ssc-367 - Ship Structure Committee

Typically, the sea state represented by joint probabilities of
significant wave heights and characteristic periods (scatter
diagram) is applied to the transfer function to produce the stress
range spectrum. Integration of the spectra provides a number of
statistical parameters, such as the bandwidth, the zero-uncrossing
frequency, etc., allowing development of short-term probability
density functions.
The short-term probabilitydensity function of the stress range for
each significant wave height and its characteristic period is
generally defined by using a Rayleigh distribution. For this
assumption to be valid, (1) a large number of sea states must be
used, and (2) the stress cycles can be considered narrow-banded.
Individualstress cycles are considerednarrow-bandedwhen they are
readily identifiable and there is no ambiguity in counting the
stress cycles. The wide-banded loadings exhibit smaller stress
cycles interspersed among larger stress cycles. Because it is
difficult to define the stress cycles, different cycle counting
methods result in different fatigue damage predictions.
Rainflow counting is the name of a large class of stress cycle
counting methods, including the original rainflow method, Hayes
method, range-pair counting, range-pair-range counting, ordered
overall range counting, racetrack counting and hysteresis loop
counting.
Rainflow counting and other alternatives are briefly discussed in
Sections 7.4.2 and 7.4.3, respectively, to illustrate the options
availableto upgradeMiner’s rule. However, it should be noted that
two very important variables affecting fatigue life computation
should be addressed in any attempt to upgrade Miner’s rule:
(1) S-N curves are based on constant amplitude stress blocks and
should be compared against variable amplitude results.
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