ssc-367 - Ship Structure Committee

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Fatiguetest data should be carefullyevaluated andS-N curves
should be generated by statisticalmeans to allow estimation
of failure probabilityand incorporationof conservatism into
the design. Separate S-N curves should be applicable to
different weld details and in some applications to different
profiles.
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S-N
not
curves include a level of fabricationeffects that should
be exceeded.
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The cumulative fatigue damage computation should be based on
Miner’s rule, and should consider the damaging effects of all
loadings (both global and local).
Fatigue damage assessment technology has benefitted from the
application of fatigue crack growth data and fracture mechanics
analysis of defects. In addition to predicting fatigue life,
fracture mechanics analysis allows better understanding of various
parameters that affect the behavior of welded joints. In turn,
experimental data and fracture mechanics analysis have allowed
upgrading of recommendedS-N curves (References1.5, 1.7) including
Gurney’s work on the influence of plate thickness (Reference 7.1).
7.2
S-N CURVES
The S-N curves recommended by various rules, recommendations and
codes are based on the application of constant amplitude stress
cycle on various detail/joint geometries in the laboratory until
fatigue failure. Most S-N curves for simple details (stiffener,
cutout, etc.) account for the local notch stress and can be used
with the member nominal stresses. Tubular joints of offshore
structures exhibit a wide variety of joint configurations and
details. Therefore, while the S-N curves account for several
parameters (platethickness,weld profile), theydo not account for
peak stresses, requiring the application of SCF’S on computed
nominal stresses to obtain peak (hot-spot)stresses.
7-2