Research Papers: Materials and Fabrication

Stress Intensity Factor Interaction for Subsurface to Surface Flaw Transformations Under Stress Concentration Fields

[+] Author and Article Information
Pierre Dulieu

Tractebel (ENGIE) Bvd.,
de Merckem 60,
Namur 5000, Belgium
e-mail: pierre.dulieu@tractebel.engie.com

Valéry Lacroix

Tractebel (ENGIE) Bvd,
Simón Bolívar 34-36,
Brussels 1000, Belgium
e-mail: valery.lacroix@tractebel.engie.com

Kunio Hasegawa

Center of Advanced Innovation Technologies,
Technical University of Ostrava,
17. Listopadu 15/2172,
Ostrava-Poruba 708 33, Czech
e-mail: kunioh@kzh.biglobe.ne.jp

Contributed by the Pressure Vessel and Piping Division of ASME for publication in the JOURNAL OF PRESSURE VESSEL TECHNOLOGY. Manuscript received December 11, 2017; final manuscript received August 30, 2018; published online November 12, 2018. Assoc. Editor: Andrew J. Duncan.

J. Pressure Vessel Technol 140(6), 061404 (Nov 12, 2018) (5 pages) Paper No: PVT-17-1257; doi: 10.1115/1.4041435 History: Received December 11, 2017; Revised August 30, 2018

If a single subsurface flaw is detected that is close to a component's free surface, a flaw-to-surface proximity rule is used to determine whether the flaw should be treated as a subsurface flaw, or transformed to a surface flaw. The transformation from subsurface to surface flaw is adopted as flaw-to-surface proximity rules in all fitness-for-service (FFS) codes. These proximity rules are applicable when the component's free surface is without a stress concentration. On the other hand, subsurface flaws have been found under notches, such as roots of bolts, toes in welded joints, or geometrical discontinuities of components. The stress intensity factors of the subsurface flaws are affected by the stress concentrations caused by the notches. The stress intensity factor of the subsurface flaw increases with increasing stress concentration factor of the notch and decreasing ligament distance between tip of the subsurface flaws and the notch, for a given notch width. Such subsurface flaws are transformed to surface flaws at a distance from the notch tip for conservative evaluations. This paper shows the interactions of stress intensity factors of subsurface flaws under stress concentration fields. Based on the interaction, a flaw-to-surface proximity criterion is proposed for a circular flaw under the stress concentration field induced by a notch.

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Fig. 1

Transformation from subsurface to surface flaw at flaw characterization

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Fig. 2

Model dimensions for circular flaw near notch root

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Fig. 3

Cross section at notch with circular flaw

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Fig. 4

Applied stress correction to reach constant net-section stress without and with various notch depths

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Fig. 5

Interaction of stress intensity factor K1/K0* at point 1 as a function of ligament distance S/a for different stress concentration factor α and for notch width 2b = 2a

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Fig. 6

Equivalent ligament distance S/a at K1/K0* = 1.07 as a function of stress concentration factor α and for notch width 2b = 2a

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Fig. 7

Proposal of proximity criterion on transformation from subsurface to surface flaw under notches



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