Dissertations / Theses on the topic 'Fatigue life of metals'
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Nowicki, Timothy. "Statistical model prediction of fatigue life for diffusion bonded Inconel 600 /." Online version of thesis, 2008. http://hdl.handle.net/1850/7984.
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Erasmus, Daniel Jacobus. "The fatigue life cycle prediction of a light aircraft undercarriage." Thesis, Nelson Mandela Metropolitan University, 2010. http://hdl.handle.net/10948/1527.
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The reliability of systems relies heavily on accurate fatigue life prediction of related components. Fatigue life prediction is a complicated process requiring the correct methodology to determine accurate and reliable predictions. The Palmgren – Miner damage accumulation hypothesis is widely used in determining the fatigue life of components exposed to variable loading conditions. Modifications have been made to this hypothesis trying to achieve a greater degree of accuracy, of these the Liu – Zenner modification has been the most successful. In this report the systematic process of fatigue life prediction using the Liu – Zenner modification to achieve reliable results is calculated. A representative stress time history measured in service on the component forms the basis for defining a flight cycle which is the chosen unit in which to express the fatigue life. Rainflow cycle counting performed on the stress time history allowed the formulation of a load spectrum to which the component is exposed in one cycle. Combining the load spectrum with the developed SN curve of the component and using the Liu – Zenner modification to the Palmgren – Miner rule a reliable fatigue life in cycles is predicted.
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Heffern, Thomas V. "Probabilistic modeling and simulation of metal fatigue life prediction." Thesis, Monterey, Calif. : Springfield, Va. : Naval Postgraduate School ; Available from National Technical Information Service, 2002. http://library.nps.navy.mil/uhtbin/hyperion-image/02sep%5FHeffern.pdf.
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Thesis (M.S. in Aeronautical Engineering)--Naval Postgraduate School, September 2002.Thesis advisor(s): Ramesh Kolar, E. Roberts Wood. Includes bibliographical references (p. 113). Also available online.
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Peters, Donald Michael Dirk. "The effects of shot peening on low cycle fatigue life of 7075-T6 aluminium alloy round bar." Thesis, Nelson Mandela Metropolitan University, 2014. http://hdl.handle.net/10948/2929.
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The aim in this dissertation was to improve our understanding of the effectiveness of shot peening in prolonging fatigue life, of 7075-T6 Aluminium Alloy round bar, taking into consideration surface residual stress, microstructural and micro-hardness parameters. Three point bending, high stress, moderately low cycle, fatigue tests were conducted to study the effects of shot peening and associated surface residual compressive stresses on fatigue life. The influence of shot peening on the microstructure was explored, including the application of mechanical small plastic straining and surface skimming, to vary the surface residual compressive stresses and induce strain hardening. Tests were performed to measure residual stress-depth distribution, plastic straining, micro-hardness, and the microstructure analysed on scanning electron microscopy (SEM) fractographs. The Juvinall and Marshek life prediction model was used in conjunction with the Gerber equation for non-zero mean stress applications to generate a proposed life prediction model for this material which is user-friendly. The proposed life prediction model has a linear equation format with the flexibility to conservatively accommodate most of the various types, and combinations, of treatments applied in this research by the use of customised constants. The results show that there was good correlation between actual and predicted fatigue life as well as useful insights into the role of the microstructure in explaining fatigue life behaviour.
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Müller, Ruan. "Characterising the stress-life response of mechanical formed AISI-1008 steel plate components." Thesis, Nelson Mandela Metropolitan University, 2012. http://hdl.handle.net/10948/d1008102.
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The main purpose of this research project was to determine the fatigue-life behaviour of AISI 1008 sheet steel which has been mechanically formed to a radius of curvature of 120mm and then to correlate the fatigue-life behaviour to that of the parent or “as manufactured” material. During the forming process it was felt important to induce plastic strain through stretch-bending by clamping the sides of a plate sample’s (width) edges in the bending fixture before being bent by a single acting mechanical press. It was determined through actual testing that there was a decrease in fatigue-life when the mechanical formed data was compared to fatigue data of the parent material. Standard fatigue mathematical models were used to relate the actual fatigue data. Due to the material being cold formed to a radius of curvature of 120mm, residual stresses induced during the forming process played an essential role during the fatigue-life prediction calculations. The maximum relieved stress in the parent material was compressive in nature having a magnitude of 11percent of the “as manufactured” yield strength (265 MPa). For the mechanical formed material compressive residual stresses were measured on the outer surface while tensile stresses were measured on the inner surface. The difference between actual number of cycles to failure to that calculated using the standard mathematical models for the parent material, ranged between 48 percent and 18 percent and for the mechanical formed samples between 35 percent and 1percent, depending on the strain amplitude used. An important aspect of this study was to determine the criteria required for mathematical modelling of the parent material as testing occurred between the limit of proportionality and yield point. Although this aspect requires further investigation the mathematical results obtained during this study were considered to be acceptable.
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Beheshti, Milad. "Fatigue life prediction of threaded pipe connection." Thesis, Brunel University, 2017. http://bura.brunel.ac.uk/handle/2438/15588.
