ssc-367 - Ship Structure Committee

complete the three-phasestable crack growth is always greater than
the design fatigue life.
The basic characteristicsof defects and the fatigue phenomena may
be summarized as:
Eventhe most thorough inspectionsat the fabricationfacility
will not reveal very small defects (less than 0.5 mm).
These defects will grow when subjectedto cyclic stresses due
to environmentalloads, structure dynamics (vortex shedding,
machinery vibrations, etc.), temperature changes, etc.
Repeated cyclic stresses and defect growth are additive,
making the fatigue damage cumulative.
In most cases, fatigue is insensitive to the presence of
constant loads. Consequently, stress ranges (i.e., peak-topeak
values) are used to characterize fatigue stresses.
Although a small number of extreme stress ranges may
contribute to fatigue damage, most fatigue damage is due to
the occurance of a large number of small stress ranges.
Poor structuraldesign details will amplify peak stresses.
Distortions and residual stresses introduced during original
fabrication (as well as extensive repair efforts) often
adversely affect material resistance to crack growth.
Corrosion and ocean environment adversely affect material
resistance to crack growth.
Asimplifiecl summary of fatigue phenomena is presented on Figure 2-
.
1.
2-2