ssc-367 - Ship Structure Committee

2.
OVERVIEW OF FATIGUE
2.1
FATIGUE PHENOMENA
Metal structures subjected to variable or repeated loads can fail
without ever reachingtheir staticstrengthdesign loads. This type
of failure,which consistsof the formationand growth of a crack or
cracks, has come to be known as “fatigue”.
Failures observed due to the growth of defects subjected to cyclic
loadings is due to a very complex phenomena, affected by many
parameters. Any environment or condition that results in cyclic
Ioading and reversalof componentstressesmay cause fatiguedamage.
Cyclic stresses are typically caused by machinery vibrations,
temperature changes and wind and wave actions. But although
vibrations and temperaturechanges may be important to fatigue in a
local component, these loadings are not a major concern in the
global behavior of typical marine structures. Thus, the overview
presented in this section addresses wave and wind environments,
excitation forces on mobile and stationary structures and the
response of these structures to excitation forces.
A defect subjected to a large number of cyclic stresses undergoes
three phases of stable crack growth:
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Crack initiation, or development of a defect into a
macroscopic crack.
Crack propagation, or development of a crack into a critical
size.
Cracked weldment residual strength exceedence.
The relative durations of these three phases depend on many
variables,includingmaterialproperties,defectgeometry, structure
stiffness, stress cycle magnitudes, distribution and sequence,
operating environment and maintenance. The objective is to prevent
fatigue failure by designing to ensure that the time required to
2-1