20 results
Search Results
2. Damage assessment in structural metallic materials for advanced nuclear plants.
- Author
-
Hoffelner, Wolfgang
- Subjects
COGENERATION of electric power & heat ,ALLOYS ,NUCLEAR fuel claddings ,IRRADIATION ,METALLIC composites ,CREEP (Materials) ,STRAINS & stresses (Mechanics) - Abstract
Future advanced nuclear plants are considered to operate as cogeneration plants for electricity and heat. Metals and alloys will be the main portion of structural materials employed (including fuel claddings). Due to the operating conditions these materials are exposed to damaging conditions like creep, fatigue, irradiation and its combinations. The paper uses the most important alloys: ferritic-martensitic steels, superalloys, oxide dispersion strengthened steels and to some extent titanium aluminides to discuss its responses to these exposure conditions. Extrapolation of stress rupture data, creep strain, swelling, irradiation creep and creep–fatigue interactions are considered. Although the stress rupture- and the creep behavior seem to meet expectations, the long design lives of 60 years are really challenging for extrapolations and particularly questions like negligible creep or occurrence of diffusion creep need special attention. Ferritic matrices (including oxide dispersion strengthened (ODS), steels) have better irradiation swelling behavior than austenites. Presence and size of dispersoids having a strong influence on high-temperature strength bring only insignificant improvements in irradiation creep. A strain-range-separation based approach for creep–fatigue interactions is presented which allows a real prediction of creep–fatigue lives. An assessment of capabilities and limitations of advanced materials modeling tools with respect to damage development is given. [ABSTRACT FROM AUTHOR]
- Published
- 2010
- Full Text
- View/download PDF
3. A comparative assessment of creep property predictions for a 1CrMoV rotor steel using the CRISPEN, CDM, Omega and Theta projection techniques.
- Author
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Evans, M.
- Subjects
ROTORS ,STEEL ,CREEP (Materials) ,STRAINS & stresses (Mechanics) ,MECHANICS (Physics) ,PRESSURE - Abstract
This paper assesses the relative accuracy of a number of techniques that are capable of predicting a wide range of creep properties. The techniques studied in this paper include the 4-Θ, 6-θ, CRISPEN, Omega and CDM methodologies. The parameters of these models were estimated from short-term creep property data on 1CrMoV steel and these estimated models were then used to predict the known longer-term creep properties of this steel. It was found that the CDM approach yielded predicted times to failure that were wholly inadequate. The 6-θ approach was best for predicting long term times to failure. The best minimum creep rate predictions came from using either the 4-Θ, or the CRISPEN or the Omega technique. Finally, times to small strains were best predicted using the 6-θ and CRISPEN techniques. [ABSTRACT FROM AUTHOR]
- Published
- 2004
- Full Text
- View/download PDF
4. Predicting times to low strain for a 1CrMoV rotor steel using a 6-θ projection technique.
- Author
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Evans, M.
- Subjects
CREEP (Materials) ,STEEL analysis ,STRAINS & stresses (Mechanics) ,PREDICTION models ,PARAMETER estimation - Abstract
The θ projection method of creep analysis is known to produce the poorest predictions of creep properties at low strains. This paper applies a recently suggested modification of the θ concept to 1CrMoV rotor steel where long term data exists to enable an assessment of this modification to be made. The modification takes the form of two additional θ terms that allow the initial stages of any creep curve to be modelled more accurately. The paper shows that the resulting 6-θ approach produces predictions of long-term failure times and minimum creep rates that are as good as those obtained using the traditional 4-θ approach. Unlike the 4-θ approach, the 6-θ approach is also shown to be capable of accurately predicting times to very low strains (0.05% and 0.1%) at stress levels as low as 77 MPa (well below the lowest stress of 230 MPa used in the theta analysis). For times to 1.0% strain or more the 4-θ and 6-θ techniques give similar short and long-term predictions. [ABSTRACT FROM AUTHOR]
- Published
- 2000
- Full Text
- View/download PDF
5. Rubber toughening of plastics Part XIII Dilatational yielding in PA6.6/EPR blends.
- Author
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Bucknall, C. B. and Lazzeri, A.
