ssc-367 - Ship Structure Committee

DNB = (f. T/K) (2/2 U)m r (f+ 1)
where
n = f. T
T= design life
Wirsching (Reference 7.23) proposes that the fatigue damage be
expressed as:
o = a ‘NB
where A is the adjustmentfactorto fatiguedamage predicted based on
a narrow-band stress. Thus, the rainflow counting effect to fatigue
damage can be incorporated directly if J is known. An empirical
formula proposed by Wirsching is as follows:
A (E, m) = a(m) + [1-a(m)] (LE)b(m)
where
a(m) = 0.926 - 0.033 m
b(m) = 1.587m - 2.323
Thus the fatigue damage obtained by incorporating the narrow-band
adjustment factor, A provides a closed-form formulation. The
empirical formula allows fatigue damage predictions quite close to
those obtained by incorporatingthe direct rainflow method.
The A parameter introduced by Wirsching is an equivalent rainflow
adjustment factor intendedto correct the slight conservatism of the
Rayleigh distribution. Whether a closed-form or a numerical
integrationis carriedout, short-termstatisticsand the probability
density function allow obtaining of partial damage, weighting and
summing of all damages.
7-18