ssc-367 - Ship Structure Committee

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Fatiguestrength is not analyzedbut determinedfrom laboratorytest
specimens. The experimental work that allows the definition of
fatigue strength and the S-N curves require substantial further
work. Some of the basic variables contributing to the uncertainty
fatigue strength include the effects of:
Geometry (weld profile, toe discontinuity, etc.)
Defect type, size and location
Definition of fatigue failure (Nl, N2) in S-N data
Size on S-N data
Assumption of a linear model and log normal distribution for
N
Environment (corrosion,cathodic protection, etc.)
Load amplitude and sequence
Fabrication residual stresses
Post-fabricationprocedures to increase fatigue strength
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Due to large uncertainties in each of the items listed, the fatigue
strength data show a very large scatter, requiring the use of
somewhat conservative S-N curves. The available test data on high
stress range-low cycle fatigue failure is limited. Thus, the S-N
curves for the 1000 to 10,000 cycle range are less reliable than the
high cycle ranges.
While additional work is necessary to better define geometric
variations, the recent research has shown that there are also some
uncertainties regarding the:
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Beneficialeffect of weld profilewithout weld toe grindingor
remelting
Assumption of catholically protected joints in sea water
having the same fatigue strength in air
Classification of joints based on geometry rather than load
pattern
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