- Subjects
ETHYLENE-propylene rubber ,POLYAMIDES ,STRAINS & stresses (Mechanics) ,CAVITATION ,CREEP (Materials) ,MATERIALS science - Abstract
Tensile tests were carried out at both constant stress and constant extension rate on polyamide 6.6 blends containing 0–20 wt% of maleinated ethylene-propylene rubber (EPR-g-ma), using specimens conditioned at 50% RH. Longitudinal and lateral strain measurements on creep specimens showed that blends containing EPR-g-ma exhibited dilatation at tensile stresses above 27 MPa. This observation is consistent with the predictions of the particle cavitation model proposed in an earlier paper (A. Lazzeri and C. B. Bucknall, J. Mater. Sci. 28 (1993) 6799). In standard tensile tests on PA6.6/EPR-g-ma blends, cavitation preceded yielding (defined by the 1% offset strain). Creep and yield data obtained over a range of temperatures and strain rates were correlated using the Eyring equation. The onset of dilatation in these PA6.6/EPR blends was marked by a change in deformation kinetics, which is analysed using a modified version of Gurson's equation. [ABSTRACT FROM AUTHOR]
- Published
- 2000
- Full Text
- View/download PDF
6. L12-phase cutting during high temperature and low stress creep of a Re-containing Ni-base single crystal superalloy.
- Author
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Kostka, A., Mälzer, G., Eggeler, G., Dlouhy, A., Reese, S., and Mack, T.
- Subjects
HEAT resistant alloys ,DISLOCATIONS in metals ,CREEP (Materials) ,STRAINS & stresses (Mechanics) ,DEFORMATIONS (Mechanics) - Abstract
In the present study, we investigate dislocation processes in a Ni-base single crystal superalloy (LEK94) after high temperature and low stress creep. Specimens were creep deformed at 240 MPa and 980 °C to a strain of 20%. We use diffraction contrast transmission electron microscopy (TEM) to show that two γ-channel dislocations with different Burgers vectors combine and form a super dislocation that shears the γ′-particle. This type of cutting event has now been observed for three single crystal superalloys with different alloy chemistry (CMSX-4, CMSX-6 and LEK94 in the present work) and we therefore conclude that it represents a generic elementary dislocation process which governs high temperature (around 1000 °C) and low stress (around 240 MPa) creep. The present paper provides microstructural evidence for this type of cutting processes and discusses the results in the light of previous work published in the literature. [ABSTRACT FROM AUTHOR]
- Published
- 2007
- Full Text
- View/download PDF
7. Fifty years of Harper–Dorn creep: a viable creep mechanism or a Californian artifact?
- Author
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Kumar, Praveen, Kassner, Michael E., and Langdon, Terence G.
- Subjects
CREEP (Materials) ,BULK solids flow ,ALUMINUM ,STRAINS & stresses (Mechanics) ,TEMPERATURE - Abstract
Fifty years ago, in a series of classic creep experiments conducted at the University of California in Berkeley, Harper and Dorn obtained unique experimental data revealing the possibility of a new and heretofore unrecognized flow process occurring in pure aluminum when tested at low stresses and at temperatures very close to the melting temperature. This flow mechanism, subsequently designated Harper–Dorn creep, has been the center of much argument and speculation in the ensuing years. The present paper looks back over the last half-century and charts the various developments in attempts to obtain a more detailed understanding of whether Harper–Dorn creep is (or is not) a viable creep process. Examples are presented for both metals and non-metals. It is concluded that, although it appears Harper–Dorn creep may occur only under restricted conditions associated with high purity materials and low initial dislocation densities, nevertheless there is good evidence supporting the validity of this creep mechanism as a viable and unique flow process. [ABSTRACT FROM AUTHOR]
