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Materials and Fabrication

Verification of the Estimation Methods of Strain Range in Notched Specimens Made of Mod.9Cr-1Mo Steel

[+] Author and Article Information
Masanori Ando, Nobuchika Kawasaki

Japan Atomic Energy Agency,
4002 Narita, Oarai,
Ibaraki 311-1393, Japan

Yuichi Hirose

Mitsubishi heavy industry, Ltd.,
5-717-1 Fukahori, Nagasaki,
Nagasaki 851-0392, Japan

Shingo Date

Mitsubishi heavy industry, Ltd.,
2-1-1 Shinhama, Arai, Takasago,
Hyogo 676-8686, Japan

Sota Watanabe

Mitsubishi heavy industry, Ltd.,
1-1-1 Wadamisaki, Hyogo, Kobe,
Hyogo 652-8585, Japan

Yasuhiro Enuma

Mitsubishi FBR systems,
2-34-17, Jingumae, Shibuya,
Tokyo 150-0001, Japan

Contributed by the Pressure Vessel and Piping Division of ASME for publication in the JOURNALOF PRESSURE VESSEL TECHNOLOGY. Manuscript received March 28, 2011; final manuscript received March 22, 2012; published online October 18, 2012. Assoc. Editor: Osamu Watanabe.

J. Pressure Vessel Technol 134(6), 061403 (Oct 18, 2012) (12 pages) doi:10.1115/1.4006902 History: Received March 28, 2011; Revised March 22, 2012

Several methods of estimating strain range at a structural discontinuity have been developed in order to assess component reliability. In a component design at elevated temperature, estimation of strain range is required to evaluate the fatigue and creep-fatigue damage. Therefore, estimation of strain range is one of the most important issues when evaluating the integrity of a component during its lifetimes. To verify the methods of estimating strain range for discontinuous structures, low cycle fatigue tests were carried out with notched specimens. All the specimens were made of Mod.9Cr-1Mo steel, because it is a candidate material for a primary and secondary heat transport system components of Japan Sodium-cooled Fast Reactor (JSFR). Displacement control fatigue tests and thermal fatigue tests were performed by ordinary uniaxial push–pull test machine and equipment generating the thermal gradient in the notched plate by induction heating. Several notch radii were employed to vary the stress concentration level in both kinds of tests. Crack initiation and propagation process during the tests were observed by a digital microscope and the replica method to define the failure cycles. Elastic and inelastic finite element analyses were also performed to estimate strain range for predicting fatigue life. Then, these predictions were compared with the test results. Several methods such as stress redistribution locus (SRL) method, simple elastic follow-up (SEF) method, Neuber's law, and the procedures employed by elevated temperature design codes were applied. Through these comparisons, the applicability and conservativeness of these strain range estimation methods, which is the basis of the fatigue and creep-fatigue life prediction, are discussed.

Copyright © 2012 by ASME
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References

Figures

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Fig. 1

Configuration of notched bar specimens for mechanical fatigue tests

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Fig. 2

Configuration of the notched plate specimen for thermal fatigue tests and overview of the thermal gradient measurement tests

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Fig. 3

Variation of the thermal history in the thermal gradient measurement tests

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Fig. 4

Examples of 2D and 3D models and boundary conditions for FEA: (a) The sector model of a notched bar specimen (ρ = 11.2 mm), (b) the sector model of a notched plate specimen (R = 3 mm)

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Fig. 5

Experimental results of mechanical fatigue tests with notched bar specimens normalized by nominal fatigue lives in DDS [22]

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Fig. 6

Comparison of the experimental results and estimated strain range in the mechanical fatigue tests with notched bar specimens: (a) ρ = 1.6 mm, (b) ρ = 11.2 mm, and (c) ρ = 40.0 mm

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Fig. 7

Comparison of N25% drop between experimental results and the results predicted by various strain range estimation methods in the mechanical fatigue tests with notched bar specimens

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Fig. 8

Comparison of N1mm crack between experimental results and the results predicted by various strain range estimation methods in the mechanical fatigue tests with notched bar specimens

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Fig. 9

Comparison of the accumulated conservativeness in the prediction procedure for fatigue life by DDS

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Fig. 10

Relationship between the experimental number of cycles corresponding to crack length on the surface reaching 1 mm and estimated strain range in thermal fatigue test with notched plate specimen

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Fig. 11

Comparison of N1mm crack between experimental results and predictions by various strain range estimation methods in thermal fatigue test with notched plate specimen

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Fig. 12

Relationship of the normalized stress (σepe) and normalized strain (ɛepe)

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Fig. 13

Situations of the plastic strain at the horizontal cross section calculated by inelastic FEA

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Fig. 14

Concept of the SRL method

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Fig. 15

Concept of the simple elastic follow-up method

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Fig. 16

Illustration of the strain concentration factor based on the elastic follow-up concept employed by the JSME FR code

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