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Research Papers: Materials Technology

Evaluation of Weld Root Failure Using Battelle Structural Stress Method

[+] Author and Article Information
Jeong K. Hong

Battelle,
505 King Avenue,
Columbus, OH 43201-2696
e-mail: hong@battelle.org

Contributed by the Ocean Offshore and Arctic Engineering Division of ASME for publication in the Journal of Offshore Mechanics and Arctic Engineering. Manuscript received July 18, 2011; final manuscript received July 13, 2012; published online February 25, 2013. Assoc. Editor: Xin Sun.

J. Offshore Mech. Arct. Eng 135(2), 021404 (Feb 25, 2013) (7 pages) Paper No: OMAE-11-1067; doi: 10.1115/1.4007329 History: Received July 18, 2011; Revised July 13, 2012

Weld related fatigue failure is one of the most common concerns in welded structures. From the fatigue design point of view, weld toe failure is preferable to weld root failure. Base plate thickness is a controlling parameter for weld toe failure, while weld metal size is a controlling parameter for weld root failure. However, controlling the weld metal size is not easy because the actual weld penetration and weld leg size vary along a weld and from weld to weld. Therefore, analyzing fatigue test data for weld root failure tends to enlarge scatter band due to variability in weld penetration and weld leg size when the nominal weld size is considered. The structural stress based weld fatigue master S-N curve adopted by 2007 ASME section VIII Div. 2 and new API 579/ASME FFS-1 was constructed by incorporating only clearly defined weld toe fatigue data. In this article, a simplified structural stress procedure was developed and a design master S-N curve for weld root failure was established based on the published fatigue test data. Consequently, the mean design master S-N curve for weld root failure is downshifted relative to the mean master S-N curve for weld toe failure, and has a wider scatter band. To be conservative, a crack path along weld throat is recommended for structural stress calculation. Also, the transverse shear stress effects on structural stress calculation can be ignored.

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References

Figures

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Fig. 1

Through-thickness structural stress definition (a) local stresses from the FE model; (b) structural stress or far-field stress; (c) self-equilibrating stress

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Fig. 2

Mesh-insensitivity demonstration for calculating equivalent far-field stress at weld toe (a) model definition; (b) representative FE models with fine (0.16t/0.1t) and coarse mesh (0.8t/t); (c) comparison of calculated far-field stresses at the weld toe between new method and output of FE models

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Fig. 3

Observation of an example of weld root failure for a load-carrying cruciform fillet joint under cyclic tension loading. (a) A picture of typical weld root failure [11]; (b) proposed hypothetical crack path: Crack Path①.

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Fig. 4

Joint types and loading directions for weld root failure from [11-17]

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Fig. 5

Sketch of fillet weld connection

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Fig. 6

Correlation of weld root failure data using the engineering shear stress range (nominal stress range in fillet weld)

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Fig. 7

Correlation of weld root failure data using the normal stress range in the base plate

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Fig. 8

Sketch of stress components in the fillet weld

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Fig. 9

Definitions of structural stress components for weld root failure

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Fig. 10

Structural stress analysis procedure for treating weld root failure along Crack Path①

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Fig. 11

Correlation of weld root failure data using (a) effective structural stress range and (b) equivalent effective structural stress range (with considering transverse shear component) along Crack Path①

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Fig. 12

Correlation of weld root failure data using equivalent structural stress range without considering transverse shear components along Crack Path①

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Fig. 13

Structural stress analysis procedure for treating weld root failure along Crack Path②

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Fig. 19

Comparison of design S-N curves for weld toe failure and weld root failure

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Fig. 18

Proposed design S-N curve for weld root failure

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Fig. 17

Correlation of weld root crack data using equivalent structural stress range along Crack Path② for doubler plate joints and along Crack Path① for other joint types

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Fig. 16

Comparison of weld root failure fatigue data using equivalent structural stresses along different crack paths: Crack Path① and Crack Path② for selected cruciform joints and tube joints under cyclic tension loading [15,16]

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Fig. 15

Equivalent structural stress comparison between weld toe failure and weld root failure (along two hypothetical crack paths) for load carrying cruciform joints under tension loading

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Fig. 14

Equivalent structural stress comparison with and without considering the transverse shear stress component along hypothetical crack paths for load carrying cruciform joints under tension loading

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