Research Papers: Materials and Fabrication

Weld Strength Mismatch in Strain Based Flaw Assessment: Which Definition to Use?

[+] Author and Article Information
Stijn Hertelé

e-mail: Stijn.Hertele@UGent.be

Wim De Waele

e-mail: Wim.DeWaele@UGent.be

Rudi Denys

e-mail: Rudi.Denys@UGent.be

Matthias Verstraete

e-mail: Matthias.Verstraete@UGent.be

Koen Van Minnebruggen

e-mail: Koen.VanMinnebruggen@UGent.be
Soete Laboratory,
Ghent University,
Technologiepark Zwijnaarde 903,
Zwijnaarde 9052, Belgium

Anthony Horn

Swinden Technology Centre,
Tata Steel RD&T,
Moorgate, Rotherham,
South Yorkshire S60 3AR, UK
e-mail: Anthony.Horn@amec.com

Contributed by the Pressure Vessel and Piping Division of ASME for publication in the JOURNAL OF PRESSURE VESSEL TECHNOLOGY. Manuscript received August 1, 2012; final manuscript received August 22, 2013; published online October 10, 2013. Assoc. Editor: Hardayal S. Mehta.

J. Pressure Vessel Technol 135(6), 061402 (Oct 10, 2013) (8 pages) Paper No: PVT-12-1112; doi: 10.1115/1.4025343 History: Received August 01, 2012; Revised August 22, 2013

Weld strength mismatch is a key factor in the strain based assessment of flawed girth welds under tension. A strength overmatching weld shields potential flaws within the weld itself from remotely applied deformations and consequently reduces crack driving force. Although this effect has been recognized for decades, different weld strength overmatch definitions exist, and it is not yet fully established which of those is most relevant to a strain based flaw assessment. In an effort to clarify this unsolved question, the authors have performed a large series of parametric finite element analyses of curved wide plate tests. This paper provides an experimental validation of the model and subsequently discusses representative results. It is found that crack driving force is influenced by the shape of the pipe metals' stress–strain curves, which influences the representativeness of two common mismatch definitions (based on yield strength and on ultimate tensile strength). Effects of strength mismatch on strain capacity of a flawed girth weld are best described on the basis of a flow stress, defined as the average of yield and ultimate tensile strength. Based on the observations, a framework for a new strain capacity equation is proposed.

Copyright © 2013 by ASME
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Fig. 1

CWP specimen (Soete Laboratory)

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Fig. 2

Geometrical characterization of a CWP panel

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Fig. 3

Half CWP model with representative mesh

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Fig. 4

Narrow gap GMAW girth weld

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Fig. 5

Experimental and simulated CMOD response

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Fig. 6

Post mortem investigation of the notch reveals that no ductile tearing occurred

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Fig. 7

LVDT positions for strain measurements advised in the UGent guidelines for CWP testing [17]

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Fig. 9

Illustration of the ductile tearing approach: (a) Mapping. (b) Evolution of flaw depth. (c) Tangency indicates failure

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Fig. 10

Influences of different weld strength overmatch definitions ((a) OMYS, (b) OMTS, (c) OMFS) on relative strain capacity emax/em (Ramberg-Osgood is abbreviated as RO)

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Fig. 11

Influences of different weld strength overmatch definitions ((a) OMYS, (b) OMTS, (c) OMFS) on strain capacity emax (Ramberg-Osgood is abbreviated as RO). Note the clear trends in subfigure (c).

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Fig. 12

The relation between OMFS and emax is influenced by weld geometry and flaw location

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Fig. 13

Graphical representation of the proposed framework for strain capacity estimation




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