Dissertations / Theses: 'Materials – Creep'

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De, Voy Julian David James. "Failure of creep brittle materials." Thesis, University of Leicester, 1993. http://hdl.handle.net/2381/34757.

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Abdallah, Zakaria. "Creep lifing methods for components under high temperature creep." Thesis, Swansea University, 2010. https://cronfa.swan.ac.uk/Record/cronfa43065.

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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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Feng, Gang. "Creep effects in nanoindentation." Hong Kong : University of Hong Kong, 2001. http://sunzi.lib.hku.hk/hkuto/record.jsp?B23273288.

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Feng, Gang, and 封剛. "Creep effects in nanoindentation." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2001. http://hub.hku.hk/bib/B31224350.

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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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Ramteke, Ashok Lahanuji. "Multiaxial creep of isotropic and anisotropic materials." Thesis, Imperial College London, 1987. http://hdl.handle.net/10044/1/47770.

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Isogai, Takeshi. "Creep-fatigue crack growth in engineering materials." Thesis, University of Cambridge, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.627408.

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Osiroff, Ricardo. "Damorheology: creep-fatigue interaction in composite materials." Diss., Virginia Tech, 1990. http://hdl.handle.net/10919/38757.

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This investigation addresses the interaction mechanisms of time dependent material behavior and cyclic damage during fatigue loading of fiber reinforced composite laminates. A new term 'damorheology' has been coined to describe such physical behavior. The lamina has been chosen as the building block and a cross ply laminate configuration was the selected test case. The chosen material system is the Radel X/T65-42 thermoplastic composite by Amoco. The fatigue performance at the lamina level is represented by the dynamic stiffness, residual strength and fatigue life of unidirectional laminates. The time dependent behavior is represented at the lamina level by a Pseudo-Analog Mechanical model. The thermo-rheological characterization procedure combines mechanical (creep) and thermal (dynamic mechanical analysis) techniques.
Ph. D.

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Falkeström, Oskar, Kevin Coleman, and Malin Nilsson. "Micromechanical modelling of creep in wooden materials." Thesis, Uppsala universitet, Tillämpad mekanik, 2021. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-444796.

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Wood is a complex organic orthotropic viscoelastic material with acellular structure. When stressed, wood will deform over timethrough a process called creep. Creep affects all wooden structureand can be difficult, time-consuming and expensive to measure. For this thesis, a simple computer model of the woodenmicrostructure was developed. The hypothesis was that the modelledmicrostructure would display similar elastic and viscoelasticproperties as the macroscopic material. The model was designed by finding research with cell geometries ofconiferous trees measured. The model considered late- and earlywoodgeometries as well as growth rings. Rays were ignored as they onlycomposed 5-10% of the material. By applying a finite element method, the heterogeneous late- andearlywood cells could be homogenized by sequentially loading thestrain vector and calculating the average stress. The computer model produced stiff but acceptable values for theelastic properties. Using the standard linear solid method to modelviscoelasticity, the computer model assembled creep curvescomparable to experimental results. With the model sufficiently validated, parametric studies on thecell geometry showed that the elastic and viscoelastic propertieschanged greatly with cell shape. An unconventional RVE was alsotested and shown to give identical result to the standard RVE. Although not perfect, the model can to a certain degree predict theelastic and viscoelastic characteristics for wood given itscellular geometry. Inaccuracies were thought to be caused byassumptions and approximations when building the model.

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Li, Han. "The nanoindentation size effects of creep." Click to view the E-thesis via HKUTO, 2004. http://sunzi.lib.hku.hk/hkuto/record/B30696380.

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Li, Han, and 李晗. "The nanoindentation size effects of creep." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2004. http://hub.hku.hk/bib/B30696380.

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Hall, David Edward. "Analysis of crack growth in creep-brittle materials." Diss., Georgia Institute of Technology, 1995. http://hdl.handle.net/1853/17118.

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Rasiawan, Trisha. "The influence of prior creep damage on the fracture localisation in X20 CrMoV12-1 cross-weld creep tests." Master's thesis, University of Cape Town, 2017. http://hdl.handle.net/11427/27357.

