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Tesi sul tema "Creep mechanism"

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Autore: Grafiati
Pubblicato: 4 giugno 2021
Ultima modifica: 19 febbraio 2022

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1

Dok, Atitkagna. "Tertiary Creep Behavior of Landslides Induced by Extreme Rainfall: Mechanism and Application". 京都大学 (Kyoto University), 2013. http://hdl.handle.net/2433/175207.

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2

Zheng, Xiao-Qin Materials Science &amp Engineering Faculty of Science UNSW. "Packing of particles during softening and melting process". Awarded by:University of New South Wales. School of Materials Science & Engineering, 2007. http://handle.unsw.edu.au/1959.4/31517.

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Abstract (sommario):
Softening deformation of iron ore in the form of sinter, pellet, and lump ore in the cohesive zone of an ironmaking blast furnace is an important phenomenon that has a significant effect on gas permeability and consequently blast furnace production efficiency. The macroscopic softening deformation behavior of the bed and the microscopic deformation behavior of the individual particles in the packed bed are investigated in this study using wax balls to simulate the fused layer behavior of the cohesive zone. The effects of softening temperature, load pressure, and bed composition (mono - single melting particles, including pure or blend particles vs binary ??? two different melting point particles) on softening deformation are examined. The principal findings of this study are: 1. At low softening temperatures, an increase in load pressure increases the deformation rate almost linearly. 2. At higher softening temperatures, an increase in load pressure dramatically increases the deformation rate, and after a certain time there is no more significant change in deformation rate. 3. The bed deformation rate of a mono bed is much greater than that of a binary one. 4. In a binary system, the softening deformation rate increases almost proportionally with the increase in the amount of lower melting point wax balls. 5. In a mono system with blend particles, the content of the lower melting point material has a more significant effect on overall bed deformation than the higher melting point one. 6. The macro softening deformation of the bed behaves the theory of creep deformation. 7. A mathematical model for predicting bed porosity change due to softening deformation based on creep deformation theory has been developed. 8. Increase in load pressure also reduces the peak contact face number of the distribution curves, and this is more prominent with higher porosity values. 9. The contribution of contact face number to bed porosity reduction is more pronounced in a mono system than in a binary system. 10. The porosity reduction in a binary bed is more due to the contact face area increase, presumably of the lower melting point particles. 11. The mono system has a single peak contact face number distribution pattern while the binary system exhibits a bimodal distribution pattern once the higher melting point material starts to deform. 12. In a binary system, an increase in deformation condition severity tends to reduce the contact face number of the higher melting point material without having to increase the contact face number of the lower melting point material accordingly to achieve a given porosity.
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3

Mirmasoudi, Sara. "High Temperature Transient Creep Analysis of Metals". Wright State University / OhioLINK, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=wright1452693927.

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4

Niemeier, William. "Design and Testing of a Linear Compliant Mechanism with Adjustable Force Output". Scholar Commons, 2018. http://scholarcommons.usf.edu/etd/7203.

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Abstract (sommario):
This thesis presents a novel compliant mechanism with adjustable force output. The force comes from the bending of a rectangular cross section beam within the mechanism. By rotating this beam with a stepper motor, the force output of the mechanism changes. A model was made to simulate this mechanism, and a prototype was made based off of this data. A test apparatus was constructed around this mechanism, and a series of tests were performed. These tests adjusted parameters such as beam rotation speed and weight in order to characterize the system. Adjustments were made based on this information and the mechanism was refined. The results suggest the following. The speed has a negligible effect on the behavior of the system, while the weight, length of top link r3, and position of bottom stop have a significant effect. Also, there is a large, consistent amount of hysteresis in the system. This is likely caused by the beam storing torsion or friction from the slider.
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5

Yang, Xin. "The development of creep damage constitutive equations for high chromium steel based on the mechanism of cavitation damage". Thesis, University of Huddersfield, 2018. http://eprints.hud.ac.uk/id/eprint/34682/.

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Due to increasing electrical energy power supply, thermal efficiency and the desire to reduce CO2 emissions, creep-resistant high chromium steels are becoming widely developed and applied for components of electric power plants under high pressure at high temperature. The limited design factors such as strain histories, damage evolution and lifetime are important factors when creating the components of a power plant. Obtaining a long-term (100,000h, over 11 years) creep data is time consuming and costly, hence long-term creep data is very limited, and the extrapolation using the conventional empirical methods may not be reliable due to limited data (Chen et al., 2011; Shrestha et al., 2013; Ghosh et al., 2013). To design against failures, creep damage constitutive equations have the advantage of traceability from the physics based constitutive equation to the fundamental microstructural and damage behaviour. Thus, creep modelling constitutive equations for materials of the critical components of, for example, power plants and other safety critical systems, are a key issue in the research of materials. In the past decade, a range of creep damage constitutive equations have been developed to describe creep damage behaviour for high chromium steel, however, some models are only based on creep deformation (creep microstructural degradation) and are not really concerned with cavitation damage, which is a dominant factor in creep rupture; most of them are proposed based on high stress levels of high chromium steel and extended to a low stress level, the modelling results fail to explore the phenomenon of stress breakdown. Besides, the cavitation damage equations were developed on experimental data of pure metal and super alloy, the fundamental nature of the evolution of creep cavitation damage is still unclear and necessary to solve for high chromium steel. Thus, the aim of this research project was to develop a novel creep damage constitutive equation for high chromium steel based on the mechanism of cavitation damage under a wide range of stress levels. This research made contributions to the specialised knowledge on the following three aspects. Firstly, a modified hyperbolic sine law, which describes the relationship between minimum creep strain and applied stress, was applied to high chromium steel. Through which we found that the modelling results fitted better with published experimental data by NIMS in comparison with conventional functions such as power law, hyperbolic sine law and linear power law. The other two aspects of innovation in the development of creep damage constitutive equation had been achieved. Secondly, using the quantitatively analysed results of the cavity size distribution along grain boundary by the superior 3D technology of X-ray micro-tomography, a novel creep cavitation damage equation was developed and applied to describe the evolution of cavity along grain boundary in the creep process for high chromium steel. Thirdly, the novel creep damage constitutive equations, that coupled appropriate creep deformation mechanisms with the new cavitation damage equation, were successfully applied to high chromium steel under a wide range of stress level according to comparisons made between the modelling results of novel creep damage constitutive equations, classic uniaxial KRH constitutive equations and experimental data for P91 steel at 600°C and also applied to P91 steel at 625°C.
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6

