ssc-367 - Ship Structure Committee

fatigue damage predictions. Fatigue analysis assumptions also
contribute to the inaccuracyof fatigue damage predictions. As an
example, 10 to 15 stress blocks, each representing a significant
wave height and a zero-crossing point, may be used in the fatigue
analyses. Theuseof40to 50 stress blocks is desirable, but often
considered impracticalfor most analyses.
7.3.2
Alternative Rules
The ability to use servohydraulic testing machines and to apply
computer-controlled loads has allowed testing of a substantial
number of specimens subjected to variable amplitude loading
(References 7.16, 7.17, 7.18 and 7.19). Gerald et al (Reference
7.20) provide an excellent overview on variable amplitude loading.
Some analytical work carried out and many of the test results show
that Miner’s rule is realistic and conservative. However, some of
the test results also show that Miner’s rule may lead to
underpredictionof fatigue damage.
One source of discrepancymay be crack growth fluctuations. Stress
block procedures used in tests result in the application of high
tensile stresses, which can retard crack growth. Test specimens
subjected to random loadings are less likely to have similar high
tensile stresses. Another source of discrepancy is the counting of
stress cycles. Gurney (Reference 7.17) and Trufiakov (Reference
7.21) conclude that small fluctuations superimposed on each stress
cycle add substantiallyto fatigue damage.
Miner’s rule is the acceptedmethod for fatigue damage computation.
However, since alternativesto Miner’s rule have been proposed it is
beneficial to review one such rule.
Gurney proposes a damage rule by expressing the applied stress
spectrum in terms of the maximum stress range (Smax),the number of
cycles (ni) applied at proportions (pi) of smax, and its length
(1 ni)defined as the block 1ength. Gurney’s rule states:
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