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In the oil and gas industry, threaded pipe connection is frequently used to connect the casing string, drill pipe strings or production and transportation risers and pipelines. The connection is normally preloaded in order to maintain a sealed and secure connection while in service and avoid leakage. Tapered thread are a common connection and in order to introduce preload to the threaded connection when they are assembled a certain make-up torque is going to be applied. The make-up torque plus external loads result in a multiaxial stress distribution over the connection, where the threaded connections act as stress risers. Environment such as waves and currents cause dynamic loads acting on the pipe line and offshore structures. The weakest point in offshore structure is the pipe connection because of fatigue crack initiated in the connection's threads. Researchers and engineers developed a variety of patented threaded pipe connection which all claiming to improve a connection's fatigue life. The experimental data for patented designs, available in literature, is limited. Most published studies usually comprise experiments on a single connection type. For detailed fatigue analysis those published studies cannot be used since there is no uniformity in testing setup, loading conditions and damage detection technique exist. Moreover, current design curves in codes and standards lead to overly conservative or inaccurate results. The aim of this work is to provide a better understanding of the fatigue mechanisms of threaded pipe connections and to study the effect of different design features on a connection's fatigue life. The final goal is to formulate guidelines for new fatigue resistant connection designs. API connection is used as a reference in this study. Several modifications and design features are applied to the connection type. To simulate the effect of these modifications, a parametric 2D axisymmetric finite element model, ABAQUS is used. 2D finite element result are compared with a 3D model to prove its validity for both make-up. In addition, the results of the 2D axisymmetric simulation are validated by static strain gauge measurements during a make-up test and an axial tension test. The validated model is then used to evaluated the influence of the connection properties and design features on the threaded connection's behaviour. Test rigs were designed to perform axial fatigue experiment on two scales: the small-scale experiments on 1" (33.4 mm outer diameter) connections are performed in axial fatigue testing, the medium scale tests on 4.5" (114.3 mm) connections are carried out under axial tension for which a setup is developed. The majority of the performed fatigue tests are small scale experiments. Several modified configurations are tested. The S-N curve is constructed, so that the effect of certain configuration on the connection's fatigue life can be quantified. The local modification of the threaded connection's geometry as well as the connection's contact condition's contact conditions can have an important influence on the fatigue life of the connection. A beach marking technique is used to visualized the crack fronts at different moments during the tests so that exact crack shape can be seen during post-mortem analysis. The result shown that a crack initiates at the root of the last engaged thread of the male part of the connection, and propagates slowly over a large segment of the circumference, forming a long shallow crack. When the crack penetrates the pipe wall, it rapidly increases in size along two crack fronts. The shape of crack observed in beach mark analysis do not have a semi-elliptical shape as commonly used in fracture mechanics. A fatigue crack growth analysis that considers the crack as an annular flaw, is effective in describing the crack growth behaviour. The experimentally obtained S-N curves and the result from the finite element simulations are combined in multiaxial damage evolution law. The observed trend in fatigue lives of the configuration are explained by using the fatigue analysis. Using a connection's thread load distribution as a measure for its fatigue life is proven to be inaccurate. The main reason for this is that the load distribution is related to axial stresses over the connection. The fatigue life of a threaded connection is determined by the local multiaxial stress distribution and strain range around the root of the last engaged thread. These local conditions are not only the result of the load distribution, but they are also affected by the hoop stress introduced during make-up, which can additionally be affected by a changed connection stiffness. The multiaxial damage evolution law is used to analyse the influence of several features on a connection's fatigue life. It is not for all patented modifications that an increased fatigue life is predicted when applied to the API connection. The final conclusion reached is that, in order to optimize a fatigue resistant connection, several design features must be combined together. The thread shape can be optimized to obtained a low stress concentration factor and reduce the local strains at the thread root. The connection's global geometry and make-up conditions can be optimized to improve the load distribution over the threads and reduce local stresses and strains at the threads.
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Bennett, Valerie P. "A microscale study of small crack propagation in multiaxial fatigue." Diss., Atlanta, Ga. : Georgia Institute of Technology, 1999. http://hdl.handle.net/1853/23957.
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Willens, David C. "Improving the Fatigue Life of Cylindrical Thread Rolling Dies." Digital WPI, 2020. https://digitalcommons.wpi.edu/etd-dissertations/602.
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Thread rolling is a unique metal forming process which is commonly used to form screw threads on threaded fasteners and precision leadscrews at relatively high rates of speed. Threads are formed on a cylindrical blank by flat or cylindrical dies having the reverse form on them, which rotate and penetrate the blank simultaneously, to plastically deform it into a precise geometry. Thread rolling dies are exposed to a complex state of cyclical contact stresses that eventually cause the dies to fail by fatigue and wear. The stress state is not easily ascertained through standard analytical models due to complex geometry and process conditions. This research seeks to better understand the state of contact stresses present in cylindrical thread rolling dies as they form material, to aid in identifying and testing economical methods of improving thread rolling die fatigue life. Some work has been published on using FEA simulation software to model the thread rolling process, but no work has been published on using FEA software to analyze the stresses in thread rolling dies. DEFORM®-3D Forming Simulation Software by Scientific Forming Technologies Corporation in Columbus, Ohio was used to simulate the throughfeed thread rolling process and model the state of stresses in the dies. The results were compared to the Hertzian contact stress model and the Smith Liu equations for rolling and sliding friction. Fatigue life prediction methods involving S-N curves, surface fatigue strength, and Weibull probability distributions were tested using the simulation data against field results. An optimized die design was generated from a design of experiments simulating different die design geometry. Findings show that field failures correlate well to the DEFORM® simulation results. The Hertz model with Smith Liu equations improved correlation with the simulation. Fatigue life prediction models correlated reasonably well to field results using the simulation data for inputs. These findings can aid in selecting appropriate die materials, design parameters, and fatigue life treatments.
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Moore, Zachary Joseph. "Life modeling of notched CM247LC DS nickel-base superalloy." Thesis, Atlanta, Ga. : Georgia Institute of Technology, 2008. http://hdl.handle.net/1853/24813.
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Thesis (M. S.)--Mechanical Engineering, Georgia Institute of Technology, 2008.Committee Chair: Dr. Richard W. Neu; Committee Member: Dr. David L. McDowell; Committee Member: Dr. W. Steven Johnson.
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Aytekin, Oguz. "Numerical And Experimental Investigation Of Fatigue Life In Deep Drawn Parts." Master's thesis, METU, 2005. http://etd.lib.metu.edu.tr/upload/12607386/index.pdf.
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Sheet metal forming has an important place among metal forming processes. As the usage of sheet metal increases, the fatigue simulation and optimization of these parts become more important. This thesis study examines the change of the fatigue life of a sheet metal part after forming. A sphere-like shape is deep drawn and change in thickness and residual stresses are analyzed. To understand the effect of residual stresses, deep drawn parts with and without residual stress tested against the fatigue failure. In parallel, the forming process is simulated with an implicit finite element method (FEM). The success of forming simulation is discussed in the study. Thickness changes and residual stresses calculated with FEM are included in computer aided fatigue analysis. The effect of thickness changes is examined with the results of FEM analysis. The effectiveness of the whole simulation process is discussed by comparing the outputs of experiments and computational analysis.
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Shirgaokar, Manas. "Technology to Improve Competitiveness in Warm and Hot Forging: Increasing Die Life and Material Utilization." Columbus, Ohio : Ohio State University, 2008. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=osu1205347635.
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Malpertu, Jean-Louis. "Fatigue mecano-thermique d'un superalliage a base de nickel." Paris, ENMP, 1987. http://www.theses.fr/1987ENMP0075.
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Etude sur une eprouvette tubulaire en alliage in100 soumise simultanement a un cycle de temperature (600 a 1050c) et a un cycle de deformation. Determination experimentale des lois de comportement anisotherme. Influence de divers parametres sur l'amorcage et le debut de propagation des fissures. Analyse des mecanismes d'endommagement lies a la fragilisation de l'alliage par la diffusion de l'oxygene. Proposition d'un modele d'endommagement prenant directement en compte l'interaction fatigue-oxydation. Prevision de la duree de vie en fatigue isotherme a 1000c et en fatigue mecano-thermique
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Godbole, Chinmay. "The Influence of Reinforcement on Microstructure, Hardness, Tensile Deformation, Cyclic Fatigue and Final Fracture behavior of two Magnesium Alloys." University of Akron / OhioLINK, 2011. http://rave.ohiolink.edu/etdc/view?acc_num=akron1321633235.