Lv, Duchao. "A Multi-Scale Simulation Approach to Deformation Mechanism Prediction in Superalloys". The Ohio State University, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=osu1469009668.

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7

Ahmed, Sheikh Saad. "Development of Innovative Load Transfer Mechanism to Reduce Hurricane-Induced Failures in New and Existing Residential Construction". FIU Digital Commons, 2010. http://digitalcommons.fiu.edu/etd/157.

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Implicit in current design practice of minimum uplift capacity, is the assumption that the connection's capacity is proportional to the number of fasteners per connection joint. This assumption may overestimate the capacity of joints by a factor of two or more and maybe the cause of connection failures in extreme wind events. The current research serves to modify the current practice by proposing a realistic relationship between the number of fasteners and the capacity of the joint. The research is also aimed at further development of non-intrusive continuous load path (CLP) connection system using Glass Fiber Reinforced Polymer (GFRP) and epoxy. Suitable designs were developed for stud to top plate and gable end connections and tests were performed to evaluate the ultimate load, creep and fatigue behavior. The objective was to determine the performance of the connections under simulated sustained hurricane conditions. The performance of the new connections was satisfactory.
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8

Srivastava, Ankit. "Mechanics and Mechanisms of Creep and Ductile Fracture". Thesis, University of North Texas, 2013. https://digital.library.unt.edu/ark:/67531/metadc283799/.

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The main aim of this dissertation is to relate measurable and hopefully controllable features of a material's microstructure to its observed failure modes to provide a basis for designing better materials. The understanding of creep in materials used at high temperatures is of prime engineering importance. Single crystal Ni-based superalloys used in turbine aerofoils of jet engines are exposed to long dwell times at very high temperatures. In contrast to current theories, creep tests on Ni-based superalloy specimens have shown size dependent creep response termed as the thickness debit effect. To investigate the mechanism of the thickness debit effect, isothermal creep tests were performed on uncoated Ni-based single crystal superalloy sheet specimens with two thicknesses and under two test conditions: a low temperature high stress condition and a high temperature low stress condition. At the high temperature, surface oxidation induced microstructural changes near the free surface forming a layered microstructure. Finite element calculations showed that this layered microstructure gave rise to local changes in the stress state. The specimens also contained nonuniform distribution of initial voids formed during the solidification and homogenization processes. The experiments showed that porosity evolution could play a significant role in the thickness debit effect. This motivated a basic mechanics study of porosity evolution in single crystals subjected to creep for a range of stress states. The study was performed using three-dimensional finite deformation finite element analysis of unit cells containing a single initially spherical void in a single crystal matrix. The materials are characterized by a rate-dependent crystal plasticity constitutive relation accounting for both primary and secondary creep. The effect of initial void spacing and creep exponent was also explored. Based on the experimental observations and results of finite element calculations a quantitative mechanistic model is proposed that can account for both bulk and surface damage effects and assess their relative roles in the observed thickness debit effect. Another set of calculations aim at relating the crack growth resistance and fracture surface morphology to material microstructure for ductile structural metals. The process that governs the ductile fracture of structural materials at room temperature is one of nucleation, growth and coalescence of micron scale voids, and involves large plastic deformations. Experimental studies have shown that fracture surfaces in a wide variety of materials and under a wide variety of loading conditions have remarkable scaling properties. For thirty years, the hope to relate the statistical characterization of fracture surfaces to a measure of a material's crack growth resistance has remained unfulfilled. Only recently has the capability been developed to calculate sufficient amounts of three dimensional ductile crack growth in heterogeneous microstructures to obtain a statistical characterization of the predicted fracture surfaces. This development has enabled the exploration of the relation of both fracture toughness and fracture surface statistics to material properties and microstructure when the fracture mechanism is one of void nucleation, growth and coalescence. The relation of both toughness and the statistical properties of fracture surfaces in calculations of heterogeneous microstructures to various microstructural features is discussed and a remarkable correlation between fracture surface roughness and fracture toughness is shown for the first time.
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9

Zhang, Bochun. "Failure Mechanism Analysis and Life Prediction Based on Atmospheric Plasma-Sprayed and Electron Beam-Physical Vapor Deposition Thermal Barrier Coatings". Thesis, Université d'Ottawa / University of Ottawa, 2017. http://hdl.handle.net/10393/35709.