- Published
- 2007
- Full Text
- View/download PDF
8. Room temperature creep and strain-rate-dependent stress-strain behavior of pipeline steels.
- Author
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Sheng-Hui Wang, Yonggang Zhang, and Weixing Chen
- Subjects
CREEP (Materials) ,TEMPERATURE ,PIPELINES ,STRAINS & stresses (Mechanics) ,ANNEALING of metals - Abstract
Creep deformation and the effect of strain-rate on stress-strain behavior of X-52, X-70 and X-80 pipeline steels at room temperature were studied using round tension test specimens. Depending on its chemical composition and the processing condition (as-received or fully annealed), a pipeline steel may exhibit a stress-strain curve with or without a yield point. The as-received and the annealed steels with both type of yielding behavior were creep tested at a constant stress either below or past the yield point / 0.2% offset yield strength. Independent of yielding behavior, significant post-yield creep deformation was observed in all the steels. The pre-yield creep, however, is strongly dependent on the yielding behavior. In the presence of a yield point, only a minor deformation was detected in the steels subject to the pre-yield creep. In the absence of a yield point, pre-yield creep deformation occurred to a relatively large extent. For the latter case, an annealing treatment further enhanced creep deformation. A strain-rate-dependent stress-strain behavior was also observed in all thesteels that show significant creep deformation. Dislocation mechanisms responsible for the creep behavior observed in the study are also provided in the paper. [ABSTRACT FROM AUTHOR]
- Published
- 2001
- Full Text
- View/download PDF
9. Comparison of the stress relaxation and creep behavior of conventionally forged and additively manufactured René 65.
- Author
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Tiparti, Dhruv, Wessman, Andrew, Cormier, Jonathan, and Tin, Sammy
- Subjects
STRAINS & stresses (Mechanics) ,FATIGUE limit ,HEAT treatment ,STRESS relaxation (Mechanics) ,CREEP (Materials) ,MICROSTRUCTURE ,HEAT resistant alloys - Abstract
René 65 is a high strength cast & wrought (CW) Ni-base superalloy with thermomechanical performance tailored toward intermediate temperature and stress conditions (677 °C/690 MPa). René 65 Turbine disk rotor forgings are typically subject to a sub-solvus heat treatment utilized to retain a uniform and fine-grained microstructure that confers a high level of fatigue performance and strength at the expense of high-temperature creep resistance. However, the processing of René 65 via laser powder bed fusion (LPBF) can enable this material to exhibit intermediate or coarse-grained microstructures after heat treatment that possess improved creep resistance. The effect of the difference in the processing route in conjunction with heat treatment on the resulting microstructure is presented and correlated to the creep behavior (at 677 °C/690 MPa) and stress relaxation behavior (700 °C). A good correlation of relaxation and creep behaviors was found, with LPBF processed René 65 exhibiting overall improved resistance to high-temperature deformation. The primary factor with regards to microstructural difference toward the stress relaxation/creep behavior of René 65 has been identified to be the size and volume fraction of tertiary γ' present resulting from the chosen processing pathway. [ABSTRACT FROM AUTHOR]
- Published
- 2023
- Full Text
- View/download PDF
10. Time dependent recovery of oriented polyethylene.
- Author
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Gordeyev, S. and Ward, I.
- Subjects
POLYETHYLENE ,CREEP (Materials) ,MOLECULAR weights ,STRAINS & stresses (Mechanics) ,DYNAMICS - Abstract
The recovery behaviour after creep of oriented linear polyethylenes has been studied over the temperature range 20–60 °C. A range of samples was examined to identify the influence of draw ratio and molecular weight. It has been shown that in spite of significant differences in recovery strain level, the recovery kinetics are not affected over a wide range of the structural variations and experimental conditions. It is concluded that the time dependent recovery behaviour is consistent with a model where two thermally activated processes are acting in parallel. More exact values for the activation parameters for both processes of the model have been obtained by taking into account the time dependent distribution of the applied stress between these two processes. [ABSTRACT FROM AUTHOR]