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Pereira, M. F. V. T., M. Williams, and Preez W. B. Du. "Characterization of metal powder based rapid prototyping components with respect to aluminium high pressure die casting process conditions." Journal for New Generation Sciences, Vol 8, Issue 2: Central University of Technology, Free State, Bloemfontein, 2010. http://hdl.handle.net/11462/563.
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Published ArticleThis paper is based on tests performed on die component specimens manufactured by EOS-DMLS (direct metal laser sintering) and LENS (laser engineered net shape) RP (rapid prototyping) technology platforms, as well as manufactured specimens machined out of preferred standard hot work steel DIN 1.2344. These specimens resemble typical components used in metal high pressure die casting tool sets. The specimens were subjected to a programme of cyclic immersion in molten aluminium alloy and cooling in water-based die release medium. The heat checking and soldering phenomena were analyzed through periodic inspections, monitoring crack formation and evidence of surface washout. At the end of the thermal tests, mechanical strength and hardness tests were performed to assess toughness and core resistance variations in relation to the initial conditions. Finally metallographic investigations were performed through optical microscopy on all the specimens considered. The outcomes of this research will be presented and used by the CSIR for further development and application of the assessed EOS-DMLS and LENS rapid prototyping technologies in rapid die manufacturing techniques and die design principles, including time and economic feasibility criteria to be applied when considering rapid die manufacture.
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Chieragatti, Rémy. "Influence de l'orientation cristallographique sur le comportement en fatigue oligocyclique du mar-m200 monocristallin." Paris, ENMP, 1987. http://www.theses.fr/1987ENMP0024.
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Etude experimentale sur le superalliage soumis a une charge cyclique uniaxiale suivant la direction de croissance 001 et les orientations 111, 011 et 123 a 650 c et 900c. Analyse des courbes d'ecrouissage cyclique. Caracterisation des systemes de glisement associes aux grandes deformations plastiques. Description par un modele de l'influence quantitative de l'orientation sur les courbes, d'ecrouissage cyclique. Mise en evidence du role preponderant de la periode de propagation des fissures sur l'endommagement. Prevision theorique de la duree de vie en fatigue en utilisant des donnees de fissuration etablies pour une orientation donnee
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Bassaw, Benjamin. "Comportement en fatigue d'un alliage expérimental Cu-Al(2)O(3) : (base cuivre à dispersion d'alumine) élaboré par métallurgie des poudres." Poitiers, 1988. http://www.theses.fr/1988POIT2263.
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Des essais de traction-compression dans le domaine de fatigue plastique ont revele la grande stabilite cyclique du materiau. Analyse des conditions d'amorcage et de propagation des fissures. Mise en evidence du role des defauts de frittage ou des inclusions. Des essais de fissuration sont effectues dans le domaine des moyennes et basses vitesses en considerant l'effet du rapport de charge et de l'environnement. Description par la mecanique lineaire de rupture du comportement des microfissures
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Lunt, William S. "Molecular dynamics simulation of fatigue damage in metals." Thesis, Monterey, Calif. : Springfield, Va. : Naval Postgraduate School ; Available from National Technical Information Service, 2003. http://library.nps.navy.mil/uhtbin/hyperion-image/03Dec%5FLunt.pdf.
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Williams, Zachary. "Krouse Fatigue for Metals with Elevated Mean Stress." Ohio University / OhioLINK, 2020. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1597075964521893.
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Repetto, Eduardo A. Ortiz Michael. "On the fatigue behavior of ductile F.C.C. metals /." Diss., Pasadena, Calif. : California Institute of Technology, 1998. http://resolver.caltech.edu/CaltechETD:etd-01242008-133649.
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Skelly, Michael V. "Fatigue life program using strain-life methods." Thesis, Monterey, Calif. : Springfield, Va. : Naval Postgraduate School ; Available from National Technical Information Service, 1993. http://handle.dtic.mil/100.2/ADA267310.
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Ghodratighalati, Mohamad. "Multiscale Modeling of Fatigue and Fracture in Polycrystalline Metals, 3D Printed Metals, and Bio-inspired Materials." Diss., Virginia Tech, 2020. http://hdl.handle.net/10919/104944.
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The goal of this research is developing a computational framework to study mechanical fatigue and fracture at different length scales for a broad range of materials. The developed multiscale framework is utilized to study the details of fracture and fatigue for the rolling contact in rails, additively manufactured alloys, and bio-inspired hierarchical materials. Rolling contact fatigue (RCF) is a major source of failure and a dominant cause of maintenance and replacements in many railways around the world. The highly-localized stress in a relatively small contact area at the wheel-rail interface promotes micro-crack initiation and propagation near the surface of the rail. 2D and 3D microstructural-based computational frameworks are developed for studying the rolling contact fatigue in rail materials. The method can predict RCF life and simulate crack initiation sites under various conditions. The results obtained from studying RCF behavior in different conditions will help better maintenance of the railways and increase the safety of trains.
The developed framework is employed to study the fracture and fatigue behavior in 3D printed metallic alloys fabricated by selective laser melting (SLM) method. SLM method as a part of metal additive manufacturing (AM) technologies is revolutionizing the manufacturing sector and is being utilized across a diverse array of industries, including biomedical, automotive, aerospace, energy, consumer goods, and many others. Since experiments on 3D printed alloys are considerably time-consuming and expensive, computational analysis is a proper alternative to reduce cost and time. In this research, a computational framework is developed to study fracture and fatigue in different scales in 3D printed alloys fabricated by the SLM method. Our method for studying the fatigue at the microstructural level of 3D printed alloys is pioneering with no similar work being available in the literature. Our studies can be used as a first step toward establishing comprehensive numerical frameworks to investigate fracture and fatigue behavior of 3D metallic devices with complex geometries, fabricated by 3D printing.
