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Research Papers: Design and Analysis

Effects of Prior Stress Relaxation on the Prediction of Creep Life Using Time and Strain Based Methods

[+] Author and Article Information
Ken U. Snowden

Australian Nuclear Science and Technology
Organisation (ANSTO),
Locked Bag 2001,
Kirrawee 2234 NSW, Australia

David W. Dean

EDF,
British Energy,
Assessment Technology Group,
Barnett Way,
Barnwood, Gloucester GL4 3RS, UK

Contributed by the Pressure Vessel and Piping Division of ASME for publication in the Journal of Pressure Vessel Technology. Manuscript received February 24, 2011; final manuscript received December 12, 2012; published online June 11, 2013. Assoc. Editor: Osamu Watanabe.

J. Pressure Vessel Technol 135(4), 041201 (Jun 11, 2013) (8 pages) Paper No: PVT-11-1061; doi: 10.1115/1.4023726 History: Received February 24, 2011; Revised December 12, 2012

A critical requirement for both next generation conventional and nuclear plants is the development of simplified inelastic design and fitness for purposes procedures that give a reasonably accurate prediction of the complex multiaxial time dependent stress strain behavior. The accumulation of this inelastic strain in the form of coupled creep-fatigue damage over time is one of the principal damage mechanisms which will eventually lead to crack initiation in critical, high temperature equipment. Two main procedures that address creep-fatigue loading are generally used, either a time fraction or a ductility exhaustion approach. It is generally accepted that these methods enable conservative predictions within a factor of 2 to 3 and hence are reliable methods for code based design and fitness for purpose type assessments. However, for complex cycles, this may not be the case; for example, prior relaxation cycles are found to accelerate the creep rupture of the material with the result that a significant reduction in creep life can be observed. An investigation was undertaken into the influence of prior relaxation on resultant failure using a typical low alloy ferritic power station steel. Both time-based and strain based methods were used to predict the damage caused by the stress relaxation cycles followed by operation at steady state. The predictions found that while ductility exhaustion methodologies based on mean properties were adequate in predicting the failure life, time fraction methods were found to be extremely nonconservative for mean properties and only lower bound solutions provided an estimate of remaining creep life. The ASME time fraction approach, using isochronous curves was found to be extremely conservative for K = 0.67, but was able to predict similar damages to ductility exhaustion when K = 1 was used. The Monkman-Grant approach resulted in predictions that erred on the conservative side. The results have implications for both current and future conventional and nuclear power stations as it may be difficult for time based approaches to account accurately for complex cycling, shakedown conditions or stress relaxation at welds.

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References

Figures

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Fig. 1

Fitted creep rupture data for P22 at 550 °C

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Fig. 2

The ductility versus average creep rate for 2.25Cr-1Mo steels at 550 °C. εmin = 0.03 m/m

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Fig. 3

Change in initial dwell and relaxed stress as a function of cycle history

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Fig. 4

Creep curves for 2.25Cr-1Mo steel at 550 °C and 100 MPa and 140 MPa with and without prior stress-relaxation testing at 550 °C

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Fig. 5

Rupture stress versus rupture life for P22 at 550 °C with and without PSRC with the curves for the AJB2 medium and lower bound equations

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Fig. 6

Rupture ductility versus rupture life at 550 °C for P22 with and without PSRC plotted against the experimental rupture set

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Fig. 8

Average predicted lives of all methods

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Fig. 7

Rupture ductility versus rupture life at 550 °C for P22 with and without PSRC plotted against the experimental rupture set

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