ssc-367 - Ship Structure Committee

Cumulative Damaae
The use of the Palmgren-Miner 1inear damage rule is considered
appropriate by all of the recommendations, regulations and rules.
A cumulative sum of the number of cycles at each constant stress
divided by the number of cycles to failure should always be less
than l.Oforthe desired service (design)life. While this value is
directly tied to the S-N curve selected,the desirable ratio (i.e.,
safety factor) of fatigue to service life is not always specified.
The API reconunendedfatigue life is at least twice the service
life. For criticalmembers that may affect structure redundancy and
integrity,API recotmnendsthe use of higher fatigue to design life
ratios.
The DEn Guidance Notes reconunendadditional safety factors to
account for structural redundancy and the implications of fatigue
failure on the structure. However, no specific safety factor is
recommended.
4.2.3 Fatiaue Analvsis Based on Fracture Mechanics
The fatigue crack propagation analysis is typically used to assess
crack growth and fitness-for-purposeof defects discovered at the
fabrication yard. Test data on crack growth can also be used to
determine fatigue lives. The DnV CN 30.2 document (Reference 1.7)
provides a crack growth rate data and fracture mechanics-based
procedure for fatigue analysis and design.
Whether the welded joint details have surface or root defects, the
growth of such defects into fatigue cracks depends on several
factors, including joint connection geometry, cyclic stress range
history, weld profile and defect size. The equations provided to
solve for the number of cycles to reach fatigue failure containmany
parameters and allow evaluation of various joint and defect
geometries. As an example, butt weld toe defects in a connecting
plate whether in air or seawater, can be assessed with and without
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