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Research Papers: Materials and Fabrication

Flaw Tolerance Assessment for Low-Cycle Fatigue of Stainless Steel

[+] Author and Article Information
Masayuki Kamaya

Institute of Nuclear Safety System, Inc.,
64 Sata, Mihama-cho,
Fukui 919-1205, Japan
e-mail: kamaya@inss.co.jp

1Corresponding author.

Contributed by the Pressure Vessel and Piping Division of ASME for publication in the JOURNAL OF PRESSURE VESSEL TECHNOLOGY. Manuscript received July 8, 2016; final manuscript received March 1, 2017; published online April 21, 2017. Assoc. Editor: Haofeng Chen.

J. Pressure Vessel Technol 139(4), 041405 (Apr 21, 2017) (7 pages) Paper No: PVT-16-1108; doi: 10.1115/1.4036141 History: Received July 08, 2016; Revised March 01, 2017

According to Appendix L of the Boiler and Pressure Vessel Code Section XI, flaw tolerance assessment is performed using the stress intensity factor (SIF) even for low-cycle fatigue. On the other hand, in Section III, the fatigue damage is assessed using the design fatigue curve (DFC), which has been determined from strain-based fatigue tests. Namely, the stress is used for the flaw tolerance assessment. In order to resolve this inconsistency, in the present study, the strain intensity factor was used for crack growth prediction. First, it was shown that the strain range was the key parameter for predicting the fatigue life and crack growth. The crack growth rates correlated well with the strain intensity factor even for the low-cycle fatigue. Then, the strain intensity factor was applied to predict the crack growth under uniform and thermal cyclic loading conditions. The estimated fatigue life for the uniform cyclic loading condition agreed well with that obtained by the low-cycle fatigue tests, while the fatigue life estimated for the cyclic thermal loading condition was longer. It was shown that the inspection result of “no crack” can be reflected to determining the future inspection time by applying the flaw tolerance analysis. It was concluded that the flaw tolerance concept is applicable not only to the plant maintenance but also to plant design. The fatigue damage assessment using the design fatigue curve can be replaced with the crack growth prediction.

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Figures

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Fig. 1

Fatigue lives of cold worked and nonworked specimens obtained by load- or strain-controlled fatigue tests: (a) fatigue life for stress amplitude and (b) fatigue life for strain range

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Fig. 2

Change in crack surface length with normalized cycles obtained by replica investigations

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Fig. 3

Relationship between stress intensity factor range or strain intensity factor range and crack growth rate obtained under various loading conditions: (a) stress intensity factor range (ΔK) and (b) strain intensity factor range (ΔKε)

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Fig. 4

Fatigue lives estimated by crack growth analysis and those obtained by strain-controlled fatigue tests

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Fig. 5

Analyzed model for thermal fatigue caused by fluid temperature change

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Fig. 6

Strain distribution in the thickness direction obtained for ΔTf = 100 K

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Fig. 7

Change in normalized strain intensity factor with crack depth under various rise times ts

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Fig. 8

Crack initiation and growth prediction made using Eqs.(2) and (3) for Δε = 0.8%

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Fig. 9

A schematic drawing representing how to reflect inspection results to future maintenance

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Fig. 10

Relationship between the detection capability of inspection technique and residual life normalized by the duration before the inspection

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Fig. 11

Change in residual life secured by the inspection with the duration before the inspection

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