ssc-367 - Ship Structure Committee

frequency of loading identify severity of the fatigue environment.
Although a structure’sconfigurationcan be optimizedto reduce the
stress range, the site-specificenvironmental loading controls the
choice of fatigue design and analyses method.
An environmental parameter that affects fatigue is either air or
seawater. Because of the adverse effects of seawater corrosion on
fatigue strength, adesign factor is often applied for fatigue life
in a seawater environment. However, an effective cathodic
protection systemwill reduce or prevent seawater corrosion, and if
such a system is used, the design factor may be deemed unnecessary.
This approach (and its inclusion in various rules, recommendations
and standards) is based on corrosion fatigue test data on welded
plate specimenswith and without cathodic protection.
Environmentaleffects on welded flat plates have been assumed to be
the same as those on tubular joints. However, Wylde et al
(Reference 7.15) have indicated that the corrosive effect of
seawater on tubular joints may be greater than the effect on flat
plate specimens. Althoughdifficultto document, tubular jointsmay
be more susceptibleto environmentaleffects than small welded flat
plates due to scale effects, including initial flaws. Flat plates
may have longer fatigue lives as substantial time will be expended
in initiation of flaws.
7.3
FATIGUE DAMAGE COMPUTATION
State-of-the-art methodology for determining fatigue lives and
designing structures with fatigue lives in excess of the design
lives is primarily based on S-N curves and the cumulative damage
rule. The cumulative damage rule is an approach used to obtain
fatigue damage by dividing the stress range distribution into
constant amplitude stress blocks, assuming that the damage per load
cycle is the same at a given stress range.
7“11