- Published
- 1999
- Full Text
- View/download PDF
11. Viscoelastically generated prestress from ultra-high molecular weight polyethylene fibres.
- Author
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Fazal, Adnan and Fancey, Kevin
- Subjects
VISCOELASTICITY ,POLYETHYLENE fibers ,POLYMERIC composites ,VISCOELASTIC materials ,CREEP (Materials) ,STRAINS & stresses (Mechanics) - Abstract
The viscoelastic characteristics of ultra-high molecular weight polyethylene (UHMWPE) fibres are investigated, in terms of creep-induced recovery strain and force output, to evaluate their potential for producing a novel form of prestressed composite. Composite production involves subjecting fibres to tensile creep, the applied load being removed before moulding the fibres into a resin matrix. After matrix curing, the viscoelastically strained fibres impart compressive stresses to the surrounding matrix, to produce a viscoelastically prestressed polymeric matrix composite (VPPMC). Previous research has demonstrated that nylon fibre-based VPPMCs can improve mechanical properties without needing to increase mass or section dimensions. The viability of UHMWPE fibre-based VPPMCs is demonstrated through flexural stiffness tests. Compared with control (unstressed) counterparts, these VPPMCs typically show increases of 20-40 % in flexural modulus. Studies on the viscoelastic characteristics indicate that these fibres can release mechanical energy over a long-timescale and fibre core-skin interactions may have an important role. [ABSTRACT FROM AUTHOR]
- Published
- 2013
- Full Text
- View/download PDF
12. The variation of beta phase morphology after creep and negative creep for duplex titanium alloys.
- Author
-
Wang, Shing-Hoa, Lee, Hao-Hsun, Chen, Chih-Yuan, Yang, Jer-Ren, and Kao, Chin-Hai
- Subjects
CREEP (Materials) ,METAL creep ,TITANIUM alloys ,STRAINS & stresses (Mechanics) ,INTERNAL friction ,OSTWALD ripening ,MATERIALS science - Abstract
The creep resistance of SP700 (Ti–4.5Al–3V–2Mo–2Fe) is superior to Ti–6–4 (Ti–6Al–4V) at 500 °C under a constant load corresponding to an initial stress of 100 MPa. The β phase grains in the SP700 alloy prefer to orient along the loading axis in contrast to the Ti–6–4 alloy. The grain growth occurs during the stress drop incubation period. The observation of different amounts of negative creep/anelasticity upon loading is closely associated with the difference in the amount of grain/subgrain coarsening. [ABSTRACT FROM AUTHOR]
- Published
- 2009
- Full Text
- View/download PDF
13. Visco-elasticity of aramid fibres.
- Author
-
Burgoyne, C. and Alwis, K.
- Subjects
VISCOELASTICITY ,SYNTHETIC fibers ,CREEP (Materials) ,STRESS relaxation (Mechanics) ,NONLINEAR mechanics ,STRAINS & stresses (Mechanics) - Abstract
Tests are described to measure the creep and relaxation response of aramid fibres with the specific aim of determining whether the visco-elastic response is linear or non-linear. Hitherto, creep and relaxation tests have been carried out in different circumstances and at different loads, which has led to disagreement about the type of response that aramid fibres exhibit. Tests are carried out at stresses between 10% and 80% of the short-term strength of the fibres under controlled temperature and humidity conditions, and it is shown that both creep and relaxation are non-linear at stresses below 40% of the breaking load, but both are linear at stresses above this level. This result explains the contradictions in earlier work and also indicates that there may be two different processes underway in the visco-elasticity of aramids. [ABSTRACT FROM AUTHOR]
- Published
- 2008
- Full Text
- View/download PDF
14. Importance of dislocation cores in fatigue fracture.
- Author
-
Gilman, J.