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Many of Eskom's coal fired power plants have an average age of 170 000 hours and a few operating close to 300 000 hours. Main steam temperatures experienced in a power plant vary between 535-555°C. These operating conditions place main steam pipe components to operate within the creep regime. It is of utmost importance for safety and plant health that these critical components are managed to determine the remaining life and risks associated with high temperature exposure for prolonged periods of time. Non-destructive testing (NDT) methods are utilised extensively on Eskom power plants to determine the remaining life and replacement strategies for critical components. Surface replication is used as a life assessment tool for creep damage quantification of main steam pipe work. A large part of maintaining plant is repair welding on creep aged and sometimes creep aged material as entire system replacements are impractical and time consuming. By repair welding new material onto creep aged material, mechanical and microstructural properties of the creep aged material deteriorates. The study of this work is focused on characterising the as-received materials from Eskom power plants and using these creep aged materials to create cross-weld samples with virgin material. The cross-weld samples were creep-rupture tested at high temperature and low stress conditions to determine the fracture location of repair welded cross-weld samples. Once ruptured, the zone of rupture, was identified and created in a larger volume by simulation using Gleeble® thermo-mechanical equipment. The as-received base materials were subjected to different operating conditions hence contain different degrees of creep damage. The microstructural evaluation of the creep damaged material was conducted using optical microscopy, scanning electron microscopy (SEM), coupled with more advanced electron backscattered diffraction (EBSD). Microhardness and hot tensile testing were included to characterise the mechanical degradation of the as-received material. The fracture location of the creep-ruptured cross-weld samples were investigated using optical microscopy, SEM and EBSD and occurred on the outer region of the heat affected zone (HAZ) of the creep aged material. The fine grained microstructure with coarse precipitation of this region is characteristic of the fine grain heat affected zone (FGHAZ). The occurrences of voids predominantly occur in this narrow region with very few voids in the adjacent base/weld material. As this zone is of particular interest due to it being the weakest region in repair welded joints, the need to investigate it further is important. A larger testing volume of the FGHAZ was created by applying a weld thermal cycle simulation to the as-received base materials. The impact of this simulation was determined microstructurally by optical microscopy and mechanically by hardness and tensile testing. The FGHAZ has low creep resistance and is most susceptible to failure due to the small grained microstructure. Due to the numerous small grains, there is a high effective diffusion coefficient (HEDC). The multi axial stresses induced during in service/ creep testing conditions together with the HEDC causes voids to form at an accelerated rate. Significant void coalescence promotes the formation of micro cracks which in turn lead to macro crack formation and eventually failure.

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Sieburg, H. O. "Creep predictions for turbomachinery components." Master's thesis, University of Cape Town, 1989. http://hdl.handle.net/11427/18697.

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Several theories of creep and creep rupture are reviewed. Specific attention is devoted to the brittle damage theory proposed by Kachanov. Creep, damage and life predictions for rectangular or circular cross section beams under bending and tensile loads are presented. Comparison with data for a Ni Superalloy showed life predictions could be 30X in excess of experimental values. This beam model also revealed that it is imperative that no bending moments be inadvertently applied during tensile creep testing. The creep-damage material model is extended to multidimensional situations. A refinement, whereby no damage accumulates in compression, is incorporated. A User-Material subroutine for this constitutive model has been formulated, and incorporated into the ABAQUS FEM package. Several verification examples are presented; one example is the creep-damage behaviour of a notched bar in tension. The value of reference stress techniques is discussed. Reference stress estimates for a centrifugally loaded bar, as well as for a cantilever under distributed loads, are presented. These could be useful in turbine blade design. Bibliography: pages 91-92.

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Bowman, Elisabeth Therese. "The ageing and creep of dense granular materials." Thesis, University of Cambridge, 2002. https://www.repository.cam.ac.uk/handle/1810/251841.

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Saad, Abdullah Aziz. "Cyclic plasticity and creep of power plant materials." Thesis, University of Nottingham, 2012. http://eprints.nottingham.ac.uk/12538/.

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The thermo-mechanical fatigue (TMF) of power plant components is caused by the cyclic operation of power plant due to startup and shutdown processes and due to the fluctuation of demand in daily operation. Thus, a time-dependent plasticity model is required in order to simulate the component response under cyclic thermo-mechanical loading. The overall aim behind this study is to develop a material constitutive model, which can predict the creep and cyclic loading behaviour at high temperature environment, based on the cyclic loading test data of the P91 and the P92 steels. The tests on all specimens in the study were performed using the Instron 8862 TMF machine system with a temperature uniformity of less than ±10°C within the gauge section of the specimen. For the isothermal tests on the P91 steel, fully-reversed, strain-controlled tests were conducted on a parent material of the steel at 400, 500 and 600˚C. For the P92 steel, the same loading parameters in the isothermal tests were performed on a parent material and a weld metal of the steels at 500, 600 and 675°C. Strain-controlled thermo-mechanical fatigue tests were conducted on the parent materials of the P91 and the P92 steels under temperature ranges of 400-600°C and 500-675°C, respectively, with in-phase (IP) and out-of-phase (OP) loading. In general, the steels exhibit cyclic softening behaviour throughout the cyclic test duration under both isothermal and anisothermal conditions. The cyclic softening behaviour of the P91 steel was further studied by analyzing stress-strain data at 600°C and by performing microstructural investigations. Scanning electron microscope (SEM) and transmission electron microscope (TEM) images were used to investigate microstructural evolution and the crack initiation of the steel at different life fractions of the tests. The TEM images of the interrupted test specimens revealed subgrain coarsening during the cyclic tests. On the other hand, the SEM images showed the initiation of microcracks at the end of the stabilisation period and the cracks were propagated in the third stage of cyclic softening. A unified, Chaboche, viscoplasticity model, which includes combined isotropic softening and kinematic hardening with a viscoplastic flow rule for time-dependent effects, was used to model the TMF behaviour of the steels The constants in the viscoplasticity model were initially determined from the first cycle stress-strain data, the maximum stress evolution during tests and the stress relaxation data. Then, the initial constants were optimized using a least-squares optimization algorithm in order to improve the general fit of the model to experimental data. The prediction of the model was further improved by including the linear nonlinear isotropic hardening in order to obtain better stress-strain behaviour in the stabilisation period. The developed viscoplasticity model was subsequently used in the finite element simulations using the ABAQUS software. The focus of the simulation is to validate the performance of the model under various types of loading. Simulation results have been compared with the isothermal test data with different strain ranges and also the anisothermal cyclic testing data, for both in-phase and out-of-phase loadings. The model’s performance under 3-dimensional stress conditions was investigated by testing and simulating the P91 steel using a notched specimen under stress-controlled conditions. The simulation results show a good comparison to the experimental data.

