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Research Papers: Materials and Fabrication

Rules for the Assessment of Interacting Cracks Under Creep Conditions

[+] Author and Article Information
Fu-Zhen Xuan1

Key Laboratory of Safety Science of Pressurized System, MOE, School of Mechanical Engineering, East China University of Science and Technology, 130 Meilong Street, P.O. Box 402, Shanghai 200237, P. R. Chinafzxuan@ecust.edu.cn

Jun Si, Shan-Tung Tu

Key Laboratory of Safety Science of Pressurized System, MOE, School of Mechanical Engineering, East China University of Science and Technology, 130 Meilong Street, P.O. Box 402, Shanghai 200237, P. R. China

1

Corresponding author.

J. Pressure Vessel Technol 132(1), 011405 (Dec 23, 2009) (7 pages) doi:10.1115/1.4000358 History: Received January 05, 2009; Revised September 03, 2009; Published December 23, 2009; Online December 23, 2009

Multiple interacting cracks are generally treated as an enveloping single crack if the prescribed criterion is satisfied in the current fitness-for-service codes. To examine the applicability of the existing criteria under creep regime, in this report, creep crack interaction effects of plates containing collinear cracks or parallel cracks are first analyzed under the framework of reference stress method and finite element computations. An equivalent crack growth rate criterion is then proposed to develop a congruous multiple crack treatment rule for different growth mechanisms. The current combination rule for multiple cracks is examined and modified in terms of the proposed new criterion. The results reveal that the creep crack interaction factor is a combined function of creep exponent n of materials, limit load ratio of cracked structures, and interaction of stress intensity factor. More pronounced interaction is observed for both interacting cracks in the same plane and that in different planes. Although a higher intensity of creep crack interaction is observed, a greater value of γcreep is permissible compared with that of linear-elastic crack interaction denoted by SIF. Accordingly, a modified combination rule for collinear cracks of S/a=3.0 is proposed to exclude the nonconservativeness, while the current alignment rule for offset parallel cracks of H/a=2.0 is still practicable in terms of the proposed criterion.

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Figures

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Figure 1

Flow chart for multiple crack treatment in fitness-for service codes

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Figure 2

Sketch map of multiple interacting cracks: (a) offset parallel cracks and (b) collinear cracks

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Figure 3

Finite element model of offset parallel cracks for FE analysis

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Figure 4

Finite element model of collinear cracks for FE analysis

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Figure 5

Evaluation of C(t)-integral along different path with time for inner tip of twin collinear cracks

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Figure 6

Comparison of the normalized SIF from the present FE analysis with Rooke and Cartwright’s solution: (a) collinear cracks and (b) parallel cracks

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Figure 7

Comparisons between the predicted C∗ and FE results for two collinear cracks in a plate: (a) inner crack tip and (b) outer crack tip

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Figure 8

Interaction factor γSIF of double offset cracks under linear-elastic regime

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Figure 9

Interaction factor γcreep of double offset cracks under creep regime

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Figure 10

Interaction factors γcreep for various offset distance and creep coefficient n

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