- Subjects
BRITTLENESS ,MATERIAL fatigue ,FRACTURE mechanics ,DEFORMATIONS (Mechanics) ,ELASTIC solids ,CREEP (Materials) ,STRESS-strain curves ,STRAINS & stresses (Mechanics) - Abstract
It is pointed out that proposed mechanisms of fatigue fracture should be consistent with Coffin’s Law for constant strain-amplitude cyclic deformation. Most proposed mechanisms are not. The fracture of dislocation cores along their glide planes is consistent with the fact that the law describes the behavior during the first ¼ cycle as well as that after a very large number of cycles. Experimental evidences of the relative weakness of edge dislocation cores is described. Analysis of this weakness by Bullough, and in this article, discussed. It is concluded that this effect plays an important role in fatigue fracture. [ABSTRACT FROM AUTHOR]
- Published
- 2008
- Full Text
- View/download PDF
15. Superplastic power-law creep of Sn–40%Pb–2.5%Sb peritectic alloy.
- Author
-
Mahmudi, R. and Rezaee-Bazzaz, A.
- Subjects
STRAINS & stresses (Mechanics) ,CREEP (Materials) ,ALLOYS ,DEFORMATIONS (Mechanics) ,STRUCTURAL analysis (Engineering) - Abstract
The power law-creep behavior of superplastic Sn–40Pb–2.5Sb alloys with different grain sizes has been investigated at room temperature. Stress exponent values for these alloys have been determined by indentation creep, conventional creep and uniaxial tension tests in order to evaluate the correspondence of indentation creep results with conventional tests. In all cases, the indentation results were in good agreement with each other and with those of the tensile and conventional creep tests. The average stress exponent values of about 2.6 and 3.0 corresponding to the strain rate sensitivity (SRS) indices of 0.33–0.39, depending on the grain size of the materials, indicate that the grain boundary sliding is the possible mechanism during creep deformation of Sn–Pb–Sb alloys. Within limits, the indentation tests are thus considered useful to acquire information on the creep behavior of small specimens of these soft tin–lead–antimony alloys at room temperature. It is also demonstrated that the indentation creep test provides a convenient method to measure SRS and thereby to assess the ability of a material to undergo superplastic deformation. [ABSTRACT FROM AUTHOR]
- Published
- 2007
- Full Text
- View/download PDF
16. Nonlinear creep behavior of viscoelastic polycarbonate.
- Author
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Jazouli, Said, Luo, Wenbo, Brémand, Fabrice, and Vu-Khanh, Toan
- Subjects
POLYCARBONATES ,VISCOELASTICITY ,CREEP (Materials) ,AXIAL flow ,STRAINS & stresses (Mechanics) ,VISCOSITY - Abstract
A study of the tension behavior of polycarbonate (PC) under room temperature and various sustained loads is presented. Time-dependent axial elongations and transverse contractions of the specimen were simultaneously measured at nine different stress levels, from 15.89 to 59.4 MPa, and modeled according to a time-stress superposition principle. The test duration was only one hour. It was shown that creep compliance vs. log time curves at different stresses can be horizontally shifted to form a smooth master curve for one year at a reference stress of 30.97 MPa. Moreover, the stress shift factors for axial extension creep curves and transverse contraction creep curves are found to be identical for the stress levels considered, and this is verified using the Poisson's ratio measurements. [ABSTRACT FROM AUTHOR]
- Published
- 2006
- Full Text
- View/download PDF
17. A critical damage criterion for creeping solids.
- Author
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Phaniraj, C., Choudhary, B., Raj, Baldev, and Rao, K.
- Subjects
STRAINS & stresses (Mechanics) ,CREEP (Materials) ,DUCTILITY ,TRANSPARENT solids ,MECHANICS (Physics) ,PRESSURE - Abstract
The article cites a critical damage criterion for creeping solids. Creep damage is the progressive reduction in the material's ability to resist stress and is manifested as an increase in creep rate during tertiary creep eventually resulting in failure. In this article, authors identify that Monkman-Grant Ductility (MGD) is the critical strain at which true tertiary creep damage sets in and further introduces the concept of the time at which MGD is reached along the creep curve and the total secondary creep ductility is exhausted. Researchers concluded that the end of first stage corresponds to the time at which creep cavities attain a critical size and the second stage is the true tertiary creep.
