Research Papers: Materials and Fabrication

Low Cycle Fatigue Evaluation of Pipe Bends With Local Wall Thinning Considering Multi-Axial Stress State

[+] Author and Article Information
Yoshio Urabe

Japan Nuclear Safety Institute,
5-36-7, Shiba, Minato-ku,
Tokyo 108-0014, Japan
e-mail: urabe.yoshio@genanshin.jp

Koji Takahashi

Division of Materials Science and
Chemical Engineering,
Faculty of Engineering,
Yokohama National University,
79-5, Tokiwadai, Hodogaya,
Yokohama 240-8501, Japan
e-mail: ktaka@ynu.ac.jp

Hisanori Abe

Yokohama National University,
79-5, Tokiwadai, Hodogaya,
Yokohama 240-8501, Japan
e-mail: abe-hisanori-br@ynu.jp

1Corresponding author.

Contributed by the Pressure Vessel and Piping Division of ASME for publication in the JOURNAL OF PRESSURE VESSEL TECHNOLOGY. Manuscript received February 18, 2014; final manuscript received October 21, 2014; published online February 20, 2015. Assoc. Editor: David L. Rudland.

J. Pressure Vessel Technol 137(4), 041404 (Aug 01, 2015) (9 pages) Paper No: PVT-14-1028; doi: 10.1115/1.4028889 History: Received February 18, 2014; Revised October 21, 2014; Online February 20, 2015

Low cycle fatigue tests and finite element (FEM) analysis were conducted using 100A pipe bend specimens made of STPT410 carbon steel with and without local wall thinning local wall thinning was machined on the inside of the elbow and was prepared at extrados, crown, and intrados. The parameters of the wall thinning were same (the wall thinning ratio = 0.5, the wall thinning angle = 180 deg, and the wall thinning length = 100 mm) in the all test cases. The pipe bend specimens were subjected to the prescribed cyclic in-plane bending displacement with constant internal pressure of 0–12 MPa. Also, low cycle fatigue tests using sound pipe bend specimens were carried out for comparison. According to the test results, low cycle fatigue strength of wall thinned pipe bend specimens was not so different, regardless of location of wall thinning. Low cycle fatigue strength of the pipe bend specimens was beneath the best fit fatigue curve and its reason can be explained quantitatively by a proposed cumulated damage rule introducing ductility exhaustion considering multi-axial stress state. The validity of the new proposed cumulative damage rule was also confirmed by the another sample analysis using other reference data obtained by pre-overloaded in-plane cyclic bending tests.

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Fig. 1

Shape and geometry of pipe bend with local wall thinning. (a) Pipe bend specimen, (b) wall thinning at extrados (t: nominal wall thickness), (c) wall thinning at intrados, and (d) wall thinning at crown

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Fig. 2

Typical example of failure behavior of pipe bend (E-P6-D20, Nf = 123 cycles)

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Fig. 3

True stress versus true strain curve of carbon steel STPT410

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Fig. 4

FEM model of pipe bend specimen. (a) 1/2 model of sound elbow pipe, (b) 1/2 model of elbow pipe with local wall thinning at extrados, (c) 1/2 model of elbow pipe with local wall thinning at crown, and (d) 1/2 model of elbow pipe with local wall thinning at intrados.

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Fig. 5

Analysis condition

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Fig. 6

An example of load versus load point displacement relationship (E-P12-D20)

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Fig. 7

Hoop strain history at outer surface of crown, E-P9-D20 (Nf = 119)

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Fig. 8

Relationship between hoop strain range and fatigue life

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Fig. 9

Relationship between equivalent strain range and fatigue life

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Fig. 10

Dd versus Df diagram using εθa

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Fig. 11

Modified Dd versus Df diagram using correction factor (εeq,5/εθa,5)




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