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Reyngoud, Benjamin Peter. "Hybrid materials design to control creep in pipes." Thesis, University of Canterbury. Mechanical Engineering, 2015. http://hdl.handle.net/10092/10857.

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A hybrid material design has been developed to improve creep performance in pressurized metallic pipes subjected to high temperatures. Metallic pipes were reinforced with various arrangements of external wires which have substantially greater creep resistance than the pipe material. This research was conducted to explore the field of reinforcement of piping for creep reduction, exploit the creep strength of refractory metals, and investigate structure-property relationships in architectured materials. Two basic wire reinforcement architectures were tested, simple helical windings and braided sleeves. By adjusting the architecture of the reinforcement, apparent tangential (hoop) and longitudinal stresses on the pipe are altered, thereby allowing multiaxial creep strains to be controlled. The utilization of a reinforcement layer in a hybrid layup, where it is not bonded or embedded in a matrix is a relatively unexplored field. Hybridization allows the most desirable properties to be extracted from each component and have them work together in parallel. The use of braided refractory reinforcement is also a particularly novel concept, with refractory materials for reinforcement purposes traditionally being utilized in particle, whisker and discontinuous fibre form. Rather than testing in a uniaxial stress state, the present approach to creep testing pressurized pipes at high temperature remains largely underutilized, and is especially relevant to industry applications where creep takes place in the complex, multiaxial stress state of a pressurized pipe. In a low-temperature reinforcement architecture optimization study of a brass-stainless steel system, designed for ease of fabrication and to negate oxidation issues, pipes were pressurized and creep rupture tested at 400°C. Even in an unoptimized state, braided reinforcement was observed to out-perform a simple iv helical wrap by at least 22%, giving a 10-times life extension without rupture, and a reduction in creep rate in excess of 45-times for reinforcement oriented at a 50°. A simple analytical model from reinforced pressure vessel theory predicts a neutral angle (θN) of 54.7°, at which point the reinforcement is oriented to act proportionally to the applied pressure stresses. An empirical model of effective creep rate with varying reinforcement angle was derived in the present study, and used to find that a braid angle of approximately 54.7±1.5° is optimal to minimize the effective multiaxial creep rate of a hybrid pipe under internal pressure, reducing it to the point of being negligible. The braided reinforcement was observed to be constantly shifting towards the equilibrium point of θN, but only for initial angles below θN. This concept of braid reorientation is generally associated with rapid elastic deformation or static reinforcement of systems at room temperature, and the gradual shift towards θN facilitated by creep deformation has not been reported previously. A relationship for -θ (i.e. creep rate for a given reinforcement angle) was derived, including the reduction in as θ tends to θN. Findings of this optimization study were applied to a high temperature system which served as an acceleration of reformer furnace operating conditions: 253MA pipes were reinforced with tungsten wire and creep rupture tested at 1030-1040°C. Using braided reinforcement oriented at 52.6±1.4° a life extension in excess of 700x was observed, with no signs of bulk deformation after a 309x life extension. These high temperature results were considered in light of the intended industry application, with a balance of life extension, weight reduction and increased operating temperature preferred over outright life extension for the reformer furnace application.

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Leung, Chun-Pok. "Estimation of the Ct parameter for primary creep." Diss., Georgia Institute of Technology, 1988. http://hdl.handle.net/1853/15901.

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Hop, Jørund Gimmestad. "Sodium expansion and creep of cathode carbon." Doctoral thesis, Norges teknisk-naturvitenskapelige universitet, Institutt for materialteknologi, 2003. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-346.

