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Fundamental models and testing of creep in copper
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.ORCID iD: 0000-0002-2144-6402
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Many sustainable technologies for energy production, for example, generation IV nuclear system, demand the use of materials operating at elevated temperatures for long duration of up to 60 years. Requirements that are even more stringent are found for creep exposed copper canisters for disposal of spent nuclear waste. The canisters should stay intact for thousands of years. Traditional design procedures that involve empirical extrapolation of creep data are no longer reliable for such extended times. Instead physically based material models have to be used.

The final stage of creep before rupture, tertiary creep has been handled with empirical methods with adjustable parameters in the past, which makes it difficult to safely identify the controlling mechanisms. A physically based model has been developed for copper taking the substructure, cavitation and necking into account.

To improve the understanding of the important contribution from particles to the creep strength an earlier formulated model has analyzed and further developed. The model has successfully been able to describe the temperature and stress dependence of precipitation hardening for copper-cobalt alloys, where this contribution totally dominates the creep strength.

Multiaxial stress states are crucial for practically all high temperature applications. Fundamental material models have been extended for such conditions. These models have been compared with strain and stress controlled tests for notched specimens that have been performed.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2018. , p. 149
Series
TRITA-ITM-AVL ; 2018:25
Keywords [en]
Copper; Creep tests; Multiaxial stress state; Finite element method; Basic modelling; Tertiary creep; Precipitation hardening
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:kth:diva-227968ISBN: 978-91-7729-773-4 (print)OAI: oai:DiVA.org:kth-227968DiVA, id: diva2:1205956
Public defence
2018-06-12, Kollegiesalen, Brinellvägen 8, floor 4, stockholm, 10:00 (English)
Opponent
Supervisors
Available from: 2018-05-17 Created: 2018-05-15 Last updated: 2019-08-20Bibliographically approved
List of papers
1. Slow strain rate tensile tests on notched specimens of copper
Open this publication in new window or tab >>Slow strain rate tensile tests on notched specimens of copper
2016 (English)In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 663, p. 108-115Article in journal (Other academic) Published
Abstract [en]

In this study, slow strain rate tensile tests have been performed on phosphorus alloyed copper under uniaxial and multiaxial stress states at 75 and 125 °C with two strain rates 10-6 and 10-7 s-1. Multiaxial stress states have been introduced by incorporating three different notch geometries on the uniaxial specimens. It has shown that the presence of the notches decreased the strength and ductility of copper. Ductility exhaustion was likely to be the dominant rupture mechanism. Finite element analysis was conducted to compare with the experimental results with a physically based model for stress strain flow curves without fitting parameters. The model could successfully describe the experimental data, and it could predict the dependence of acuity, temperature and strain rate in the multiaxial tests.

Place, publisher, year, edition, pages
Elsevier, 2016
Keywords
Copper, Finite element method, Multiaxial stress state, Notched specimen, Slow strain rate tensile test, Curve fitting, Ductility, Stress-strain curves, Tensile testing, Ductility exhaustion, Fitting parameters, Notched specimens, Physically based modeling, Rupture mechanism, Strength and ductilities, Strain rate
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-186988 (URN)10.1016/j.msea.2016.03.111 (DOI)000375499500014 ()2-s2.0-84961999795 (Scopus ID)
Note

QC 20160524

Available from: 2016-05-24 Created: 2016-05-16 Last updated: 2018-05-16Bibliographically approved
2. Basic modelling of tertiary creep of copper
Open this publication in new window or tab >>Basic modelling of tertiary creep of copper
2018 (English)In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 53, no 9, p. 6850-6863Article in journal (Refereed) Published
Abstract [en]

Mechanisms that are associated with acceleration of the creep rate in the tertiary stage such as microstructure degradation, cavitation, necking instability and recovery have been known for a long time. Numerous empirical models for tertiary creep exist in the literature, not least to describe the development of creep damage, which is vital for understanding creep rupture. Unfortunately, these models almost invariably involve parameters that are not accurately known and have to be fitted to experimental data. Basic models that take all the relevant mechanisms into account which makes them predictive have been missing. Only recently, quantitative basic models have been developed for the recovery of the dislocation structure during tertiary creep and for the formation and growth of creep cavities. These models are employed in the present paper to compute the creep strain versus time curves for copper including tertiary creep without the use of any adjustable parameters. A satisfactory representation of observed tertiary creep has been achieved. In addition, the role of necking is analysed with both uniaxial and multiaxial methods.

Place, publisher, year, edition, pages
SPRINGER, 2018
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-223768 (URN)10.1007/s10853-017-1968-7 (DOI)000424874900048 ()2-s2.0-85040081835 (Scopus ID)
Funder
Swedish Nuclear Fuel and Waste Management Company, SKB, 16884
Note

QC 20180307

Available from: 2018-03-07 Created: 2018-03-07 Last updated: 2018-05-16Bibliographically approved
3. Fundamental Modelling of Mechanisms Contributing to Tertiary Creep in Copper AT 215 and 250°C
Open this publication in new window or tab >>Fundamental Modelling of Mechanisms Contributing to Tertiary Creep in Copper AT 215 and 250°C
2018 (English)In: Proceedings of the ASME 2018 Pressure Vessels and Piping Conference, 2018Conference paper, Published paper (Refereed)
Abstract [en]

Extensive creep tests have been performed on oxygen free copper with 50 ppm phosphorus at both low and high temperatures. It is the candidate material for storage of spent nuclear fuel in Sweden. Basic models without fitting parameters have been formulated to reproduce primary and secondary creep. For a long time, only empirical models existed for fitting of tertiary creep. To understand the role of creep damage, including recovery, cavitation and necking, basic models that do not involve adjustable parameters are in urgent demand. Only recently, basic models taking the relevant mechanisms into account have been developed. These models were used to predict the tertiary creep for copper at 75°C. The modelled results were compared with experimental creep curves and good agreement has been found. In the present paper, the models are applied to creep tests at higher temperatures (215 and 250°C). A similar representation with good accuracy is obtained. This demonstrates that the fundamental model for back stress is applicable for the higher temperature tests as well. 

National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-228012 (URN)2-s2.0-85056835819 (Scopus ID)
Conference
ASME 2018 Pressure Vessels and Piping Conference, July 15-20, 2018, Prague, Czech Republic
Note

QC 20180523

Available from: 2018-05-16 Created: 2018-05-16 Last updated: 2019-04-02Bibliographically approved
4. Creep Strength Contribution due to Precipitation Hardening in Copper-Cobalt Alloys
Open this publication in new window or tab >>Creep Strength Contribution due to Precipitation Hardening in Copper-Cobalt Alloys
(English)Manuscript (preprint) (Other academic)
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-228009 (URN)
Note

QC 20180523

Available from: 2018-05-16 Created: 2018-05-16 Last updated: 2018-05-23Bibliographically approved
5. Creep Tests on Notched Specimens of Copper
Open this publication in new window or tab >>Creep Tests on Notched Specimens of Copper
2018 (English)Manuscript (preprint) (Other academic)
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-228007 (URN)
Note

QC 20180523

Available from: 2018-05-16 Created: 2018-05-16 Last updated: 2018-05-23Bibliographically approved

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