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Research Papers: Materials and Fabrication

Development of Creep-Fatigue Evaluation Method for Modified 9Cr-1Mo Steel

[+] Author and Article Information
Shigeru Takaya

Japan Atomic Energy Agency,
4002 Narita, O-arai,
Ibaraki 311-1393, Japan
e-mail: takaya.shigeru@jaea.go.jp

Yuji Nagae, Tai Asayama

Japan Atomic Energy Agency,
4002 Narita, O-arai,
Ibaraki 311-1393, Japan

1Corresponding author.

Contributed by the Pressure Vessel and Piping Division of ASME for publication in the JOURNAL OF PRESSURE VESSEL TECHNOLOGY. Manuscript received November 25, 2013; final manuscript received January 11, 2014; published online February 27, 2014. Assoc. Editor: Marina Ruggles-Wrenn.

J. Pressure Vessel Technol 136(3), 031404 (Feb 27, 2014) (8 pages) Paper No: PVT-13-1199; doi: 10.1115/1.4026497 History: Received November 25, 2013; Revised January 11, 2014

This paper describes a creep–fatigue evaluation method for modified 9Cr-1Mo steel, which has been newly included in the 2012 edition of the Japan Society of Mechanical Engineers code for design and construction of fast reactors (JSME FRs code). In this method, creep and fatigue damages are evaluated on the basis of Miner's rule and the time fraction rule, respectively, and the linear summation rule is employed as the failure criterion. The conservativeness of this method without design factors was investigated using material test results, and it was shown that the time fraction approach can conservatively predict failure life if margins on the initial stress of relaxation and the stress relaxation rate are embedded. In addition, the conservatism of prediction tends to increase with time to failure. Comparison with the modified ductility exhaustion method, which is known to have good failure life predictability in material test results, shows that the time fraction approach predicts failure lives to be shorter in long-term strain hold conditions, where material test data are hardly obtained. These results confirm that the creep–fatigue evaluation method in the JSME FRs code has implicit conservatism in addition to explicit margins in the design procedures such as design factor.

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References

Figures

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Fig. 3

Comparison of observed and predicted lives by the time fraction approach using actual stress relaxation

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Fig. 2

Accumulated damages at failure estimated by the time fraction approach using actual stress relaxation

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Fig. 1

Evaluated test data. (a) Strain ranges and hold times, (b) strain ranges and time to failure.

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Fig. 11

Ratios of calculated final stress using a monotonic stress–strain curve and creep strain equation to observed final stress. (a) Time to failure, (b) strain range, and (c) hold time.

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Fig. 4

Ratios of predicted life by the time fraction approach using actual stress relaxation to observed life. (a) Time to failure, (b) strain range, and (c) hold time.

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Fig. 5

Stress–strain relationship at 550 °C

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Fig. 6

Comparison of experimental stress relaxation at half-life cycle and calculated curve using measured initial stress at half-life cycle, and creep strain rate of the as-received material

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Fig. 7

Accumulated damages at failure estimated by the time fraction approach with margins on initial stress and creep strain rate

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Fig. 8

Comparison of observed and predicted lives by the time fraction approach with margins on initial stress and creep strain rate

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Fig. 9

Ratios of predicted life by the time fraction approach with margins on initial stress and creep strain rate to observed life. (a) Time to failure, (b) strain range, and (c) hold time.

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Fig. 10

Ratios of calculated initial stress using a monotonic stress-strain curve to observed stress. (a) Time to failure, (b) strain range, and (c) hold time.

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Fig. 12

Comparison of observed and predicted lives by the modified ductility exhaustion method using actual stress relaxation histories

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Fig. 13

Dependence of stress relaxation on the time coefficient in the creep strain equation, αc

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Fig. 14

Comparison of observed and predicted lives by the modified ductility exhaustion method using estimated stress relaxation histories

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Fig. 15

Ratios of predicted life by the modified ductility exhaustion method to observed life. (a) Time to failure, (b) strain range, and (c) hold time.

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Fig. 16

Extrapolation results of creep—fatigue life predictions by the modified ductility exhaustion method and the time fraction approach

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