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An apparatus to measure compressive creep in carbon materials has been developed. Using the final experimental set-up five material properties could be measured in each electrolysis experiment. Creep, sodium expansion, compressive strength and E-modulus were measured for 3 commercial cathode materials at 25 and 980 °C with and without electrolysis. The sodium diffusion coefficient (D) was calculated from the sodium expansion results. Filler materials for cathode blocks, i.e., certain anthracite and petrol coke qualities, were exposed to sodium vapour to examine crack evolution. Creep The three commercial cathode materials were found to deform with time under compression at 25 °C, 980 °C and during electrolysis at 980 °C. Only samples from one block for each quality was studied, so care must be taken before extending the ranking to all classes of cathode materials. The ranking from low to high creep at 980 °C and during electrolysis was: Semigraphitized < Anthracitic < Semigraphitic The creep is larger during electrolysis than at 980 °C for all materials and the increase in creep from virgin to electrolysed material at 980 °C is largest for the anthracitic material. Repeated loadings did not influence the shape of the creep strain curve, which could be described by the expression (time)n. The magnitude of the creep strain ranged from 0.01 to 0.07 % with a load of 20 MPa held for one hour. The largest measured creep was approximately 0.35 % after 20 hours of electrolysis in the semigraphitic material. The stress-strain diagram of the anthracitic material is unchanged before and after electrolysis and exhibits a more linear behaviour than in the other materials. The stress-strain diagram of the semigraphitic and semigraphitized materials changes after loading and tend to increase after electrolysis. Cracks All anthracite grains cracked to some extent after being exposed to sodium vapour at 800 °C. The lowest heat treated grains cracked the most. Cracks through grains were also found in the commercial material during electrolysis. In the petrol cokes only grains calcined to 1500 °C with a structure characterised by a gradient from mosaic to flow was observed to crack after exposure to sodium vapour at 800 °C. Diffusion coefficient The diffusion coefficient of sodium in carbon during electrolysis has been calculated with three different solutions of Fick’s law and is found to increase with current density and graphitic character of the material. Two of the calculations were based on the expansion of the sample (penetration from bottom and radial penetration) and one on a rather few measured sodium concentrations. The diffusion coefficient was calculated to be in the range 8-10-5 to 5-10-4 cm2/s at current densities from 0.06 to 0.88 A/cm2, which is around 10 times larger than reported before (Table 2.1). The cryolite ratio did not influence D as the saturation time for samples in acidic or basic melt was the same. The ranking from larger to smaller diffusion coefficient in the studied materials was Semigraphitized > Semigraphitic > Anthracitic The diffusion coefficient increased with heat treatment temperature in some laboratory produced materials.

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Elmansy, N. M. "Deformation of bituminous highway pavement materials." Thesis, University of Bradford, 1986. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.373221.

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Nishida, Kenji. "Creep-fatigue failure of engineering materials at elevated temperatures." Thesis, Imperial College London, 1989. http://hdl.handle.net/10044/1/47589.

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Çelikin, Mert. "The creep behaviour of Magnesium-Manganese based alloys." Thesis, McGill University, 2012. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=110533.

