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Research Papers: Materials and Fabrication

Low-Cycle Fatigue of Pressurized Steel Elbows Under In-Plane Bending

[+] Author and Article Information
George E. Varelis

Department of Mechanical Engineering,
University of Thessaly,
Volos, Greece
e-mail: gevareli@mie.uth.gr

Spyros A. Karamanos

Department of Mechanical Engineering,
University of Thessaly,
Volos, Greece
e-mail: skara@mie.uth.gr

1Corresponding author.

Contributed by the Pressure Vessel and Piping Division of ASME for publication in the JOURNAL OF PRESSURE VESSEL TECHNOLOGY. Manuscript received July 23, 2013; final manuscript received March 23, 2014; published online September 15, 2014. Assoc. Editor: Xian-Kui Zhu.

J. Pressure Vessel Technol 137(1), 011401 (Sep 15, 2014) (10 pages) Paper No: PVT-13-1116; doi: 10.1115/1.4027316 History: Received July 23, 2013; Revised March 23, 2014

The present study examines the mechanical behavior of steel process piping elbows, subjected to strong cyclic loading conditions. The work is numerical, supported by experimental data on elbow specimens subjected to in-plane cyclic bending, with or without internal pressure, resulting in failure in the low-cycle-fatigue range. The investigation of elbow behavior is conducted using rigorous finite element analysis accounting for measured elbow geometry and the actual material properties. An advanced cyclic plasticity material model is employed for the simulation of the tests. Emphasis is given on the value of local strain and its accumulation at the critical elbow location where cracking occurs. Based on the cyclic stress–strain curve of the material and the strain-based fatigue curve from the test data, the use of Neuber's formula leads to a fatigue analysis and design methodology, offering a simple and efficient tool for predicting elbow fatigue life.

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References

Figures

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Fig. 1

Experimental setup: (a) nonpressurized specimens tested at TU Delft [27] and (b) pressurized specimens tested at CSM [28]

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Fig. 2

(a) Crack location at the elbow “flank” and (b) ovalization at the elbow middle section [25]

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Fig. 3

General view of the finite element model

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Fig. 4

Schematic representation of the constitutive model and definition of congruent point s¯ on the memory surface

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Fig. 5

Material curve from strain-controlled cyclic material tests [30] and model predictions

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Fig. 6

Comparison of numerical and experimental results for test no. 5, (P = 0 MPa and Δl = ±200 mm); (a) load–displacement curve and (b) flattening–displacement curve

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Fig. 7

Comparison of numerical and experimental results for test no. 9 (P = 3.2 MPa and Δl = ±200 mm); (a) load–displacement curve and (b) flattening–displacement curve

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Fig. 8

Comparison of numerical and experimental results for test no. 11 (P = 7 MPa and Δl = ±200 mm); (a) load–displacement curve and (b) flattening–displacement curve

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Fig. 9

Comparison of numerical and experimental results for test no. 13 (P = 12 MPa and Δl = ±200 mm); (a) load–displacement curve and (b) flattening–displacement curve

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Fig. 10

Numerical results for the range of cross-sectional flattening for the first load cycle of specimens with end-displacement Δl = ±200 mm and Δl = ±300 mm

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Fig. 11

Strain concentration at the elbow critical region (elbow flank)

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Fig. 12

Evolution of hoop strains ranges at the critical region for end-displacement Δl = ±150 mm; numerical versus experimental results

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Fig. 13

Numerical predictions for the evolution of strain at the critical region; end-displacement range Δl = ±200 mm: (a) hoop direction and (b) longitudinal direction

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Fig. 14

Fatigue life curve and BPVC code predictions

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Fig. 15

Cyclic material stress–strain curve, from strain-controlled tests on P355N steel elbow material [30]

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Fig. 16

Schematic representation of the fatigue life prediction using the simplified method

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Fig. 17

Fatigue life predictions using the simplified method, as compared with test values

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