M.Sc. Thesis Sharif University of Technology ; Adibnazari, Saeed (Supervisor)
Abstract
In many mechanical components, contact loading has a major role in stress field analysis. In this thesis, a combination of crack growth simulation and unilateral contact problem was carried out numerically. The Finite Element Method provides a powerful numerical tool to analyze contact problems. In the traditional FEM discontinuities should not cross through elements, so crack will grow through boundaries of elements, therefore, the model should remesh in each step of crack growth in order to align the mesh along the boundaries of discontinuities. This limitation causes major errors and computational costs. To overcome this limitation, the extended Finite Element Method (XFEM) will be used...
Application of Isogeometric Method in Modeling and Analyzing Crack Growth Problems
, M.Sc. Thesis Sharif University of Technology ; Khoei, Amir Reza (Supervisor)
Abstract
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...
Modeling of Cohesive Crack Propagation with Energy Method using XFEM
, M.Sc. Thesis Sharif University of Technology ; Khoei, Amir Reza (Supervisor)
Abstract
Crack propagation in materials is an attractive problem in engineering because of the impact on the safety as well as economic issues. Much research studies have been done on the crack initiation, crack propagation criteria and path in the materials with different characteristics and conditions. Crack modeling depending on the material properties in brittle and quasi-brittle materials is done as Linear Elastic Fracture Mechanics (LEFM) and cohesive crack, respectively. The aim of this thesis is the modeling of crack propagation using energy method and comparing it with the cohesive crack. In order to model this problem, it is necessary to solve the governing equilibrium equation of the...
Numerical and Experimental Study on Optimization of Overload arameters for Fatigue Life Increase
, M.Sc. Thesis Sharif University of Technology ; Saadat Foumani, Mahmoud (Supervisor)
Abstract
Fatigue life and crack growth retardation due to periodic tensile overload that superposed on constant amplitude cycles has been investigated. The tests were performed using C(T) specimens made of 7075 aluminium. All periodic overload tests were carried out at the same spacing between overloads and the effect of overload ratio on fatigue life has been investigated. Results of experiments revealed that the at low and high overload ratios, crack growth rate is relatively high and there is an optimum OLR that maximize interaction and thus minimize FCGR. Whit OCR-1=50, overload ratio of 1.85 led to maximum interaction and resulted in 219% increase in specimen's life relative to CA loading. Also...
Analysis of Fatigue Cracks in Frame 360 Section 41 of B747
, M.Sc. Thesis Sharif University of Technology ; Adibnazari, Saeid (Supervisor)
Abstract
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 the first stage of this project, the remaining life time of the frame has been estimated using theoretical relationships. In...
Theoretical and Experimental Study of Turbine Discs under Severe Spin and Temperature Fluctuations
, M.Sc. Thesis Sharif University of Technology ; Durali, Mohammad (Supervisor)
Abstract
Gas turbine discs must be tested before they are installed in an engine for their mechanical properties and resistance to crack growth. Such test is usually done in a device called Spin Tester in which high rotational speed and temperature fluctuations can be applied to a disc. In this project a plug-in software is developed by which life of gas turbine discs under sever fluctuations of rotating speed and temperature can be assessed. The tests performed in the software are; stress analysis, fatigue analysis, crack initiation point, and finally 3-D fatigue crack growth path. The software is written using Python programming in ABAQUS. The software adds a sub-menu in ABAQUS Plugins as "Engine...
Three-dimensional cohesive fracture modeling of non-planar crack growth using adaptive FE technique
, Article International Journal of Solids and Structures ; Volume 49, Issue 17 , September , 2012 , Pages 2334-2348 ; 00207683 (ISSN) ; Moslemi, H ; Sharifi, M ; Sharif University of Technology
2012
Abstract
In this paper, the three-dimensional adaptive finite element modeling is presented for cohesive fracture analysis of non-planer crack growth. The technique is performed based on the Zienkiewicz-Zhu error estimator by employing the modified superconvergent patch recovery procedure for the stress recovery. The Espinosa-Zavattieri bilinear constitutive equation is used to describe the cohesive tractions and displacement jumps. The 3D cohesive fracture element is employed to simulate the crack growth in a non-planar curved pattern. The crack growth criterion is proposed in terms of the principal stress and its direction. Finally, several numerical examples are analyzed to demonstrate the...
Optimization of composite patch repair for inclined crack on aluminum plate using genetic algorithm
, Article 30th Congress of the International Council of the Aeronautical Sciences, ICAS 2016, 25 September 2016 through 30 September 2016 ; 2016 ; 9783932182853 (ISBN) ; Abedian, A ; Firooz, S ; Sharif University of Technology
International Council of the Aeronautical Sciences
2016
Abstract
In this study, a composite patch repair will be designed for an aluminum plate with a central crack placed at 45 degrees to the applied unidirectional tensile load carried by the plate. In this state, the condition of loading is mixed mode. In the first step, behavior of the crack repaired by the composite patch under the applied tensile load is simulated by using ABAQUS commercial software. The crack growth process is modeled with the extended finite element method (XFEM) and the cohesive zone model (CZM) is used to model any damage progression in the adhesive of the composite patch repair. The shape of the five layer composite patch is assumed to be elliptical. For design optimization, the...
Correlation between isothermal crystallization properties and slow crack growth resistance of polyethylene pipe materials
, Article Polymer Testing ; Volume 80 , 2019 ; 01429418 (ISSN) ; Pircheraghi, G ; Rashedi, R ; Sepahi, A ; Sharif University of Technology
Elsevier Ltd
2019
Abstract
In this study, three PE100 polyethylene materials with fairly different short chain branch distributions were used to characterize the relation between thermal properties and creep test failure time. The samples were thermally characterized using isothermal and non-isothermal differential scanning calorimetry (DSC). The three resins showed different behavior after fitting on the Avrami equation. N-100J2 sample, in which short chain branches (SCBs) are located on longer molecule chains, has a lower chain mobility and higher crystallization time, while N-100J1 has the opposite crystallization properties. Also, these samples had different Avrami index, n. Microstructural evaluation of these...
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...
Three-Dimensional Cohesive Modeling of Curved Crack Growth in Quasi-brittle Material Using Adaptive Technique
, M.Sc. Thesis Sharif University of Technology ; Khoei, Amir Reza (Supervisor)
Abstract
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...
Modeling of Hydraulic Fracturing in Fractured Saturated Porous Media Using the Extended Finite Element Method
, M.Sc. Thesis Sharif University of Technology ; Khoei, Amir Reza (Supervisor)
Abstract
In current study, we investigate crack propagation is saturated porous media by using the extended finite element method. The porous media is simulated by using well-known u-p formulation. In this regard the equilibrium equation of the bulk is employed together with momentum and mass balance equations of the fluid phase. The XFEM, known as one of the best methods in crack simulations, will be employed for numerical modelings. According to the method, crack propagation does not involve any changes in meshing, yet by using the concept of enrichment the elements are enabled to possess crack interfaces inside themselves. Thus by crack growth, only discritization alters and new DOFs are inserted...