Composite materials are fabricated by combining the attractive mechanical properties of materials into one system. A combination of materials with different mechanical properties, size, geometry, and order of different phases can lead to fabricating a new material with a wide range of properties. A fundamental problem in engineering is how to find the design that exhibits the best combination of these properties. Biological composites like bone, nacre, and teeth attracted much attention among the researchers. These materials are constructed from simple building blocks and show an uncommon combination of high strength and toughness. By inspiring from simple building blocks in bio-inspired materials, we have simulated fracture behavior of a pre-designed composite material consisting of soft and stiff building blocks. The results show a better performance of bio-inspired composites compared to their building blocks. Furthermore, an optimization methodology is implemented into the designing the bio-inspired composites for the first time, which enables us to perform the bio-inspired material design with the target of finding the most efficient geometries that can resist defects in their structure. This study can be used as an effective reference for creating damage-tolerant structures with improved mechanical behavior.Doctor of Philosophy
The goal of this research is developing a multiscale framework to study the details of fracture and fatigue for the rolling contact in rails, additively manufactured alloys, and bio-inspired hierarchical materials. Rolling contact fatigue (RCF) is a major source of failure and a dominant cause of maintenance and replacements in many railways around the world. Different computational models are developed for studying rolling contact fatigue in rail materials. The method can predict RCF life and simulate crack initiation sites under various conditions and the results will help better maintenance of the railways and increase the safety of trains. The developed model is employed to study the fracture and fatigue behavior in 3D printed metals created by the selective laser melting (SLM) method. SLM method as a part of metal additive manufacturing (AM) technologies is revolutionizing industries including biomedical, automotive, aerospace, energy, and many others. Since experiments on 3D printed metals are considerably time-consuming and expensive, computational analysis is a proper alternative to reduce cost and time. Our method for studying the fatigue at the microstructural level of 3D printed alloys can help to create more fatigue and fracture resistant materials. In the last section, we have studied fracture behavior in bio-inspired materials. A fundamental problem in engineering is how to find the design that exhibits the best combination of mechanical properties. Biological materials like bone, nacre, and teeth are constructed from simple building blocks and show a surprising combination of high strength and toughness. By inspiring from these materials, we have simulated fracture behavior of a pre-designed composite material consisting of soft and stiff building blocks. The results show a better performance of bio-inspired structure compared to its building blocks. Furthermore, an optimization method is implemented into the designing the bio-inspired structures for the first time, which enables us to perform the bio-inspired material design with the target of finding the most efficient geometries that can resist defects in their structure.
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Wertz, John Nicholas. "Isothermal Fatigue Life Prediction Techniques." The Ohio State University, 2013. http://rave.ohiolink.edu/etdc/view?acc_num=osu1363195868.
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Fernandes, Paulo Jorge Luso. "Fatigue and fracture of metals in liquid-metal environments." Thesis, University of Cambridge, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.337963.
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Fan, Zhengxuan. "Atomistic simulation of fatigue in face centred cubic metals." Thesis, Université Paris-Saclay (ComUE), 2016. http://www.theses.fr/2016SACLX076/document.
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La fatigue induite par chargement cyclique est un mode d'endommagement majeur des métaux. Elle se caractérise par des effets environnementaux et de grandes dispersions de la durée de vie qui doivent être mieux comprises. Les matériaux analysés sont de type cfc : aluminium, cuivre, nickel et argent. Le comportement de marches naturellement créées en surface par le glissement cyclique de dislocations est examiné par simulations en dynamique moléculaire sous vide et sous environnement oxygène pour le cuivre et le nickel. Un phénomène de reconstruction est observé sur les marches en surface, qui peut induire une forte irréversibilité. Trois mécanismes de reconstruction des marches apparues en surface sont observés et décrits. L’irréversibilité de ces marches est ensuite analysé. Elles sont irréversibles pour des chargements expérimentaux, sauf arrivée de dislocations de signe opposé sur un plan de glissement directement voisin.Avec arrivée de dislocations sur des plans non voisins, l'irréversibilité s’accumule cycle par cycle et il est possible de reproduire l’apparition de fissures en surface dont la profondeur augmente graduellement.Un environnement oxygène modifie la surface (début d’oxydation) mais pas l’irréversibilité parce que l’oxygène n’a pas d'influence majeure sur les différents mécanismes liés à l’évolution du relief.Une estimation grossière de l'irréversibilité est faite pour des dislocations coin pures dans une bande de glissement persistante pour les matériaux dits ondulés. On obtient un facteur d’irréversibilité entre 0,5 et 0,75 pour le cuivre, sous vide et sous l’environnement oxygène, en accord avec des mesures récentes en microscopie à force atomique.La propagation de fissures est simulée en environnement inerte. Les fissures peuvent se propager à cause de l'irréversibilité des dislocations générées, liée à leurs interactions allant jusqu’à la création de jonctionsFatigue is one of the major damage mechanisms of metals. It is characterized by strong environmental effects and wide lifetime dispersions which must be better understood. Different face centred cubic metals, Al, Cu, Ni, and Ag are analyzed. The mechanical behaviour of surface steps naturally created by the glide of dislocations subjected to cyclic loading is examined using molecular dynamics simulations in vacuum and in air for Cu and Ni. An atomistic reconstruction phenomenon is observed at these surface steps which can induce strong irreversibility. Three different mechanisms of reconstruction are defined. Surface slip irreversibility under cyclic loading is analyzed. All surface steps are intrinsically irreversible under usual fatigue laboratory loading amplitude without the arrival of opposite sign dislocations on direct neighbor plane.With opposite sign dislocations on non direct neighbour planes, irreversibility cumulates cycle by cycle and a micro-notch is produced whose depth gradually increases.Oxygen environment affects the surface (first stage of oxidation) but does not lead to higher irreversibility as it has no major influence on the different mechanisms linked to surface relief evolution.A rough estimation of surface irreversibility is carried out for pure edge dislocations in persistent slip bands in so-called wavy materials. It gives an irreversibility fraction between 0.5 and 0.75 in copper in vacuum and in air, in agreement with recent atomic force microscopy measurements.Crack propagation mechanisms are simulated in inert environment. Cracks can propagate owing to the irreversibility of generated dislocations because of their mutual interactions up to the formation of dislocation junctions
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Zhao, Tianwen. "Fatigue of aluminum alloy 7075-T651 /." abstract and full text PDF (UNR users only), 2009. http://0-gateway.proquest.com.innopac.library.unr.edu/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:3342620.
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Thesis (Ph. D.)--University of Nevada, Reno, 2008."December, 2008." Includes bibliographical references (leaves 76-83). Library also has microfilm. Ann Arbor, Mich. : ProQuest Information and Learning Company, [2009]. 1 microfilm reel ; 35 mm. Online version available on the World Wide Web.
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Glass, Bradley Smyth. "Continuum mechanics approaches to the study of fracture and fatigue in metals." Faculty of Engineering, 2004. http://ro.uow.edu.au/theses/264.
