ssc-367 - Ship Structure Committee

D is the cumulative damage,
k is the number of stress blocks,
n is the number of stress cycles in stress block i with
constant stress range, and
N is the number of cycles to failure at constant stress range.
Although the linear Palmgren-Minerdamage rule is extensively used,
the significance of constant-amplitudeloading and the sequence of
loading (i.e.,large stress blocks during the beginning rather than
toward the end of design life) may be important to correct
assessmentof fatigue damage. This subject is discussed further in
Section 7.
Fatique Life Evaluation
Fatiguedamage and fatigue life should redetermined at all critical
hot-spot stress areas. While one or two areas may be targeted on a
plate and stiffenerinterface,at least eight points are recommended
on a tubularmember. If eight points, spaced at45 degree intervals
around the circumference, are chosen, relatively accurate hot-spot
stresses and fatigue damage data will be obtained. Typically,
fatigue damage (D) is calculated on an annual basis. The fatigue
life (L) is then determinedby taking the inverseof the accumulated
damage ratio (D).
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