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Using experimentally measured temperature-process-dependent model parameters, the failure analysis and life prediction were conducted for Atmospheric Plasma Sprayed Thermal Barrier Coatings (APS-TBCs) and electron beam physical vapor deposition thermal barrier coatings (EB-PVD TBCs) with Pt-modified -NiAl bond coats deposited on Ni-base single crystal superalloys. For APS-TBC system, a residual stress model for the top coat of APS-TBC was proposed and then applied to life prediction. The capability of the life model was demonstrated using temperature-dependent model parameters. Using existing life data, a comparison of fitting approaches of life model parameters was performed. The role of the residual stresses distributed at each individual coating layer was explored and their interplay on the coating’s delamination was analyzed. For EB-PVD TBCs, based on failure mechanism analysis, two newly analytical stress models from the valley position of top coat and ridge of bond coat were proposed describing stress levels generated as consequence of the coefficient of thermal expansion (CTE) mismatch between each layers. The thermal stress within TGO was evaluated based on composite material theory, where effective parameters were calculated. The lifetime prediction of EB-PVD TBCs was conducted given that the failure analysis and life model were applied to two failure modes A and B identified experimentally for thermal cyclic process. The global wavelength related to interface rumpling and its radius curvature were identified as essential parameters in life evaluation, and the life results for failure mode A were verified by existing burner rig test data. For failure mode B, the crack growth rate along the topcoat/TGO interface was calculated using the experimentally measured average interfacial fracture toughness.
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10

Gieseke, Brian G. "Mechanics and mechanisms of creep-fatigue crack growth in Cu-1 wt% Sb". Diss., Georgia Institute of Technology, 1990. http://hdl.handle.net/1853/19982.

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11

Thornton, Gail Marilyn. "Creep behaviour and creep mechanisms of normal and healing ligaments". Thesis, National Library of Canada = Bibliothèque nationale du Canada, 2000. http://www.collectionscanada.ca/obj/s4/f2/dsk1/tape3/PQDD_0023/NQ49542.pdf.

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12

Millard, Joseph William. "Pressure Dependence Of The Strength Of Magnesite Deforming By Low Temperature Plasticity, Diffusion Creep, Or Dislocation Creep". University of Akron / OhioLINK, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=akron1526913343559104.

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13

White, Roger John. "Environmental creep mechanisms in glass/polyester composites". Thesis, University of Bath, 1985. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.353403.

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Abstract (sommario):
A previous study, looking at the creep-rupture behaviour of mixed reinforcement GRP when immersed in water, had discovered that low loads, behaviour became temperature sensitive. Since the recorded time to failure of a sample was reduced at elevated temperatures, from that predicted by a linear extrapolation of the short term creep-rupture results, this deviation caused problems in the accurate prediction of long-term design stresses. In order to improve the accuracy of long term design predictions, it was decided to study the mechanisms of creep in GRP that initiates time dependent failure. From this, it was hoped that accurate design criteria suitable for predicting GRP response over a 30 year design life from short term creep tests, could be developed. This thesis reports the results obtained from such a study. A series of creep tests were performed on mixed reinforcement GRP samples at several stress levels, both in air, and in room temperature distilled water, using a microcomputer based data collection system. In conjunction with this work, damage development in samples, due to combinations of water uptake and creep loading, was followed, using both scanning electron, and optical, microscopy. Moisture uptake measurements were undertaken under a series of load/temperature regimes, and fibre/matrix debonding followed using photographic techniques. In this way, water absorption, both in terms of uptake rate, and location within a sample, could be characterised. Tensile tests were also performed to determine the standard mechanical properties of the mixed reinforcement GRP used. It was found that a critical damage state was created at loads in excess of 50% of ultimate, but not below. This took the form of between 2 and 8 neighbouring filament breaks in the longitudinal woven rovings at weave crossover points, producing microcracks in the reinforcement. The creation of this multifilament fracture damage during primary creep, was considered to be necessary for time dependent failure to occur in air. Secondary greep strain was found to increase in discrete steps, both in air and water. This was attributed to the formation of transverse grasks in the longitudinal woven rovings, propogating from the above critical damage. In water, diffusion was found to be non-Fickian. Moisture uptake increased with increases in applied load and temperature. Water was seen to accumulate at weave cross-over points when immersed under load. This led to stress-enhanced fibre corrosion in these regions, weakening the reinforcement, and reducing the failure time from that expected at the same load level in air. The localised nature of moisture degradation was thought to result in the formation of critical fibre damage at loads below 50% of ultimate, when immersed in water. Two design criteria based on the observed creep mechanisms, have been developed for GRP that predict response when loaded in either air, or water. Both predict the existence of creep-rupture limits at low loads.
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14

Resen, Abdul-Amir S. "Biaxial creep of plastics". Thesis, University of Manchester, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.256753.

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15

Peltier, Jon Michael. "Creep rupture mechanisms in notched specimens of Rene 95". Thesis, Massachusetts Institute of Technology, 1987. http://hdl.handle.net/1721.1/120272.

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16

Morrow, Benjamin M. "Characterization and Modeling of Creep Mechanisms in Zircaloy-4". The Ohio State University, 2011. http://rave.ohiolink.edu/etdc/view?acc_num=osu1307372079.

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17

Stracey, Muhammad Ghalib. "Continuum Damage Mechanics (CDM) modelling of dislocation creep in 9-12% Cr creep resistant steels". Master's thesis, University of Cape Town, 2016. http://hdl.handle.net/11427/22994.

