ssc-367 - Ship Structure Committee

Although considered to be an emerging technology and necessitate
time consuming effort, probabilisticmethods have been effectively
utilized in some fatigue analyses. Typically, such a method
accounts for:
● .
Inaccuraciesin defining stresses due to random loadings
●
Uncertainties and observed scatter in S-N data
●
Randomness of failure in the use of simplified models
A probabilistic method recommended by Wirsching (Reference 4.11)
utilizesa fulldistributionalprocedureand the variablesdiscussed
above are assumed to have a log-normal distribution.
Adetailed analysis and design method, based on the use of a finite
element model, to determine environmental loading, vessel response
and load and stress distribution does not need to be a complete
probabilisticmethod. Daidola and Basar(Reference 4.12) d“ scussing
lack of statistical data on ship strengths and stresses recommend
development of a semiprobabilisticanalysis method which does not
require a distribution shape.
Fracture Mechanics Methods
A fracture mechanics method addresses the relationship between
defect geometry, material, and the stress history. The defect
geometry can be accurately modeled with finite elements. Stress
intensityfactorscharacterizingthe defect behavior and the fatigue
crack growth laws allow determination of defect growth
characteristics. Thus, a hypothetical or an actual defect is used
as the basis for determining the fatigue life and identifying the
necessary corrective measure.
The initial defect size and location and the stress intensity are
very important parameters in determining crack growth period to
failure. The fracturemechanics approach is a useful tool to assess
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