ssc-367 - Ship Structure Committee

Current recommendations,rules and standardsuniformlyallowthe use
of Miner’s rule to compute the cumulative damage. Applicable
cumulative damage rules are discussed in this section, followed in
Section 7.4 by a discussion of stress spectrum in the context of
fatigue damage computation.
7.3.1
Miner’s Rule
The damage for each constant stress block is defined as a ratio of
the number of cycles of the stress block required to reach failure.
Thus, the Palmgren-Minerlinear damage rule defines the cumulative
damage (D) for multiple stress blocks as equal to:
‘i
D=:T < 1.0
i=l i
As briefly discussed in Section 3.2.5, Miner’s rule can either
overpredict or underpredict the cumulative damage.
One source of inaccuracy regarding cumulative damage is the
applicationof constantamplitudestressblocks; itmay be important
to be able to predict the fatigue damage due to variable amplitude
loading. Another source of inaccuracy is the sequence of loading;
while Miner’s rule cannot account for the loading sequence,
occurrence of large amplitude loads early in fatigue life can
accelerate the rate of crack growth. Another source of inaccuracy
for wide band processes is the choice of cycle counting method,
which is further discussed in Section 7.4.
Despite these sources of potential inaccuracy,Miner’s rule is used
to compute fatigue damage because of its simplicity as well as its
ability to predict fatigue damage conservativelymost of the time.
Other uncertainties in determining wave environment, wave loading
and hot-spot stresses contribute far more to the inaccuracy of
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