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The generation of electricity to meet an ever-growing demand has become a defining characteristic of the modern world for both developed and developing nations alike. This, coupled with the intensifying concern with pollution and its effects on the environment has put immense pressure on how quickly and efficiently power is produced. Being the most prevalent source of electricity generation, coal fired power plants have been subject to increasing scrutiny and study in an effort to improve the efficiency at which they operate. Hence, coal fired power plants are being run at increased temperatures and pressures such as those observed in Super-critical and Ultra-super-critical plants. This has by extension put excessive demand on materials used in these plants specifically within the boiler and superheater pipe sections where the most extreme thermodynamic conditions are experienced. The most commonly used materials for these applications are in the family of ferritic/martensitic 9-12% Cr steels chosen for their superior material properties especially during long-term exposure as coal fired power plants typically operate for over 20 years before being decommissioned. One of the lesser understood aspects of 9-12%Cr steels is with regard to their long-term material properties specifically that of creep degradation and deformation. This has been partially due to the reliance of creep life predictions in the past being based on accelerated creep testing and empirically based modelling. With the relatively recent revelations of empirically based modelling shown to be inaccurate when extrapolated to the long-term, a need has been identified amongst researchers to develop more accurate models based on physical relationships and material microstructure. Moreover, the insight obtained from modern experimental techniques and technologies as well as ever-expanding computing capabilities provide an opportunity to produce microstructurally based models with a high degree of complexity. Thus motivated, the focus of this dissertation was to develop a physically based dislocation creep model using the Continuum Damage Mechanics (CDM) approach. A dislocation CDM model was developed and implemented in the current work for uniaxial creep loading using the numerical modelling software Matlabᵀᴹ. The CDM approach was built upon fundamental dislocation theory as well as other microstructural considerations pertaining to dislocation creep including subgrain coarsening, M₂₃C₆ precipitate coarsening and stress redistribution. The CDM model was found to require calibration in order to be applied to specific 9- 12% Cr steels which was implemented using a parameter optimisation routine. The results obtained were compared with experimentally obtained, long-term creep-time and microstructural data for the 11% Cr steel CB8 and the 9% Cr steel P92. The CDM creep-time predictions were found to vary in accuracy depending upon the experimental data against which the model was calibrated. Upon further investigation, it was hypothesised that the discrepancy observed was due to the formation of the Modified Z-phase in some of the long term creep data but not in others which was based primarily on the differing creep exposure times of the various samples. The CDM creep-time predictions for P92 were found to be accurate when compared with experimental results regardless of creep exposure times. The apparent difference in the approximation of the creep deformation for the two steels was concluded as being due to the formation of the Modified Z-phase in CB8 but not in P92 as Modified Zphase formation is intrinsically linked with the Cr content of the steel.
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18

Rangarajan, Shalini. "Long-term creep of encased polymer liners". Morgantown, W. Va. : [West Virginia University Libraries], 2002. https://etd.wvu.edu/etd/controller.jsp?moduleName=documentdata&jsp%5FetdId=2582.

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Thesis (M.S.)--West Virginia University, 2002.
Title from document title page. Document formatted into pages; contains xiv, 117 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 111-113).
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19

Prakash, Om. "Creep deformation of metal analogue materials". Thesis, University of Cambridge, 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.239561.

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20

Jones, Kimberly A. "The creep behavior of aluminum alloy 8009". Thesis, Georgia Institute of Technology, 1993. http://hdl.handle.net/1853/19630.

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21

Sorsh, Frans. "Assessment of creep damage in Fe-Ni-Cr alloys". Thesis, KTH, Hållfasthetslära (Avd.), 2019. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-259516.

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It is only a matter of time before components working in high temperature environments fail due to creep. Design for creep is therefore of vital importance to maximize the lifetime of components and reduce costs that may arise from maintenance and replacement of components. This thesis aims to use metallographical methods and finite element modeling to assess creep damage in a hydrogen reformer. The decommissioned reformer, made of Fe-Ni-Cr alloys, was investigated thoroughly via replica testing, hardness measurements as well as finite element modeling of the welds. An extended literature review was performed to gain a better understanding of creep in Fe-Ni-Cr alloys, welds and the modeling of creep generally. The microstructures of samples from the reformer were analyzed and mapped out in terms of creep damage which were then compared to a creep analysis of the welds with a simulation time of 100 000 h. The FE results yielded high stresses and creep strains with a maximum of 0.95% in the boundaries of the welds which gave realistic representations of strain distributions when compared to the metallographical results. Hardness measurement indicated that a relatively narrow zone with altered mechanical properties is present along the weld boundaries. This area, called the heat affected zone, was found to be most affected by creep with microcracks reaching maximum lengths of 2 mm. The creep strains obtained from simulation did not fall in line with the observed creep damage, it was thus concluded that a material model that considers tertiary creep would yield a more realistic representation in FEM for Fe-Ni-Cr alloys.
Det är bara en tidsfråga innan komponenter som arbetar i högtemperaturförhållanden misslyckas pga kryp. Att designa med avseende på kryp är därmed viktigt för att maximera livslängden och reducera kostnader som kan komma från underhåll och från utbyte av komponenter. I detta examensarbete används metallografiska metoder och finita element modellering för att bedöma krypskador i en vätgasreformer. Vätgasreformern, som är tillverkad i Fe-Ni-Cr legeringar, togs ur drift och undersöktes metallografiskt med hjälp av replikprovning och hårdhetsprovning samt med finita element modellering av svetsar. En djupgående litteraturstudie utfördes för att öka förståelsen av kryp, specifikt i Fe-Ni-Cr legeringar och även modellering av kryp generellt. Mikrostrukturen från utvalda provbitar undersöktes och krypskador kartläggdes för att sedan jämföra med en krypanalys av svetsarna där 100 000 timmars kryp simulerades. Krypanalysen resulterade i höga spänningar och kryptöjningar upp till maximalt 0.95% i svetsgränserna vilket gav realistiska representationer av töjningsdistributionen jämfört med metallografiska resultaten. Hårdhetsmätningar indikerade att ett smalt område med förändrade mekaniska egenskaper fanns utmed svetsgränserna. Detta område, den värmepåverkade zonen, var mest utsatt för krypskador med mikrosprickor uppemåt 2 mm i längd. Kryptöjningar som erhölls från simuleringen gav inte en tillräckligt bra uppskattning av kryptöjningarna – de krypskador som observerades motsvarar lokalt högre töjning. Slutsatsen är att en materialmodell som tar hänsyn till tertiärkryp skulle i det här fallet ge en mer realistisk representation i FEM för Fe-Ni-Cr legeringar.
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22