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This thesis investigates continuum mechanics based means of metal failure assessment. A basic science approach was employed throughout the study to examine the fundamental relationships responsible for metal failure. The extension of previously existing continuum mechanics based theories to encompass a wider range of application was considered in this thesis. Research was conducted as two separate studies which examine specific aspects of the metal failure spectrum, namely failure due to monotonic loading, and fatigue failure due to cyclic loading. The failure due to monotonic loading research was conducted to examine the influence of hydrostatic stress on metal ductility. A fundamental relationship in the form of a monotonic failure criterion was proposed based on a relationship between equivalent plastic fracture strain and hydrostatic stress. An experimental program incorporating uniaxial tensile testing of notched specimens was conducted to examine the proposed relationship for the hydrostatic tensile stress range. Finite element analyses were produced to confirm the mechanical properties and obtain the stress-strain state present at specimen failure. A good correlation was established between the load-displacement results obtained from experiment and finite element analysis, providing confirmation of the stress-strain data. The stress-strain results confirmed the existence of a relationship between hydrostatic stress and ductility in the form of a monotonically decreasing value of equivalent plastic fracture strain with increasing hydrostatic tensile stress. The relationship determined was in accordance with the trend indicated by various researchers for the hydrostatic compressive stress range. The potential application of such a criterion to finite element methods was amply demonstrated from this research. The fatigue failure due to cyclic loading research examined the application of energy based methods to fatigue life characterisation. Based on the hypothesis that irreversible damage may be attributed entirely to plastic deformation, the application of the plastic strain energy approach to the entire fatigue life spectrum was pursued. For application to high cycle fatigue, a thermodynamic approach was developed to allow plastic strain energy determination beyond the range of application of conventional mechanical measurement. Thermodynamic models consisting of varying degrees of free surface contribution to heat dissipation were developed as possible representations of the high cycle fatigue damage process. An experimental program was conducted incorporating mechanical and thermodynamic means of measurement. Thermodynamic measurement was achieved via an experimental apparatus incorporating precision temperature measurement and thermal isolation at the specimen surface. Assuming an appropriate thermodynamic model, a finite difference analysis allowed a quantitative determination of plastic strain energy. Close agreement was indicated from comparison of the low cycle fatigue plastic strain energy results obtained from mechanical and thermodynamic measurement. A qualitative determination of plastic strain energy for high cycle fatigue was achieved, subject to confirmation of the thermodynamic model. The qualitative assessment verified the existence of measurable plastic strain energy during high cycle fatigue.
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Hansen, Robert C. "Thermal and mechanical fatigue of 6061 Al - P100 Gr metal matrix composite." Thesis, Monterey, California : Naval Postgraduate School, 1990. http://handle.dtic.mil/100.2/ADA238795.
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Thesis (M.S. in Mechanical Engineering)--Naval Postgraduate School, September 1990.Thesis Advisor(s): Dutta, Indranath ; Mitra, Shantanu. "September 1990." Description based on title screen as viewed on December 21, 2009. DTIC Identifier(s): Fatigue (mechanics), thermal fatigue, metal matrix composites, laminates, bending, ultimate strength, fiber reinforced composites, theses. Author(s) subject terms: Aluminum-graphite composite, bend fatigue, thermal fatigue. Includes bibliographical references (p. 64-65). Also available in print.
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Ralph, William Carter. "Assessment of hole drilling procedures on resulting fatigue lives." Thesis, Available online, Georgia Institute of Technology, 2004:, 2003. http://etd.gatech.edu/theses/available/etd-04072004-180146/unrestricted/ralph%5Fwilliam%5Fc%5F200312%5Fms.pdf.
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Morrissey, Ryan J. "Frequency and mean stress effects in high cycle fatigue of Ti-6A1-4V." Thesis, Georgia Institute of Technology, 1997. http://hdl.handle.net/1853/17095.
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Jin, Ohchang. "The characterization of small fatigue crack growth in PH13-8 Mo stainless steel." Diss., Georgia Institute of Technology, 2000. http://hdl.handle.net/1853/19633.
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Yoon, Kee Bong. "Characterization of creep fatigue crack growth behavior using C[superscript](t[superscript]) parameter." Diss., Georgia Institute of Technology, 1990. http://hdl.handle.net/1853/17523.
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Dallmeier, Johannes. "Experimental analysis and numerical fatigue modeling for magnesium sheet metals." Doctoral thesis, Technische Universitaet Bergakademie Freiberg Universitaetsbibliothek "Georgius Agricola", 2016. http://nbn-resolving.de/urn:nbn:de:bsz:105-qucosa-209124.
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The desire for energy and resource savings brings magnesium alloys as lightweight materials with high specific strength more and more into the focus. Most structural components are subjected to cyclic loading. In the course of computer aided product development, a numerical prediction of the fatigue life under these conditions must be provided. For this reason, the mechanical properties of the considered material must be examined in detail. Wrought magnesium semifinished products, e.g. magnesium sheet metals, typically reveal strong basal textures and thus, the mechanical behavior considerably differs from that of the well-established magnesium die castings. Magnesium sheet metals reveal a distinct difference in the tensile and compressive yield stress, leading to non-symmetric sigmoidal hysteresis loops within the elasto-plastic load range. These unusual hysteresis shapes are caused by cyclic twinning and detwinning. Furthermore, wrought magnesium alloys reveal pseudoelastic behavior, leading to nonlinear unloading curves. Another interesting effect is the formation of local twin bands during compressive loading. Nevertheless, only little information can be found on the numerical fatigue analysis of wrought magnesium alloys up to now.
The aim of this thesis is the investigation of the mechanical properties of wrought magnesium alloys and the development of an appropriate fatigue model. For this purpose, twin roll cast AM50 as well as AZ31B sheet metals and extruded ME21 sheet metals were used. Mechanical tests were carried out to present a comprehensive overview of the quasi-static and cyclic material behavior. The microstructure was captured on sheet metals before and after loading to evaluate the correlation between the microstructure, the texture, and the mechanical properties. Stress- and strain-controlled loading ratios and strain-controlled experiments with variable amplitudes were performed. Tests were carried out along and transverse to the manufacturing direction to consider the influence of the anisotropy. Special focus was given to sigmoidal hysteresis loops and their influence on the fatigue life. A detailed numerical description of hysteresis loops is necessary for numerical fatigue analyses. For this, a one-dimensional phenomenological model was developed for elasto-plastic strain-controlled constant and variable amplitude loading. This model consists of a three-component equation, which considers elastic, plastic, and pseudoelastic strain components. Considering different magnesium alloys, good correlation is reached between numerically and experimentally determined hysteresis loops by means of different constant and variable amplitude load-time functions.