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The concern for weight-reduction in vehicles has increased substantially in recent decades due to fuel consumption regulations. Extending the use of magnesium (Mg), the lightest structural metal, in automotive powertrain applications has been one of the routes to reduce vehicle weights. In this doctoral work, the creep behaviour of Mg-Mn based systems with cerium (Ce) and/or strontium (Sr) additions is studied in order to shed light into the new design principles for the development of creep-resistant Mg alloys. Transmission electron microscopy (TEM) is used to characterize the microstructures (the orientation, coherency, crystal structure and the morphology of the phases) as well as the features of creep deformation (twins, dislocations, sub-grains). In the Mg-Mn binary system, the dynamic precipitation of α-Mn was observed upon thermal exposure and during creep. Two different orientation relationships (OR) were determined between α-Mn (rods) precipitated upon heat treatment and the α-Mg matrix. The creep mechanisms were determined to be dislocational processes based on the activation energy (Qc) values (Qc (pure Mg) = 105 and 168kJ/mole; Qc (Mg-1.4Mn) = 127 and 154kJ/mole) obtained via long-term creep tests at different temperatures. The creep strengthening effect of Mn was due to the dynamic precipitation of α-Mn on dislocations (acting as heterogeneous nucleation sites) during creep. In the Mg-Ce-Mn ternary system, the intradendritic precipitation of Mg12Ce occurred heterogeneously where α-Mn precipitates acted as nucleants. The nucleation and growth of the α-Mn and Mg12Ce precipitates were mutually affected by the presence of the other. The dislocational processes were found to be rate-controlling, hence, the refinement in the Mg12Ce precipitate size via α-Mn precipitates enhanced the creep resistance by providing effective dislocation pinning. In the Mg-Sr-Mn ternary system, a new orientation relationship (OR) was determined between the Mg17Sr2 and α-Mn phases, which resulted in differing morphologies of the dynamically precipitated α-Mn depending on the region of precipitation (interdendritic and intradendritic regions). Creep deformation was again found to be dependent on dislocational processes: pipe diffusion at low temperatures and either dislocation climb or activated cross-slip at the higher temperatures (Qc (JM51) = 99 and 234kJ/mole; Qc (JM52) = 93 and 135kJ/mole). The dynamic precipitation of α-Mn on the basal dislocations of the intradendritic regions enhanced creep resistance via pinning by coherency strain fields. Mg-Sr-Mn-Ce alloys exhibiting high creep resistance were developed. The design principles were based on the results obtained from the creep behaviour studies of the pure Mg, Mg-Mn, Mg-Ce-Mn, Mg-Sr-Mn systems, as well as thermodynamic calculations by FactSage. The microstructure of the alloys consisted of the Mg17Sr2 intermetallic phase at the interdendritic regions which strengthened the grain boundaries, and dissolved Mn and Ce which resulted in the dynamic co-precipitation of Mg12Ce and α-Mn (Sr) in intradendritic regions during creep. The creep strain of the quaternary alloys was around to be four times lower than the strain of the ternary alloys.
L'intérêt pour la réduction du poids des véhicules automobiles a augmenté de façon significative au cours de la présente décennie principalement à cause des restrictions sur la consommation d'essence. L'augmentation de l'utilisation du magnésium (Mg), le métal structurel le plus léger, pour les applications automobiles, les composantes des groupes propulseurs, a ainsi été une des avenues utilisées pour solutionner le problème de réduction de poids des véhicules. Dans ce travail de doctorat, le comportement au fluage des systèmes de base Mg-Mn avec des additions de cérium (Ce) et/ou de strontium (Sr) est étudié afin de faire la lumière sur de nouveaux principes de développement d'alliages de magnésium résistant au fluage. La microscopie à transmission électronique (MET) est utilisée pour caractériser les microstructures (l'orientation, la cohérence, la structure cristalline et la morphologie des phases) ainsi que les caractéristiques des déformations provoquées par le fluage (mâcles, dislocations, sous-grains). Dans le système binaire Mg-Mn, une précipitation dynamique de la phase α-Mn a été observée suite à une exposition thermique et au cours du fluage. Deux différentes relations d'orientation (OR) ont été déterminées entre les tiges de α-Mn qui ont précipitées suite au traitement thermique et dans la matrice α-Mn. Les mécanismes de déformation au fluage ont été déterminés comme étant des procédés basés sur les valeurs de l'activation d'énergie (Qc), Qc Mg pur = 105 et 168kJ/mole; Qc Mg-1.4Mn = 127 et 154kJ/mole, obtenues par le biais d'essais de fluage effectués à long terme à différentes températures. L'effet de renforcement au fluage du Mn a été causé par la précipitation dynamique de la phase α-Mn sur les dislocations agissant ainsi comme des sites de nucléation lors du fluage. Dans le système ternaire Mg-Ce-Mn, une précipitation interdendritique de Mg12Ce a été produite de façon hétérogène à l'endroit où les précipités α-Mn ont agits comme agents de nucléation. La nucléation et la croissance des précipités α-Mn et Mg12Ce ont été affectées mutuellement par la présence de l'un et de l'autre. Les procédés de dislocation ont donc été trouvés par conséquent comme étant un taux de contrôle, le raffinement de la grosseur du précipité Mg12Ce via les précipités α-Mn qui a amélioré la résistance au fluage par l'approvisionnement d'une méthode effective de fixation des dislocations. Dans le système ternaire Mg-Sr-Mn, une nouvelle relation d'orientation a été déterminée entre les phases Mg17Sr2 et α-Mn qui a résulté dans la différentiation de morphologies du précipité α-Mn en fonction de la région de précipitation (régions interdendritiques et intradendritiques). La déformation au fluage a été trouvée dépendante des procédés de disclocations: diffusion de défauts à basse température et l'un ou l'autre de la montée de dislocation ou des glissements activés entrecroisés à une température plus élevée (Qc (JM51) = 99 et 234 kJ/mole; Qc (JM52) = 93 et 135 kJ/mole). La précipitation dynamique de la phase α-Mn sur les dislocations de base des régions interdendritiques a amélioré la résistance au fluage par le biais de la fixation des champs d'allongement cohérents. Les alliages Mg-Sr-Mn-Ce démontrant une résistance élevée au fluage ont été développés. Les principes de conception ont été basés sur les résultats à partir du comportement au fluage des systèmes, Mg pur, Mg-Mn, Mg-Ce-Mn et Mg-Sr-Mn ainsi que des calculs thermodynamiques effectués à partir de la méthode FactSage. La microstructure des alliages a été constituée de la phase intermétallique Mg17Sr2 aux régions interdendritiques qui ont renforcé les bordures de grains et qui ont dissout le Mn et le Ce lesquels ont favorisé la co-précipitation dynamique de Mg12Ce et α-Mn dans les régions intradendritiques pendant le fluage. Les allongements obtenu lors du fluage des alliages quaternaires ont été quatre fois plus bas que les allongements des alliages ternaires.

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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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25

Whitt, Harrison Collin. "Creep and Creep-fatigue Deformation Studies in 22V and P91 Creep-strength EnhancedFerritic Steels." The Ohio State University, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=osu1555603135480185.

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26

Hastie, Robert L. "The effect of physical aging on the creep response of a thermoplastic composite." Diss., This resource online, 1991. http://scholar.lib.vt.edu/theses/available/etd-07282008-134037/.

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27

Gul, Rizwan Mahmood 1967. "Bending fatigue and creep of tough matrix laminates." Thesis, Massachusetts Institute of Technology, 1994. http://hdl.handle.net/1721.1/17388.

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28

Mishra, Surendra. "Stereological assessment of the evolution of microstructural damage during creep." Thesis, Georgia Institute of Technology, 1992. http://hdl.handle.net/1853/32779.

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29

Ho, Kwang-Il. "An anisotropic continuum damage model for creep-dominated, multiaxial loading histories." Diss., Georgia Institute of Technology, 1987. http://hdl.handle.net/1853/20043.

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30

Schöberle, Bernd Michael. "Evaluation of viscoelastic materials for MEMS by creep compliance analysis." Tönning Lübeck Marburg Der Andere Verl, 2008. http://d-nb.info/98953068X/04.

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31

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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32

Syed, Asif S. A. "Time dependent micro deformation of materials." Thesis, University of Oxford, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.362108.