Huleux, Vincent. "Comparaison des caractéristiques microstructurales et des propriétés en fluage de superalliages monocristallins à base de nickel de différentes générations". Thesis, Paris Sciences et Lettres (ComUE), 2019. http://www.theses.fr/2019PSLEM027.

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Pour améliorer le rendement des moteurs, les parties les plus chaudes, notamment les aubes monocristallines de la turbine haute pression, réalisées en superalliages à base de nickel, doivent faire face à une amplification des sollicitations thermomécaniques. L’objectif de l’étude est de compléter les connaissances sur le rôle des différents éléments d’alliage sur la stabilité microstructurale et les propriétés en fluage, et d’en approfondir la compréhension. Sur la base de ces éléments de réflexion, des critères de composition chimique peuvent être établis pour répondre aux futures exigences de résistance aux contraintes thermomécaniques en service.Cinq superalliages industrialisés de compositions chimiques différentes appartenant aux trois premières générations, ont été étudiés : AM1, René N5, CMSX-4, CMSX-4 Plus et CMSX-10K. Les températures des essais de fluage ont été choisies pour couvrir toute la gamme d’utilisation de ces alliages. Un comparatif détaillé, basé sur les évolutions microstructurales et les mécanismes de déformation se produisant au cours des essais aux différentes températures et sous différentes contraintes, a été réalisé.L’étude s’est plus particulièrement focalisée sur l’analyse des spécificités du CMSX-4 Plus, alliage de troisième génération récemment développé, dont les propriétés en fluage à haute température sont remarquables, et qui pourrait répondre aux besoins des motoristes.On s’est ainsi attaché à :− l’optimisation des paramètres de traitement thermiques de l’alliage ;− l’analyse approfondie de l’effet de la désorientation primaire du monocristal sur sa résistance en fluage à basse température et sous contrainte élevée ; − l’analyse de l’évolution de la mise en radeaux des précipités γ’, de la formation d’un réseau de dislocations ainsi que de la précipitation de phases TCP en fluage à haute température
Single-crystal nickel-based superalloys have been developed in order to increase the resistance of aeronautic turbine blades to high temperatures and stresses to improve their efficiency. The objective of this study is to increase knowledges about the role of the various alloying elements on microstructural stability and creep properties. Based on these considerations, chemical composition criteria can be established to meet to the next requirements for thermomechanical resistance in service.Five industrialized superalloys with different chemical compositions (from the first to the third generation) were tested: AM1, René N5, CMSX-4, CMSX-4 Plus et CMSX-10K. Temperatures of the creep tests have been chosen to cover the entire range of service temperature of these alloys. A detailed comparison has been performed, based on the microstructural evolutions and the deformation mechanisms occurring during these tests at various temperatures and under various stresses.The study focused on the analysis of the CMSX-4 Plus, a third-generation alloy which has been recently developed, and which shows remarkable high-temperature creep properties.We focused on:- the optimization of the heat treatment parameters for this alloy;- the analysis of the effect of the primary crystal misorientation on its creep resistance at low temperature and high stress;- the analysis of the evolution of γ' precipitates rafting, the formation of the dislocation network as well as the precipitation of TCP phases during creep at high temperature
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23

Boal, Joseph T. "Biaxial creep and recovery of engineering plastics". Thesis, University of Manchester, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.277699.

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24

Brady, Edward William. "The creep of type 310N stainless steel". Thesis, University of Liverpool, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.284258.

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25

Hamilton, Benjamin Carter. "Creep crack growth behavior of aluminum alloy 2519-T87". Thesis, Georgia Institute of Technology, 1994. http://hdl.handle.net/1853/20500.

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26

Cheikh-Ali, Askar D. "A study of deformation mechanisms of creep and superplasticity in zinc /". Thesis, McGill University, 2004. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=85060.

