ssc-367 - Ship Structure Committee

The S-N curves that can be used directly with the nominal stresses
most often apply to ship structuredetails. Munse’s SSC-318 report
(Reference 1.3) documents the S-N curves for 69 ship structure
details and refers to earlierwork by Jordan and Cochran (Reference
7.2) on in-service performance of ship structure details.
Tubular offshore components have more complex geometries and are
subjected to corrosive ocean environment, requiring careful
assessment of all parameters contributing to fatigue failure and
selection of appropriateS-N curves.
Many design, fabrication and in-service factors affect the fatigue
lives of details/joints. Fatigue cracks in welded joints often
initiate at weld discontinuities introduced during fabrication.
Weld quality problems that contributeto the degradation of fatigue
strength include:
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Planar defects in the body of the weld
Incompletepenetration
Imperfectweld root quality
Imperfectweld toe profile
Development of an embrittled heat affected zone (HAZ)
Fatigue assessment requires definition of the number of applied
stress cycles (N). Welded details/joints subjected to repeated
cyclic stresseswill go through several stages of crack growth. For
each hot-spot stress range (s), failure is assumed to go through
three stages:
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First discernible surface cracking (Nl)
First through-wall cracking (N2)
Extensive cracking and end of testing (N3)
Ideally, cracks should be large enough to detect, yet not large
enough to cause failure and alterationof load path. To ensure that
cracks are repairable, the number of cycles to failure in fatigue
7-3