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33

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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34

Wright, Richard J. "Bolt bearing creep behavior of highly loaded polymer matrix composites at elevated temperatures." Thesis, Georgia Institute of Technology, 1997. http://hdl.handle.net/1853/17362.

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35

Gill, Yancy. "Creep crack growth characterization of SA-106 C carbon steel." Diss., Georgia Institute of Technology, 1994. http://hdl.handle.net/1853/20039.

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36

Ward, Andrew L. "The influence of the accumulation of deformation on the failure of polyethylene pipe materials." Thesis, Manchester Metropolitan University, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.386054.

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37

Ranade, Ajit. "Barrier and Long Term Creep Properties of Polymer Nanocomposites." Thesis, University of North Texas, 2004. https://digital.library.unt.edu/ark:/67531/metadc5563/.

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The barrier properties and long term strength retention of polymers are of significant importance in a number of applications. Enhanced lifetime food packaging, substrates for OLED based flexible displays and long duration scientific balloons are among them. Higher material requirements in these applications drive the need for an accurate measurement system. Therefore, a new system was engineered with enhanced sensitivity and accuracy. Permeability of polymers is affected by permeant solubility and diffusion. One effort to decrease diffusion rates is via increasing the transport path length. We explore this through dispersion of layered silicates into polymers. Layered silicates with effective aspect ratio of 1000:1 have shown promise in improving the barrier and mechanical properties of polymers. The surface of these inorganic silicates was modified with surfactants to improve the interaction with organic polymers. The micro and nanoscale dispersion of the layered silicates was probed using optical and transmission microscopy as well as x-ray diffraction. Thermal transitions were analyzed using differential scanning calorimetry. Mechanical and permeability measurements were correlated to the dispersion and increased density. The essential structure-property relationships were established by comparing semicrystalline and amorphous polymers. Semicrystalline polymers selected were nylon-6 and polyethylene terephthalate. The amorphous polymer was polyethylene terphthalate-glycol. Densification due to the layered silicate in both semicrystalline and amorphous polymers was associated with significant impact on barrier and long term creep behavior. The inferences were confirmed by investigating a semi-crystalline polymer - polyethylene - above and below the glass transition. The results show that the layered silicate influences the amorphous segments in polymers and barrier properties are affected by synergistic influences of densification and uniform dispersion of the layered silicates.

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38

Vasatis, Ioannis Platon. "The creep rupture behavior of notched bars of IN-X750." Thesis, Massachusetts Institute of Technology, 1986. http://hdl.handle.net/1721.1/60721.

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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1986.
MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE
Bibliography: leaves 191-205.
by Ioannis Platon Vasatis.
Ph.D.

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39

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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40

Weyer, Royden. "The modelling of damage due to diffusional creep in high chromium steels." Master's thesis, University of Cape Town, 2016. http://hdl.handle.net/11427/22961.

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Understanding the creep deformation of high chromium steels in use in modern power plants has become important in predicting the behaviour and stability of these materials over their operational lifetime. At the deformation rates and conditions recorded in modern power plants, diffusional creep by vacancy migration is seen to be the dominant creep mechanism. However, the understanding of diffusional creep in particle stabilized materials is heavily incomplete. The aim of this project was to model the damage caused by diffusional creep, while considering the microstructure of high chromium steels and the evolution of this microstructure. This problem is addressed by expanding the existing Nabarro-Herring theory on lattice diffusion into a spatially resolved FEM model using MATLAB. This model focussed on adapting the Nabarro-Herring creep model to handle vacancy concentration changes over time. This allowed the model to produce the primary, secondary and tertiary creep stages present in experimental creep tests. As for microstructure, the focus was on adding precipitates (one of the strongest creep strengthening mechanisms) and voids (the largest cause of material damage). During creep exposure, precipitates were subject to coarsening while voids were subject to growth. The primary creep stage was formed by the initial rapid flux of vacancies into the body of the grain, due to large chemical potential gradients. A dynamic equilibrium of vacancy concentration would form within the grain, leading to the secondary creep stage. The creep rate produced was similar to that of the existing theory and it was found to decrease with the introduction of precipitates. This was evaluated by analysing the stress gradients caused by hard particles in a softer matrix. These stress fields lowered the stress in the grain boundaries and thus resulted in fewer vacancies being generated. Coarsening led to a reduction in the stress field distribution and thus resulted in creep strength loss in the material. The inclusion of voids was shown to decrease the initial creep rate, with void growth lessening this effect and leading to the tertiary creep stage. The initial strengthening was due to the void surface replacing the grain boundary as a source of vacancies. As the void surface is a very inefficient source, fewer vacancies were generated, resulting in lower diffusion rates. A slight steady increase in the creep rate over time was shown with the inclusion of void growth. The increase in vacancy generation was caused by the higher stress fields around voids. Initially the stress increase due to a loss in area was accounted for as a stress concentration around the void. Once this void grew too large in relation to the grain size, the stress concentration no longer accounted for all of the stress increase due to load bearing area loss. This resulted in the damage equation coming into play, causing a rapid increase in the stress throughout the grain and leading to the rapid tertiary creep stage.