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Different types of zinc bicrystals were tested under creep conditions. Observations of grain boundary sliding along boundaries with different misorientations were made. It was established that Sigma = 9 coincidence boundary slides faster than boundaries of general type. Grain boundary sliding along coincidence boundary was activated at higher stress and only in the presence of intragranular slip. Sliding along coincidence and near-coincidence boundaries is accompanied by regular boundary migration whereas sliding along general boundary is not. The sliding-migration behavior of coincidence boundaries was explained in terms of DSC-dislocations associated with steps. Sliding along general boundaries was described in terms of non-DSC grain boundary dislocations having infinitesimal Burgers vector.
It is established in bicrystals with individual boundaries that crystallographic slip can increase significantly the rate of grain boundary sliding. Two mechanisms of stimulation of sliding by slip are proposed. Intragranular slip can make a direct contribution to grain boundary sliding in incompatible conditions of deformation. Identical deformation of grains can increase the rate of sliding only by its accommodation at boundary irregularities. The close relationship between sliding and slip at incompatible deformation makes intragranular slip strongly dependent on boundary structure and geometry of slip and independent of the Schmid factor.
It is demonstrated that deformation of textured fine-grained Zn-1.1%Al alloy in the direction favorable for intragranular slip decreases the contribution of grain boundary sliding to the total strain. Deformation in the direction unfavorable for slip increases its contribution. This shows that grain boundary sliding and intragranular slip are concurrent and independent processes during superplastic deformation.
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27

Thirumalai, Neeraj Srinivas. "Low temperature creep of titanium alloys : microstructure, deformation mechanisms and modeling /". The Ohio State University, 2000. http://rave.ohiolink.edu/etdc/view?acc_num=osu1488196234911249.

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28

Segle, Peter. "Numerical simulation of weldment creep response". Doctoral thesis, KTH, Materials Science and Engineering, 2002. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-3359.

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In-service inspections of high temperature pressureequipment show that weldments are prone to creep and fatiguedamage. It is not uncommon that severely damaged weldments arefound even before the design life of the component has beenreached. In order to improve this situation action has beentaken during the last decades, both from industry, universitiesand research institutes, aiming at an enhanced understanding ofthe weldment response.

The work presented in this thesis focuses on numericalsimulation of weldment creep response. For a more profoundunderstanding of the evolution of creep damage in mismatchedlow alloy weldments, simulations are performed using thecontinuum damage mechanics, CDM, concept. Both design and lifeassessment aspects are addressed. The possibility to assessseam welded pipes using results from tests of cross-weldspecimens taken out from the seam is investigated. It is foundthat the larger the cross-weld specimen the better thecorrelation. The advantage to use the CDM concept prior to aregular creep analysis is also pointed out. In order to developthe CDM analysis, a modified Kachanov-Rabotnov constitutivemodel is implemented into ABAQUS. Using this model, a secondredistribution of stresses is revealed as the tertiary creepstage is reached in the mismatched weldment.

Creep crack growth, CCG, in cross-weld compact tension, CT,specimens is investigated numerically where a fracturemechanics concept is developed in two steps. In the first one,the C*value and an averaged constraint parameter areused for characterising the fields in the process zone, whilein the second step, the creep deformation rate perpendicular tothe crack plane and a constraint parameter ahead of the cracktip, are used as characterising parameters. The influence oftype and degree of mismatch, location of starter notch as wellas size of CT specimen, is investigated. Results show that notonly the material properties of the weldment constituentcontaining the crack, but also the deformation properties ofthe adjacent constituents, influence the CCG behaviour.Furthermore, the effect of size is influenced by the mismatchof the weldment constituents.

A circumferentially cracked girth weld with differentmismatch is assessed numerically by use of the fracturemechanics concept developed. The results show that type anddegree of mismatch have a great influence on the CCG behaviourand that C*alone cannot characterise crack tip fields.Corresponding R5 assessments are also performed. Comparisonwith the numerical investigation shows that the assumption ofplane stress or plane strain conditions in the R5 analysis isessential for the agreement of the results. Assuming the formerresults in a relatively good agreement for the axial stressdominated cases while for the hoop stress dominated cases, R5predicts higher CCG rates by an order of magnitude.

Keywords:ABAQUS, constraint effect, continuum damagemechanics, creep, creep crack growth, design, design code,finite element method, fracture mechanics, life assessment,mismatch, numerical simulation, weldment

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29

Dempers, Richard William. "Multi-axial creep behaviour of gamma titanium aluminide". Thesis, Imperial College London, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.368165.

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30

O'Dwyer, John Gerard. "Creep performance of SiCp reinforced aluminium alloy 2618A". Thesis, Imperial College London, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.313719.

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31

Hague, James R. B. "Effect of moisture content changes on wood creep". Thesis, Bangor University, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.358374.

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32

Stewart, Calvin. "A Hybrid Constitutive Model For Creep, Fatigue, And Creep-Fatigue Damage". Doctoral diss., University of Central Florida, 2013. http://digital.library.ucf.edu/cdm/ref/collection/ETD/id/6023.

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In the combustion zone of industrial- and aero- gas turbines, thermomechanical fatigue (TMF) is the dominant damage mechanism. Thermomechanical fatigue is a coupling of independent creep, fatigue, and oxidation damage mechanisms that interact and accelerate microstructural degradation. A mixture of intergranular cracking due to creep, transgranular cracking due to fatigue, and surface embrittlement due to oxidation is often observed in gas turbine components removed from service. The current maintenance scheme for gas turbines is to remove components from service when any criteria (elongation, stress-rupture, crack length, etc.) exceed the designed maximum allowable. Experimental, theoretical, and numerical analyses are performed to determine the state of the component as it relates to each criterion (a time consuming process). While calculating these metrics individually has been successful in the past, a better approach would be to develop a unified mechanical modeling that incorporates the constitutive response, microstructural degradation, and rupture of the subject material via a damage variable used to predict the cumulative “damage state” within a component. This would allow for a priori predictions of microstructural degradation, crack propagation/arrest, and component-level lifing. In this study, a unified mechanical model for creep-fatigue (deformation, cracking, and rupture) is proposed. It is hypothesized that damage quantification techniques can be used to develop accurate creep, fatigue, and plastic/ductile cumulative- nonlinear- damage laws within the continuum damage mechanics principle. These damage laws when coupled with appropriate constitutive equations and a degrading stiffness tensor can be used to predict the mechanical state of a component. A series of monotonic, creep, fatigue, and tensile-hold creep-fatigue tests are obtained from literature for 304 stainless steel at 600°C (1112°F) in an air. Cumulative- nonlinear- creep, fatigue, and a coupled creep-fatigue damage laws are developed. The individual damage variables are incorporated as an internal state variable within a novel unified viscoplasticity constitutive model (zero yield surface) and degrading stiffness tensor. These equations are implemented as a custom material model within a custom FORTRAN one-dimensional finite element code. The radial return mapping technique is used with the updated stress vector solved by Newton-Raphson iteration. A consistent tangent stiffness matrix is derived based on the inelastic strain increment. All available experimental data is compared to finite element results to determine the ability of the unified mechanical model to predict deformation, damage evolution, crack growth, and rupture under a creep-fatigue environment.
Ph.D.
Doctorate
Mechanical and Aerospace Engineering
Engineering and Computer Science
Mechanical Engineering
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33