For a numerical fatigue life analysis, an energy based fatigue parameter has been developed. It is denoted by “combined strain energy density per cycle” and consists of a summation of the plastic strain energy density per cycle and the 25 % weighted tensile elastic strain energy density per cycle. The weighting represents the material specific mean stress sensitivity. Applying the energy based fatigue parameter on modeled hysteresis loops, the fatigue life is predicted adequately for constant and variable amplitude loading including mean strain and mean stress effects. The combined strain energy density per cycle achieves significantly better results in comparison to conventional fatigue models such as the Smith-Watson-Topper model. The developed phenomenological model in combination with the combined strain energy density per cycle is able to carry out numerical fatigue life analyses on magnesium sheet metals.
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Conner, Brett P. (Brett Page) 1975. "Contact fatigue : life prediction and palliatives." Thesis, Massachusetts Institute of Technology, 2002. http://hdl.handle.net/1721.1/16866.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2002.Includes bibliographical references (p. 125-135).
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Fretting fatigue is defined as damage resulting from small magnitude (0.5-50 microns) displacement between contacting bodies where at least one of the bodies has an applied bulk stress. The applicability and limits of a fracture mechanics based life prediction is explored. Comparisons are made against highly controlled experiments and less controlled but more realistic experiments using a novel dovetail attachment fixture. Surface engineering approaches are examined from a mechanics perspective. Using a new tool, depth sensing indentation, the mechanical properties of an aluminum bronze coating are determined. Fretting fatigue experiments are performed on specimens coated with aluminum bronze and on specimens treated with low plasticity burnishing. Low plasticity burnishing is a new method of introducing beneficial compressive residual stresses without significant cold work at the surface. A mechanics based approach to the selection of palliatives is addressed.
by Brett P. Conner.
Ph.D.
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Ramesh, Aashish, and Gaurav Kalkur. "Fatigue life validation of aircraft materials." Thesis, Linköpings universitet, Mekanik och hållfasthetslära, 2020. http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-170893.
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Fatigue is one of the critical design aspects with immense significance where thefatigue life of a material can be stated as the number of cycles that a componentcan withstand under a particular type of loading without failure. The design processhas to include fatigue analysis in order to predict failure due to fatigue. This helpsin maintenance and servicing of a component reducing the chance of failure duringoperation of the component. Increased efficiency of predictive maintenance improvesthe life of the component.This thesis aims to study the relationship between the experimental, analytical andnumerical solutions of two high strength aluminium alloys and one steel alloy fortheir life in aircraft applications covering the effects of geometrical irregularities. Italso aims to answer convergence between the numerical and the analytical methodwhen compared with each other. The simulations are carried out for three materialsamong many used in aircraft and industrial applications (Al 7050-T7451, Al 7075-T6 and AISI 4340 Steel) for a pre-defined set of geometries. The stress field andthe stress concentration factor variations are also studied to identify their effects onfatigue life.The results from this work forms a strong background for the future research alongside SAAB or any other industries using these materials for their structures to findout the failure or predicting it accurately. Also, integral structures can be analysedin detail using this thesis as a base.
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Hedberg, Lundblad Louise, and Anna Lund. "Comparison of Fatigue Life Evaluation Methods." Thesis, KTH, Hållfasthetslära, 2021. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-298345.
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The aim of this thesis is to investigate a selected set of fatigue life calculation methods and evaluate if they are suitable for fatigue life estimation of truck components at Scania. Failure due to fatigue can be cause by road induced vibrations, which is an inevitable phenomenon trucks are exposed to. By estimating when and where these components will fail, they can be designed to reduce the amount of failure per vehicle. Three types of fatigue life calculation methods, namely equivalent stress methods, critical plane methods and spectral methods, have been evaluated. These are methods for calculating fatigue life in both the time domain and the frequency domain. The chosen calculation methods have been evaluated based on their sensitivity to input parameters, their accuracy on predicting fatigue life and their ability to find the critical areas where the components are most likely to fail. The methods have also been compared to a method already implemented at Scania. To evaluate the methods, two different components were used. The first component was designed to give a multiaxial stress state and the other was a real truck component where fatigue data had been collected from a shake rig test at Scania. It was found that all investigated methods were successful in finding critical areas where failure will occur. However, the resulting estimated fatigue life had a very low accuracy. To draw any conclusions about the accuracy of the fatigue life estimations, a model that better reflects the dynamics of the real truck component is needed. Therefore, the conclusion is that the chosen methods can be used for finding critical areas in a component but not to determine the absolute time to failure for the model used. However, the method already implemented at Scania was equally successful in finding the critical areas and it has a much shorter computational time than the methods in the time domain. Since it is already implemented and used, the Scania method is recommended for the purpose of finding the critical areas of a component. A sensitivity study was conducted in order to investigate the influence of a variation of material parameters on the fatigue life calculated with the different methods. This study showed that the SN-curve parameters are important for the resulting fatigue life of methods that consider the endurance limit, and, therefore, that the choice of SN-curve is important. Since the road induced vibrations in this study caused load signals where the majority of the cycles were found below the endurance limit, methods that account for the endurance limit have to be used for calculations on components experiencing similar conditions. Furthermore, it was found that the resulting stress signal from the FE-analysis using input data from the shake rig test was non-Gaussian, this makes the results from all the chosen frequency domain methods invalid. To use these methods, they need to be extended to consider non-Gaussian signals.Syftet med detta examensarbete är att undersöka ett antal utvalda metoder för utmattningsberäkning och utvärdera om dessa är lämpliga för att uppskatta livslängden på lastbilskomponenter hos Scania. Haveri på grund av utmattning kan orsakas av vibrationer från vägen, ett fenomen som påverkar komponenter på lastbilar. Genom att uppskatta när och var dessa komponenter går sönder kan de konstrueras för att minska antalet haverier. Olika typer av metoder för utmattningsberäkning i både tidsdomänen och frekvensdomänen har utvärderats. Dessa inkluderade ekvivalenta spännings-metoder, kritiska plan-metoder samt spektrala metoder. Metoderna har utvärderats med avseende på deras känslighet för variation i materialparametrar, hur den beräknade livslängden skiljer sig mot verkliga tester och hur bra de är på att hitta de kritiska områdena på en lastbilskomponent. Detta har även jämförts mot en beräkningsmetod som redan används på Scania. Två olika komponenter användes för att utvärdera metoderna. En av komponenterna var designad för att ge ett multiaxiellt spänningstillstånd och en var en riktig lastbilskomponent med data uppmätt från ett skaktest på Scania. Alla studerade metoder fann de kritiska områdena där utmattningsbrott riskerar att uppstå. Däremot visade det sig att beräkningsmetoderna inte lyckades estimera livslängder som låg i närheten av de som uppmättes under testet i skakriggen. En mer verklighetsnära modell vilken bättre motsvarar de dynamiska egenskaperna av systemet behövs för att kunna dra en slutsats om modellernas träffsäkerhet gällande estimeringen av livslängden. För ändamålet att hitta kritiska områden rekommenderas metoden som redan används hos Scania, eftersom denna var lika framgångsrik att hitta dessa, men gjorde det på en avsevärt kortare tid. Därutöver identifierades att spänningssignalen från FE-analysen, där indata från skakriggen användes, inte var gaussisk. Detta innebär att signalen inte uppfyller kraven för de spektrala metoderna och därmed att resultaten från beräkningarna på lastbilskomponenten inte går att använda för att dra några slutsatser. Känslighetsanalysen visade att de metoder som tar hänsyn till utmattningsgränsen är känsliga för ändringar i SN-parametrar. Detta beror på att många cykler, för det studerade lastfallet, låg nära utmattningsgränsen och att antalet cykler som ingick i beräkningarna därför påverkades stort av SN-parametrarna. Eftersom de vibrationer som uppstår då lastbilar framförs på vägar kan ge upphov till många cykler med amplituder nära utmattningsgränsen bör endast metoder som kan ta hänsyn till utmattningsgränsen användas vid dessa fall.