- Published
- 2005
- Full Text
- View/download PDF
18. Rate-dependent fracture toughness of pure polycrystalline ice.
- Author
-
Xu, X., Jeronimidis, G., Atkins, A. G., and Trusty, P. A.
- Subjects
ICE ,POLYCRYSTALS ,STRAINS & stresses (Mechanics) ,MECHANICS (Physics) ,FRACTURE mechanics ,MATERIAL plasticity ,CREEP (Materials) - Abstract
A series of three-point bend tests using single edge notched testpieces of pure polycrystalline ice have been performed at three different temperatures (-20°C, -30°C and -40°C). The displacement rate was varied from 1 mm/min to 100 mm/min, producing the crack tip strain rates from about 10
-3 to 10-1 s-1 . The results show that (a) the fracture toughness of pure polycrystalline ice given by the critical stress intensity factor (KIC ) is much lower than that measured from the J—integral under identical conditions; (b) from the determination of KIC , the fracture toughness of pure polycrystalline ice decreases with increasing strain rate and there is good power law relationship between them; (c) from the measurement of the J—integral, a different tendency was appeared: when the crack tip strain rate exceeds a critical value of 6 × 10-3 s-1 , the fracture toughness is almost constant but when the crack tip strain rate is less than this value, the fracture toughness increases with decreasing crack tip strain rate. Re-examination of the mechanisms of rate-dependent fracture toughness of pure polycrystalline ice shows that the effect of strain rate is related not only to the blunting of crack tips due to plasticity, creep and stress relaxation but also to the nucleation and growth of microcracks in the specimen. [ABSTRACT FROM AUTHOR]- Published
- 2004
- Full Text
- View/download PDF
19. Cavitation asymmetry in silicon nitride by scanning laser acoustic microscopy.
- Author
-
Lofaj, F. and Luprano, V.
- Subjects
SILICON nitride ,CAVITATION ,ASYMMETRY (Chemistry) ,CREEP (Materials) ,DEFORMATIONS (Mechanics) ,STRAINS & stresses (Mechanics) ,ACOUSTIC microscopy - Abstract
Scanning laser acoustic microscopy (SLAM) was used to visualize creep damage distribution in the gas-pressure-sintered silicon nitride after creep at 1300°C in 4-point bending. SLAM revealed asymmetrical distribution of creep damage beneath the tensile surface and in a narrow zone that spread continuously across the neutral axis toward the compression surface. Scanning and transmission electron microscopy confirmed the presence of cavities in tensile zone of bending bar. The combination of the homogeneous cavity development in the zone of tensile stresses and formation of the damage zone ahead of main crack was proposed to explain such cavity distribution. Current SLAM observation is a direct evidence of cavitation asymmetry in vitreous bonded ceramics. Such distribution supports the model of simultaneous presence of cavitation and non-cavitation creep mechanisms in silicon nitride and similar ceramics. [ABSTRACT FROM AUTHOR]
- Published
- 2003
- Full Text
- View/download PDF
20. Application of an ultrasonic technique to creep cavitation in silicon nitride.
- Author
-
Cao, J. W., Lofaj, F., and Okada, A.
- Subjects
SILICON nitride ,ULTRASONIC waves ,ELASTICITY ,CAVITATION ,CREEP (Materials) ,TESTING-machines ,STRAINS & stresses (Mechanics) - Abstract
A non-destructive method based on measurements of ultrasonic wave velocities and Young's modulus is proposed for quantification of creep cavitation in silicon nitride. Tensile creep tests of silicon nitride were conducted at 1400°C in air and the tests were periodically interrupted to measure the longitudinal and transverse ultrasonic wave velocities and Young's modulus. The velocities and Young's modulus decreased linearly with tensile creep strain. The volume fraction of cavities was estimated from the values of the ultrasonic wave velocities and Young's modulus, and compared with the cavity volume predicted from tensile creep strain. The dependence of Young's modulus on volume fraction of cavities is discussed. [ABSTRACT FROM AUTHOR]
- Published
- 2001
- Full Text
- View/download PDF
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