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41

Leppard, Claire Louise. "Mathematical modelling of some mechanical properties of construction materials." Thesis, Coventry University, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.313143.

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42

He, Junjing. "High temperature performance of materials for future power plants." Doctoral thesis, KTH, Materialvetenskap, 2016. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-191547.

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Increasing energy demand leads to two crucial problems for the whole society. One is the economic cost and the other is the pollution of the environment, especially CO2 emissions. Despite efforts to adopt renewable energy sources, fossil fuels will continue to dominate. The temperature and stress are planned to be raised to 700 °C and 35 MPa respectively in the advanced ultra-supercritical (AUSC) power plants to improve the operating efficiency. However, the life of the components is limited by the properties of the materials. The aim of this thesis is to investigate the high temperature properties of materials used for future power plants. This thesis contains two parts. The first part is about developing creep rupture models for austenitic stainless steels. Grain boundary sliding (GBS) models have been proposed that can predict experimental results. Creep cavities are assumed to be generated at intersection of subboundaries with subboundary corners or particles on a sliding grain boundary, the so called double ledge model. For the first time a quantitative prediction of cavity nucleation for different types of commercial austenitic stainless steels has been made. For growth of creep cavities a new model for the interaction between the shape change of cavities and creep deformation has been proposed. In this constrained growth model, the affected zone around the cavities has been calculated with the help of FEM simulation. The new growth model can reproduce experimental cavity growth behavior quantitatively for different kinds of austenitic stainless steels. Based on the cavity nucleation models and the new growth models, the brittle creep rupture of austenitic stainless steels has been determined. By combing the brittle creep rupture with the ductile creep rupture models, the creep rupture strength of austenitic stainless steels has been predicted quantitatively. The accuracy of the creep rupture prediction can be improved significantly with combination of the two models. The second part of the thesis is on the fatigue properties of austenitic stainless steels and nickel based superalloys. Firstly, creep, low cycle fatigue (LCF) and creep-fatigue tests have been conducted for a modified HR3C (25Cr20NiNbN) austenitic stainless steel. The modified HR3C shows good LCF properties, but lower creep and creep-fatigue properties which may due to the low ductility of the material. Secondly, LCF properties of a nickel based superalloy Haynes 282 have been studied. Tests have been performed for a large ingot. The LCF properties of the core and rim positions did not show evident differences. Better LCF properties were observed when compared with two other low γ’ volume fraction nickel based superalloys. Metallography study results demonstrated that the failure mode of the material was transgranular. Both the initiation and growth of the fatigue cracks were transgranular.

QC 20160905

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43

Ventakesh, T. A. 1970. "Reactive infiltration processing and compression creep of NiAl and NiAl composites." Thesis, Massachusetts Institute of Technology, 1998. http://hdl.handle.net/1721.1/50458.

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Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1998.
Includes bibliographical references (p. 116-118).
Reactive infiltration processing of bulk and composite NiAl was investigated with powder and wire preforms of nickel. Inhomogeneous microstructures were often obtained with powder preforms because their high surface-to-volume ratio, low permeability, and irregular infiltration paths lead to simultaneous infiltration and reaction. Homogenous NiAl could be obtained with nickel-wire preforms which had a lower surface-to-volume ratio, higher permeability, and regular infiltration paths, because infiltration was completed before the onset of reaction. Composites with continuous tungsten (W) and sapphire fibers were also successfully fabricated by reactive infiltration, while composites with molybdenum particulates and short-fibers showed significant dissolution in NiAl. The high-temperature uni-axial compression creep behavior of uni-directionally reinforced continuous fiber composite materials was investigated using NiAl-W as a model system for the case where both the NiAl matrix and the W fiber underwent plastic deformation by creep. The creep behavior of the constituents NiAl and W and NiAl composites reinforced with 5-20 volume % W was characterized at 1025 °C and 715 °C. At 1025°C, the NiAl-W composites exhibited three stage creep behavior with distinct primary, secondary, and tertiary creep, wherein the composite creep-rate decreased monotonically, remained constant, and increased rapidly, respectively. At 715C, the NiAl-W composites exhibited insignificant primary and tertiary creep but significant secondary creep. Microstructurally, primary and secondary creep were characterized by pure uni-axial compression of W fibers while brooming, bulging, buckling, and kinking were four fiber deformation modes that contributed to tertiary creep. The composite primary creep was modeled by solving for transient stress-states while loads transferred from the weaker phase (matrix) to the stronger phase (fiber) as the composite transitioned from the elastic state present at loading to steady-states attained at later times. The effects of primary creep of the constituents on the primary creep of the composite were also captured. Composite primary creep strains were predicted to be significant at high applied composite stresses and for high fiber volume fraction composites, while the composite primary time was uniquely related to the composite steady-state creep-rate by a power-law at a given temperature and for the stress range investigated. Good correlation between the primary creep model predictions and experiments was obtained when the observed composite steady-state creep behavior converged to the McLean steady-state. The composite secondary creep was observed to correlate reasonably well with the rule-of-mixtures model developed by McLean. The composite tertiary creep was modeled by solving for global or local kink-band evolution with composite deformation respectively contributing to fiber buckling or kinking. The model predicted the critical threshold strain for the onset of tertiary stage to be most sensitive to the initial kink angles while being relatively insensitive to the initial kink-band heights and varied inversely with the volume fraction of fiber in the composite. Reasonable correlation between the model and experiments was obtained when the observed composite steady-state correlated well with the McLean steady-state.
by T.A. Venkatesh.
Ph.D.

