Originally Posted by DumnMad
Exactly, fatigue failure is caused by cyclic loading and ocurrs faster at higher stresses.
Overload the bolts a few times from grounding reduces the fatigue life by leaving residual stress in the steel
, then cyclic loading from wave action can finish it off.
Toughness, or the ability to absorb energy, is needed for a keel
to survive an impact.
The comment 'fatigue occurs faster at higher stresses' requires some clarification.
At lower frequencies with longer duration loads then strain is often the primary failure mode. A keel
strike is a good example of this type of loading.
, temperature has a significant bearing on the toughness of steel. The titanic suffered catastrophic failure because the glass transition temperature of its hull
steel was above the sea temperature. Even welds in high strength steels can be shown to have vastly different properties to the parent metal.
Failure is usually a mix of cyclic loadings. Miners law is one means we can use to understand a failure comprised of a range of loads. A single
large impact load can consume a large percentage of the useful life of a steel structure.
When an overload occurs then yielding, deformation and crack propagation become significant.
also acts to reduce toughness in varying forms.
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