Gunn, Russell Michael. "Non-linear analysis of arch dams including an anisotropic damage mechanics based constitutive model for concrete". Thesis, University of Brighton, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.299219.

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34

Hampton, Christopher N. "Strength and creep testing for artificial ground freezing". Thesis, University of Nottingham, 1986. http://eprints.nottingham.ac.uk/12176/.

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Artificial ground freezing (AGF) provides a means by which excavations can be given temporary or permanent structural support. It may also be used to control the movement of groundwater without the risk of pollution of potable aquifers. As AGF is called upon to strengthen ground at ever increasing depths, the design process needs to be adapted to account for the greater stresses encountered. In strong materials, the prime consideration is the short term strength of the materials and closed-form formulae can be used in design. In weaker materials, the time dependent creep behaviour of the frozen ground predominates and more complex analysis techniques have been devised (e. g. finite elements). Previous works in this field have been chiefly concerned with uniaxial states of stress. In this thesis, consideration is given to the problem of modelling creep under triaxial stress conditions. An introduction is followed by an outline of the general applications and design procedures currently used in ice wall design. Descriptions are then given of a selection of soils and weakly cemented rocks which have been incorporated into a programme of tests to investigate both short and long term strength behaviour. The apparatus available at the start of this project was suitable for uniaxial and low pressure triaxial tests only. Equipment subsequently developed to extend the confining pressure capability to 12 MPa, is described in detail. Short term strength tests show the increase in strength on freezing of ground materials is almost entirely due to the cohesion contributed by the ice matrix. Analysis of the creep test results leads to the development of a new empirically based triaxial creep equation for frozen soils. A sensitivity analysis of the parameters in this equation is followed by its application to a simplified design. Suggestions for further work in this field are included.
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35

Milligan, Walter W. Jr. "Deformation modeling and constitutive modeling for anisotropic superalloys". Diss., Georgia Institute of Technology, 1988. http://hdl.handle.net/1853/19922.

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36

Lin, Qiaoxing, e 林僑興. "Strength degradation and damage micromechanism of granite under long-term loading". Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2006. http://hub.hku.hk/bib/B37115406.

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37

Staley, James T. "Mechanisms of creep crack growth in a Cu-1 wt.% Sb alloy". Thesis, Georgia Institute of Technology, 1988. http://hdl.handle.net/1853/10098.

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38

Unocic, Raymond Robert. "On the Creep Deformation Mechanisms of an Advanced Disk Ni-base Superalloy". The Ohio State University, 2008. http://rave.ohiolink.edu/etdc/view?acc_num=osu1218556287.

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39

Grover, Parmeet Singh. "An accelerated test procedure for creep-fatigue crack growth testing". Thesis, Georgia Institute of Technology, 1993. http://hdl.handle.net/1853/20188.

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40

Walker, Nigel Stuart. "Type IV creep cavitation in low alloy ferritic steel weldments". Thesis, University of Bristol, 1997. http://hdl.handle.net/1983/efa6973c-9a3d-4a95-8297-61f12cbde92d.

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41

Green, Stuart. "Creep behaviour of short glass fibre reinforced nylon 66". Thesis, University of Liverpool, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.279696.

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42

Hamilton, Benjamin Carter. "Creep behavior of aluminum alloys C415-T8 and 2519-T87". Diss., Georgia Institute of Technology, 1997. http://hdl.handle.net/1853/20497.

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43

Horník, Vít. "Základní mechanismy únavového a kombinovaného poškození únava-creep niklových superslitin MAR-M 247 a IN 713LC". Doctoral thesis, Vysoké učení technické v Brně. Fakulta strojního inženýrství, 2021. http://www.nusl.cz/ntk/nusl-433626.

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The thesis is focused on clarifying fatigue damage mechanisms and fatigue-creep damage mechanisms of MAR-M 247 and IN 713LC polycrystalline Ni-based superalloys. This thesis begins with basic information about nickel-based superalloys and their microstructure, followed by a description of fatigue and creep mechanisms and their mutual interaction. The next part contains experimentally obtained results describing the behavior of MAR-M 247 and IN 713LC superalloys under various sets of conditions. Three testing temperatures - 800, 900 and 950 °C were used for the measurement of fatigue properties under symmetrical loading cycle, because in the temperature range 800 – 950 °C, the mechanism of fatigue crack propagation of both superalloys should change from the originally crystallographic at "lower" temperatures (800 °C) to non-crystallographic at "higher" temperatures (950 °C). In addition the effect of processing technology on fatigue properties was studied on the superalloy IN 713LC. High-frequency cyclic loading (about 120 Hz) with high mean stress at elevated temperatures was applied to induce fatigue-creep interaction. The combined fatigue-creep loading was performed on the IN 713LC superalloy at 800 °C and on the MAR-M 247 superalloy at 900 °C.
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44

Tang, Alice A. "The creep behaviour of cross-weld specimens and welded pipes". Thesis, University of Nottingham, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.342069.