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Kirmani, Ghulam Ashraf-Ul-Harmain. "Single overload fatigue crack growth retardation : an implementation of plasticity induced closure." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1997. http://www.collectionscanada.ca/obj/s4/f2/dsk2/ftp02/NQ32752.pdf.
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Bian, Lichun, and 卞立春. "Fatigue crack growth analysis of metallic plates with an inclinedsemi-elliptical crack." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2001. http://hub.hku.hk/bib/B31242844.
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Cretegny, Laurent. "Use of atomic force microscopy for characterizing damage evolution during fatigue." Diss., Georgia Institute of Technology, 2000. http://hdl.handle.net/1853/20141.
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Kucz, Daniel Allen. "Analysis of distortion-induced fatigue cracking in a steel trapezoidal box girder bridge." Access to citation, abstract and download form provided by ProQuest Information and Learning Company; downloadable PDF file, 177 p, 2009. http://proquest.umi.com/pqdweb?did=1885755701&sid=9&Fmt=2&clientId=8331&RQT=309&VName=PQD.
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KJERENGTROEN, LIDVIN. "RELIABILITY ANALYSIS OF SERIES STRUCTURAL SYSTEMS (PROBABILITY, DESIGN, FATIGUE)." Diss., The University of Arizona, 1985. http://hdl.handle.net/10150/187909.
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Reliability analysis of series structural systems with emphasis on problems typical for metal fatigue is addressed. Specific goals include the following: (1) Given the distribution of strength of the components and the distribution of external loads on the system what is the probability of failure of the system? (2) Given the target safety index for the system, what would be the target safety index for the components? Exact solutions in the analysis of series structural systems only exists for some special problems. Some of these special problems are investigated. In particular some special cases of the problem of unequal element reliabilities are considered and some interesting observations are made. Numerical integration is in general required even when an exact solution exists. A correction or adjustment factor is developed for an important class of problems. This factor makes it possible to relate element and system probabilities of failure without numerical integration. However in most cases no exact solution to the structural series system problem exists. Approximations by for instance bounds on the probability of failure or Monte Carlo simulation has been the only way of approximating solutions. These two methods are generally not good approximation schemes since they are either too crude or too expensive. In this dissertation an approximation scheme for analysis of series systems where no exact solution exists is developed. The method only requires a simple numerical integration if the component safety index and the correlation coefficient between failure modes is known. Numerous examples are used to verify the method against known exact results and excellent estimates are obtained. Applications by practical examples is also given. In the appendix the problem of convergence of fatigue life distribution is also summarized.
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Dunn, DeRome Osmond. "Modeling cycle and time dependent creep/relaxation effects on fatigue lives of notched members." Diss., Virginia Polytechnic Institute and State University, 1991. http://hdl.handle.net/10919/54823.
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Mechanical cyclic variations in mean stress and strain amplitude is a well-known occurrence for metals even at room temperature. Many fatigue analysis procedures ignore these variations. Fatigue analysis which included both time and cycle dependent mechanical material behavior for metals at room temperature had not been previously studied except for the case of creep. An investigation studying transient mechanical effects on Ti-6Al-4V titanium and 7475-T651 Al alloys was done to determine how great an effect transients at room temperature would have on fatigue life under cyclic conditions. The mechanical material response was modeled using viscoplasticity constitutive laws and Neuber’s rule eliminating the need for finite element modeling of uniaxially loaded notched members. However, the Nenber’s modeling may be used with any material constitutive law. The procedures for fatigue damage used cycle counting to compute strain amplitude and mean stress. Since a large amount of fatigue data is reported as strain-life curves, the fatigue analysis was developed using this fatigue data although it did not include transients. If favorable results are obtained, development of modeling and testing to include transients in strain-life fatigue data could be avoided, and the existing fatigue data base utilized.
Experimental work was undertaken and nonlinear optimization techniques used to compute model constants for the two alloys. However, small amounts of rate dependence was found for cyclic strain control testing. The viscoplasticity models became stiff when rate dependence was low causing numerical problems, and model constants for the viscoplastic constitutive law could not be determined since convergence was not achieved. Also, only small amounts of transient static stress relaxation was observed for extended hold periods.
Finally, experimental verification was done for the local surface stresses in a notched member under load using advanced x-ray stress equipment. Measurements during brief pauses were made over a cycle. From the x-ray results, an anomalous surface behavior was observed. The surface yielded before the bulk material with the lower surface yielding seeming to be time dependent in nature. Since rolled plates of the alloys were used, texture was measured and studied in the form of pole figures, and extreme texture was found for both alloys. However. successful x-ray measurements were made for the alloys studied even though assuming linear d-spacing versus sin²ψ. Finally. x-ray measurements for a cycled notched member, exhibited relaxation of mean stress and not relaxation of residual stress.Ph. D.
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Ding, Fei. "A general approach for fatigue life prediction /." abstract and full text PDF (free order & download UNR users only), 2006. http://0-gateway.proquest.com.innopac.library.unr.edu/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:3250682.
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Thesis (Ph. D.)--University of Nevada, Reno, 2006."December, 2006." Includes bibliographical references (leaves 77-94). Online version available on the World Wide Web. Library also has microfilm. Ann Arbor, Mich. : ProQuest Information and Learning Company, [2006]. 1 microfilm reel ; 35 mm.
