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Research Papers: Design and Analysis

Failure Analysis of Piping Systems With Thinned Elbows on Tri-Axial Shake Table Tests

[+] Author and Article Information
Tadahiro Shibutani

Center for Risk Management and Safety Science,
Yokohama National University,
Yokohama 240-8501, Japan

Izumi Nakamura

National Research Institute for Earth
Science and Disaster Prevention,
Tsukuba, Ibaraki 305-0006, Japan

Akihito Otani

IHI Corporation,
Yokohama,
Kanagawa 235-8501, Japan

Contributed by the Pressure Vessel and Piping Division of ASME for publication in the JOURNAL OF PRESSURE VESSEL TECHNOLOGY. Manuscript received July 25, 2012; final manuscript received August 21, 2014; published online October 13, 2014. Assoc. Editor: Osamu Watanabe.

J. Pressure Vessel Technol 137(1), 011205 (Oct 13, 2014) (5 pages) Paper No: PVT-12-1106; doi: 10.1115/1.4028422 History: Received July 25, 2012; Revised August 21, 2014

This paper presents a computational failure analysis of piping systems with and without thinned elbows on tri-axial shake table tests. In a previous experimental study, two piping models, a sound piping system and a degraded piping system with thinned elbows, were assessed. The sound piping system was found to fail at the elbow flank due to in-plane cyclic bending, whereas the degraded system failed at the end of the elbow due to excessive pipe ovalization. In the present study, finite element (FE) models of elbows were developed in order to carry out fracture analysis. The measured displacements of seismic motions were used as the boundary conditions for FE models. In the sound piping system, plastic strain concentrated at the flank of the elbow due to in-plane bending. The cumulative damage factor was calculated from the fatigue curve and Miner's rule. The effect of ratcheting was also considered. In the failed elbow, the calculated cumulative damage factor showed good agreement with experimental results. On the other hand, for the fracture analysis of the thinned elbow, the entire seismic loading history on the tri-axial shake table was considered, since the effect of pipe ovalization depends on loading history. The ovalization occurred at the elbow due to cumulative seismic loading. Consequently, the principal plastic strain began to concentrate at the end of the elbow. These FE results offer quantitative explanation for the observed failure modes in the degraded piping system.

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References

Figures

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Fig. 1

Test model for tri-axial shake table test [7]. (a) Configuration of test model and (b) appearance with another isometric view.

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Fig. 2

FE model for Elbow 3 (unit: mm)

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Fig. 3

FE model of Elbow 1 with wall thinning. (a) FE model and boundary conditions and (b) configuration of wall thinning.

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Fig. 4

Contour map of accumulated equivalent plastic strain on inner/outer sides of Elbow 3 (7.5 times magnified seismic wave). (a) Inner side, (b) outer side, and (c) crack observed by liquid penetrant inspection.

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Fig. 5

Evolution of strains during 7.5 times magnified seismic wave

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Fig. 6

Progressive deformation of Elbow 1 with wall thinning

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Fig. 7

Principal plastic strain on the inner side of Elbow 1. (a) 7.5 times magnified seismic wave (fifth) and (b) sinusoidal wave (first).

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Fig. 8

Evolution of total strains during sinusoidal wave motions

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Fig. 9

Overview of the crack at thinned Elbow 1 (inner surface)

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Fig. 10

Fracture surface of the crack at thinned Elbow 1

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