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Research Papers: Experimental Work

Low-Cycle Fatigue and Ratcheting Responses of Elbow Piping Components

[+] Author and Article Information
T. Hassan

Department of Civil, Construction,
and Environmental Engineering,
North Carolina State University,
Raleigh, NC 27695-7908
e-mail: thassan@ncsu.edu

M. Rahman, S. Bari

Department of Civil, Construction,
and Environmental Engineering,
North Carolina State University,
Raleigh, NC 27695-7908

1Corresponding author.

2Present address: Areva Inc., Charlotte, NC 28262.

3Present address: DTE Energy, Detroit, MI 48226.

Contributed by the Pressure Vessel and Piping Division of ASME for publication in the JOURNAL OF PRESSURE VESSEL TECHNOLOGY. Manuscript received April 28, 2014; final manuscript received November 8, 2014; published online March 25, 2015. Assoc. Editor: Reza Adibi-Asl.

J. Pressure Vessel Technol 137(3), 031010 (Jun 01, 2015) (12 pages) Paper No: PVT-14-1074; doi: 10.1115/1.4029068 History: Received April 28, 2014; Revised November 08, 2014; Online March 25, 2015

The objective of this study was to investigate low-cycle fatigue and ratcheting responses of elbows through experimental and analytical studies. Low-cycle fatigue and ratcheting damage accumulation in piping components may occur under repeated reversals of loading induced by earthquake and/or thermomechanical operation. Ratcheting and fatigue damage accumulation can cause failure of piping systems through fatigue cracks or plastic buckling. However, the ratcheting damage induced failures are yet to be understood clearly; consequently, ASME Code design provisions against ratcheting failure continue to be a controversial issue over the last two decades. A systematic set of piping component experimental responses involving ratcheting and a computational tool to simulate these responses will be essential to rationally address the issue. Development of a constitutive model for simulating component ratcheting responses remains to be a challenging problem. In order to develop an experimentally validated constitutive model, a set of elbow experiments was conducted. The loading prescribed in the experiments involved displacement-controlled or force-controlled in-plane cyclic bending with or without internal pressure. Force, displacement, internal pressure, elbow diameter change, and strains at four locations of the elbow specimens were recorded. This article presents and discusses the results from the experimental study. A sister article evaluates seven different constitutive models against simulating these elbow ratcheting and fatigue responses.

Copyright © 2015 by ASME
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References

Figures

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Fig. 1

(a) Photograph of the elbow specimen and test setup showing the actuator and load cell of the universal testing machine and (b) a sketch of the elbow specimen and test boundary conditions (dimensions are in mm).

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Fig. 2

(a) Photograph of the ΔD apparatus composed of four LVDTs and four spring loaded supports attached to a rigid polymer ring and (b) schematic of the ΔD apparatus.

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Fig. 3

Elbow specimen diameter and thickness measurement locations: (a) cross sections A–F on straight pipe and G–K on elbow, (b) elbow and straight pipe wall thickness measurement locations 1–12 (see Table 1) for each of the A–K cross sections, and (c) weld thickness measurement locations 1–8 (see Table 3).

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Fig. 4

Loading paths prescribed in the elbow experiments: (a) displacement-controlled cycle at steady internal pressure and (b) force-controlled cycle at steady internal pressure.

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Fig. 5

An elbow specimen showing through-wall fatigue crack at a flank

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Fig. 6

Responses from elbow specimen 4 (see Table 4): (a) force–displacement (P–δ), (b) positive and negative peak forces as a function of the number of cycle N, (c) flank to flank diameter change (ΔDx) versus δ, (d) positive and negative peaks of ΔDx as a function of N, (e) extrados circumferential strain (εθ) versus δ, and (f) positive and negative extrados εθ peaks as a function of N.

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Fig. 7

Mean and amplitude responses of (a) and (b) force (Pm and Pc) and (c) and (d) flank to flank diameter change (ΔDmx and ΔDax) as a function of the number of cycle N from specimens 1, 3, and 4 (see Table 4).

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Fig. 8

First cycle force–displacement (P–δ) hysteresis responses from (a) specimen 1, (b) specimen 3, and (c) specimen 4 (see Table 4)

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Fig. 9

Mean and amplitude responses of (a) and (b) circumferential strain (εmθ and εaθ) and (c) and (d) axial strain (εmx and εax) at flank as a function of the number of cycle N from elbow specimens 1, 3, and 4 (see Table 4)

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Fig. 10

Extrados circumferential strain responses: (a) mean (εmθ) and (b) amplitude (εaθ) as a function of number of cycle N from specimens 1, 3, and 4 (see Table 4)

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Fig. 11

Intrados circumferential strain responses: (a) mean (εmθ) and (b) amplitude (εaθ) as a function of number of cycle N from specimens 1, 3, and 4 (see Table 4)

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Fig. 12

Influence of displacement amplitude (δc) on elbow responses from specimens 1 and 2: (a) force amplitude (Pc), (b) flank to flank diameter change amplitude (ΔDax), (c) flank to flank diameter change mean (ΔDmx), and (d) flank circumferential strain mean (εmθ).

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Fig. 13

Elbow responses from force-controlled loading cycle: (a) force–displacement (P–δ), (b) flank to flank diameter change (ΔDx), (c) flank circumferential strain (εθ), and (d) circumferential strain ratcheting (εmθ) from specimens 5 and 6 (see Table 5).

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Fig. 14

Uniaxial loading responses of SS304L developed for model parameter determination: (a) tensile stress–strain curve, (b) force-controlled cyclic (uniaxial ratcheting) response, and (c) positive peak axial strain (εxp) ratcheting as a function of number of cycles N.

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Fig. 15

Biaxial loading responses of SS304L developed for model parameter determination: (a) axial stress–strain hysteresis loops, (b) circumferential strain (εθ) ratcheting, and (c) positive peak circumferential strain (εθp) ratcheting as a function of number of cycles N.

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Fig. 16

Stress–strain curve composed from tensile stress–strain curve and force-controlled (uniaxial ratcheting) stress–strain curve for model parameter determination

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