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44

Wang, Ziqiang Ph D. Massachusetts Institute of Technology. "Lithium deposition and stripping in solid-state battery via coble creep." Thesis, Massachusetts Institute of Technology, 2020. https://hdl.handle.net/1721.1/127717.

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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2020
Cataloged from the PDF of thesis.
Includes bibliographical references (pages 104-107).
Solid-state Li metal batteries require accommodation of electrochemically generated mechanical pressure inside Li metal. In this thesis it shows, through in situ transmission electron microscopy experiment of Li and Na deposition/stripping in mixed ionic-electronic conductor (MIEC) hollow tubules, an intriguing result that (a) Li metal can flow and retract inside 3D MIEC channels as a single crystal, (b) Coble creep dominates via interfacial diffusion along the MIEC/metal phase boundary, (c) this MIEC electrochemical tubular matrix can effectively relieve stress, maintain electronic and ionic contact, eliminate solid-electrolyte interphase (SEI) debris, reduce the possibility of "dead lithium", and allow the reversible deposition/stripping of Li metal across a distance of many microns, for 100 cycles. This thesis proposes quantitative design rules for MIEC electrochemical cell and shows that interfacial diffusion greatly liberates MIEC material choices when using ~100 nm wide and 10-100[mu]m deep channels. A centimeter-scale, ~10¹⁰ MIEC cylinders/solid electrolyte/LiFePO₄ full cell shows high capacity of ~ 164 mAh/g(LiFePO₄ and almost no degradation for over 50 cycles, starting with 1x excess Li.
by Ziqiang Wang.
Ph. D.
Ph.D. Massachusetts Institute of Technology, Department of Materials Science and Engineering

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45

FARINA, LUIS C. "Caracterizacao viscoelastica por meio de ensaios de fluencia e ruptura por fluencia de compositos polimericos de matriz de resina epoxidica e fibra de carbono." reponame:Repositório Institucional do IPEN, 2009. http://repositorio.ipen.br:8080/xmlui/handle/123456789/9391.

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Made available in DSpace on 2014-10-09T12:26:27Z (GMT). No. of bitstreams: 0
Made available in DSpace on 2014-10-09T14:04:31Z (GMT). No. of bitstreams: 0
Dissertacao (Mestrado)
IPEN/D
Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

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46

Ibanez, Alejandro R. "Modeling creep behavior in a directionally solidified nickel base superalloy." Diss., Georgia Institute of Technology, 2003. http://hdl.handle.net/1853/5353.

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47

Magnusson, Hans. "Creep modelling of particle strengthened steels." Doctoral thesis, KTH, Materialteknologi, 2010. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-12235.

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Materials used in thermal power plants have to resist creep deformation for time periods up to 30 years. Material evaluation is typically based on creep testing with a maximum duration of a few years. This information is used as input when empirically deriving models for creep. These kinds of models are of limited use when considering service conditions or compositions different from those in the experiments. In order to provide a more general model for creep, the mechanisms that give creep strength have to be identified and fundamentally described. By combining tools for thermodynamic modelling and modern dislocation theory the microstructure evolution during creep can be predicted and used as input in creep rate modelling. The model for creep has been utilised to clarify the influence of aluminium on creep strength as a part of the European COST538 action. The results show how AlN is formed at the expense of MX carbonitrides. The role of heat treatment during welding has been analysed. It has been shown that particles start to dissolve already at 800ºC, which is believed to be the main cause of Type IV cracking in commercial alloys. The creep strength of these steels relies on minor additions of alloying elements. Precipitates such as M23C6 carbides and MX carbonitrides give rise to the main strengthening, and remaining elements produce solid solution hardening. Particle growth, coarsening and dissolution have been evaluated. By considering dislocation climb it is possible to determine particle strengthening at high temperatures and long-term service. Transient creep is predicted by considering different types of dislocations. Through the generation and recovery of dislocation densities an increase in work hardening during primary creep is achieved. The role of substructure is included through the composite model. Cavity nucleation and growth are analysed in order to explain the intergranular fracture and to estimate the ductility.
QC20100616

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48

Magnusson, Hans. "Creep modelling of particle strengthened steels." Licentiate thesis, Stockholm : Kungliga Tekniska högskolan, 2007. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-4436.

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49

Parks, Carl L. "An experimental approach for studying the creep behavior of thin film/ substrate interfaces." Thesis, Monterey, Calif. : Springfield, Va. : Naval Postgraduate School ; Available from National Technical Information Service, 2004. http://library.nps.navy.mil/uhtbin/hyperion/04Sep%5FParks.pdf.

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50

Al-Shakarchi, M. Y. O. "Effect of ambient temperature and thermal-cycle conditioning on the deformation of bituminous pavement materials." Thesis, University of Bradford, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.380579.

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Dissertations / Theses on the topic 'Materials – Creep'

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