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45

Khodja, Karim. "Laboratory investigations for the assessment of the mechanical behaviour of Teesside rocksalt". Thesis, University of Newcastle Upon Tyne, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.320027.

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46

Degli, Alessandrini Giulia. "Deformation mechanisms and strain localization in the mafic continental lower crust". Thesis, University of Plymouth, 2018. http://hdl.handle.net/10026.1/12799.

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The rheology and strength of the lower crust play a key role in lithosphere dynamics, influencing the orogenic cycle and how plate tectonics work. Despite their geological importance, the processes that cause weakening of the lower crust and strain localization are still poorly understood. Through microstructural analysis of naturally deformed samples, this PhD aims to investigate how weakening and strain localization occurs in the mafic continental lower crust. Mafic granulites are analysed from two unrelated continental lower crustal shear zones which share comparable mineralogical assemblages and high-grade deformation conditions (T > 700 °C and P > 6 Kbar): the Seiland Igneous Province in northern Norway (case-study 1) and the Finero mafic complex in the Italian Southern Alps (case-study 2). Case-study 1 investigates a metagabbroic dyke embedded in a lower crustal metasedimentary shear zone undergoing partial melting. Shearing of the dyke was accompanied by infiltration of felsic melt from the adjacent partially molten metapelites. Findings of case-study 1 show that weakening of dry and strong mafic rocks can result from melt infiltration from nearby partially molten metasediments. The infiltrated melt triggers melt-rock reactions and nucleation of a fine-grained (< 10 µm average grain size) polyphase matrix. This fine-grained mixture deforms by diffusion creep, causing significant rheological weakening. Case-study 2 investigates a lower crustal shear zone in a compositionally-layered mafic complex made of amphibole-rich and amphibole-poor metagabbros. Findings of case-study 2 show that during prograde metamorphism (T > 800 °C), the presence of amphibole undergoing dehydration melting reactions is key to weakening and strain localization. Dehydration of amphibole generates fine-grained symplectic intergrowths of pyroxene + plagioclase. These reaction products form an interconnected network of fine-grained (< 20 µm average grain size) polyphase material that deforms by diffusion creep, causing strain partitioning and localization in amphibole-rich layers. Those layers without amphibole fail to produce an interconnected network of fine grained material. In this layers, plagioclase deforms by dislocation creep, and pyroxene by microfracturing and neocrystallization. Overall, this PhD research highlights that weakening and strain localization in the mafic lower crust is governed by high-T mineral and chemical reactions that drastically reduce grain size and trigger diffusion creep.
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47

Shyu, Hung-Fa. "Ultrasonic system with split spectrum processing for HAZ creep damage evaluation". Thesis, Cranfield University, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.309846.

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48

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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49

Alfthan, Johan. "Micro-mechanically based modeling of mechano-sorptive creep in paper". Doctoral thesis, KTH, Solid Mechanics, 2004. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-41.

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The creep of paper is accelerated by moisture content changes. This acceleration is known as mechano-sorptive creep, which is also found in wood and some other materials. Mechano-sorptive creep has been known for several decades but it is still not well understood, and there is no generally accepted model explaining the effect.

In this thesis, it is assumed that mechano-sorptive creep is the result of transient redistributions of stresses during moisture content changes in combination with non-linear creep behaviour of the material. The stress redistributions are caused by the anisotropic hygroexpansion of the fibres, which will give a mismatch of hygroexpansive strains at the bonds and hence large stresses each time the moisture content changes. This redistribution will lead to an uneven stress state. If the creep of the material depends non-linearly on stresses this will give an increase in creep rate where the stresses are high, that is larger than the decrease of creep rate where stresses are low, so in average there will be an increase in creep rate. The stress distribution evens out as the stresses relax during creep, and the moisture content has to change again to create a new uneven stress state and maintain the accelerated creep.

Two different network models based on this mechanism are developed in this thesis. Numerical simulations show that the models produce results similar to the mechano-sorptive creep found in paper. In the first model it is assumed that creep takes place in the fibre-fibre interfaces at the bonds, in the second the creep of the fibres themselves is accelerated. The second model is further developed. Experiments verify model predictions of the dependence of the amplitude of moisture changes.

The second model shows a linear relationship between mechanical load and deformation, although creep of the fibres depends non-linearly on stresses. This linear behaviour is also found in applications. Further analysis shows that the mechanical load can be treated as a small perturbation of the internal stress state caused by moisture content changes. This can be used to develop a linearized model, from which a continuum model can be derived. This leads to a reduction of the necessary number of variables, and a significant increase in speed of calculations. Hence, this linearized continuum model can be used as a constitutive law of paper in problems with complicated geometries, for example a corrugated board box in varying humidity.

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50

Wilson, Peter. "Remanent creep life prediction in low-alloy ferritic steel power plant components". Thesis, University of Cambridge, 1991. https://www.repository.cam.ac.uk/handle/1810/221887.

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