ssc-367 - Ship Structure Committee

Following the weighting of the short-term density functions, the
long-term density functions for the structure’s design life are
obtained. While the cumulative damage may be computed through
numerical integration, an approximation is introduced to allow
application of a closed-form solution. Typically, a Weibull shape
parameter (Weibull distribution) is used in predicting cumulative
fatigue damage based on the 1ong-term, closed-form method. This
subject is discussed further in Section 6 and in a comprehensive
paper by Chen and Mavrakis (Reference7.24).
7.5
OVERVIEW AND RECOMMENDATIONS
7.5.1
Application of S-N Curves
The S-N curves used indeterminingfatiguedamage computations should
be compatible with structural details investigated. The S-N curve
including the effect of peak stresses should be used together with
nominal stresses at the detail, while the S-N curve uninfluenced by
the weld profile should be used with nominal stresses increased“by
appropriate SCFS.
Scatter in fatigue test data should also be appropriately accounted
for. One primary parameter affecting scatter of S-N data may be
plate thickness. As plate thickness increases higher localized
stresses will occur near plate surface, accelerating propagation of
fatigue cracks. Consideringthat small specimen S-Ndata needto be
adjusted for scale effects and a reasonable confidence level should
be achieved, S-N curves may be obtained assuming 95% to 97.5%
confidence level and a log normal distribution.
There are other parameters that are difficult to assess yet they
affect the crack growth and fatigue failure, causing substantial
scatter of S-N data points. One importantconsideration is the size
of initialflaw (crack)and another is the number of flaws. Although
further work is necessary, Morgan’s (Reference 7.25) findings on
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