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Papadopoulos, Ioannis. "Predicting the fatigue life of elastomer components." Thesis, Queen Mary, University of London, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.428590.
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Critz, David Karl. "Computer aided fatigue life prediction for polymers." Thesis, Massachusetts Institute of Technology, 1997. http://hdl.handle.net/1721.1/10489.
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Kihm, Frederic. "Fatigue life for non-Gaussian random excitations." Thesis, University of Southampton, 2017. https://eprints.soton.ac.uk/425929/.
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This thesis presents several developments in analysing the fatigue life of mechanical components subjected to various kinds of random excitations and the subsequent mechanical vibration. Typically components must be designed in such a way that they can withstand the effects of the exposure to environmental conditions without being damaged. Their design must be verified using laboratory testing or by Finite Element (FE) calculations. Often, design and testing are performed on the basis of specifications taken from internal, national or international standards, with the implicit assumption that if the equipment survived the particular environment it would also survive the vibrations it will see in service. Previous work in the area has developed computer-based models to predict the fatigue damage witnessed by components under random loads, but most of these are limited to only stationary Gaussian random excitations. A few have concentrated on non-Gaussian responses, not excitation, resulting from some non-linearities in the structure being activated. This thesis describes the development of original statistical analysis methods with the ability to determine extreme responses and fatigue life estimates for linear structures when subjected to non-Gaussian random excitations. The emphasis is mainly on two sources of non-Gaussian random excitations namely clipped random excitations and random excitations with a high kurtosis value. Fatigue damage from specific sine-on-random excitations is also studied. In all cases, theoretical formulations were derived to obtain fatigue life estimates without the need for long time domain realisations. Such a statistical approach is particularly suited for simulating the fatigue damage induced during a random test on a shaker system. One of the main benefits of being able to assess the fatigue life in the case of a structure subjected to a leptokurtic random excitation is to create an accelerated test definition. The idea is to associate a specific kurtosis value to a given Power Spectral Density (PSD) in order to reduce the exposure duration, while encompassing the same fatigue damage potential as the original stationary and Gaussian random test. In practice, the engineer will be able to simulate the effects of the kurtosis control capability of some commercial vibration control systems on the fatigue damage experienced by in the device under test. This process will be achieved using a FE-based fatigue analysis tool, where the user specifies the excitation PSD, the kurtosis value and an FE results file representing the frequency response function linking the excitation and the stress response at each node or element of the FE model of the test article. The stress response PSD with the associated response kurtosis are obtained and the statistical rainflow histogram is extracted. Fatigue life estimates are then derived by associating the statistical rainflow histogram with the material fatigue curve. The theoretical formulations derived are applied to examples coming from numerical simulations. The estimate of the probability density functions obtained for the response stress and fatigue life correlate well with the results obtained from time domain simulated data, showing the robustness and the accuracy of the theoretical expressions.
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Miller, Mark. "Fatigue life prediction of nickel base superalloys." Thesis, University of Southampton, 2007. https://eprints.soton.ac.uk/68693/.
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Neural networks have been used extensively in material science with varying success. It has been demonstrated that they can be very effective at predicting mechanical properties such as yield strength and ultimate tensile strength. These networks require large amounts of input data in order to learn the correct data trends. A neural network modelling process has been developed which includes data collection methodology and subsequent filtering techniques in conjunction with training of a neural network model. It has been shown that by using certain techniques to ‘improve’ the input data a network will not only fit seen and unseen Ultimate Tensile Strength (UTS) and Yield Strength (YS) data but correctly predict trends consistent with metallurgical understanding. Using the methods developed with the UTS and YS models, a Low Cycle Fatigue (LCF) life model has been developed with promising initial results. Crack initiation at high temperatures has been studied in CMSX4 in both air and vacuum environments, to elucidate the effect of oxidation on the notch fatigue initiation process. In air, crack initiation occurred at sub-surface interdendritic pores in all cases. The sub-surface crack grows initially under vacuum conditions, before breaking out to the top surface. Lifetime is then dependent on initiating pore size and distance from the notch root surface. In vacuum conditions, crack initiation has been observed more consistently from surface or close-to-surface pores - indicating that surface oxidation is in-filling/”healing” surface pores or providing significant local stress transfer to shift initiation to sub-surface pores. Complementary work has been carried out using PWA 1484 and Rene N5. Extensive data has been collected on initiating pores for all 3 alloys. A model has been developed to predict fatigue life based upon geometrical information from the initiating pores. A Paris law approach is used in conjunction with long crack propagation data. The model shows a good fit with experimental data and further improvements have been recommended in order to increase the capability of the model.
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Senne, Jolyn Louise. "Fatigue Life of Hybrid FRP Composite Beams." Thesis, Virginia Tech, 2000. http://hdl.handle.net/10919/33982.
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As fiber reinforced polymer (FRP) structures find application in highway bridge structures, methodologies for describing their long-term performance under service loading will be a necessity for designers. The designer of FRP bridge structures is faced with out-of-plane damage and delamination at ply interfaces. The damage most often occurs between hybrid plys and dominates the life time response of a thick section FRP structure. The focus of this work is on the performance of the 20.3 cm (8 in) pultruded, hybrid double web I-beam structural shape. Experimental four-point bend fatigue results indicate that overall stiffness reduction of the structure is controlled by the degradation of the tensile flange. The loss of stiffness in the tensile flange results in the redistribution of the stresses and strains, until the initiation of failure by delamination in the compression flange. These observations become the basis of the assumptions used to develop an analytical life prediction model. In the model, the tensile flange stiffness is reduced based on coupon test data, and is used to determine the overall strength reduction of the beam in accordance the residual strength life prediction methodology. Delamination initiation is based on the out-of-plane stress sz at the free edge. The stresses are calculated using two different approximations, the Primitive Delamination Model and the Minimization of Complementary Energy. The model successfully describes the onset of delamination prior to fiber failure and suggests that out-of-plane failure controls the life of the structure.Master of Science
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Wannenburg, Johann. "A study of fatigue loading on automotive and transport structures." Thesis, University of Pretoria, 2007. http://upetd.up.ac.za/thesis/available/etd-04032008-100638/.
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Pape, John Andrew. "Design and implementation of an apparatus to investigate the fretting fatigue of PH 13-8 Mo stainless steel." Thesis, Georgia Institute of Technology, 1997. http://hdl.handle.net/1853/18394.
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Li, Zongbao. "Crack detection in annular components by ultrasonic guided waves." Diss., Georgia Institute of Technology, 1998. http://hdl.handle.net/1853/15920.
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