Page 154 - Introduction to Naval Architecture

.140 STRENGTH

Built-in stresses

Taking mild steel as the usual material from which ships are built, the
plates and sections used will already have been subject to strain
before construction starts. They may have been rolled and unevenly
cooled. Then in the shipyard they will be shaped and then welded,
As a result they will already have residual stresses and strains before the
ship itself is subject to any load. These built-in stresses can be quite
large and even exceed the yield stress locally. Built-in stresses are
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difficult to estimate but in frigates it was found that welding the
longitudinals introduced a compressive stress of SOMPa in the hull
plating, balanced by regions local to the weld where the tensile
stresses reached yield.

Cracking and brittle fracture

In any practical structure cracks are bound to occur. Indeed the build
process makes it almost inevitable that there will be a range of crack-
like defects present before the ship goes to sea for the first time. This
is not in itself serious but cracks must be looked for and corrected
before they can cause a failure. They can extend due to fatigue or
brittle fracture mechanisms. Even in rough weather fatigue cracks grow
only slowly, at a rate measured in mm/s. On the other hand, under
certain conditions, a brittle fracture can propagate at about 500 m/s.
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The MVKurdistan broke in two in 1979 due to brittle fracture. The MV
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Tyne Bridge suffered a four metre crack . At one time it was thought
that thin plating did not suffer brittle fracture but this was disproved by
the experience of RN frigates off Iceland in the 1970s. It is therefore
vital to avoid the possiblity of brittle fracture. The only way of ensuring
this is to use steels which are not subject to this type of failure under
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service conditions encountered .
The factors governing brittle fracture are the stress level, crack
length and material toughness. Toughness depends upon the material
composition, temperature and strain rate. In structural steels failure at
low temperature is by cleavage. Once a crack is initiated the energy
required to cause it to propagate is so low that it can be supplied from
the release of elastic energy stored in the structure. Failure is then very
rapid. At higher temperatures fracture initiation is by growth and
coalescence of voids and subsequent extension occurs only by
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increased load or displacement . The temperature for transition from
one fracture mode to the other is called the transition temperature. It is
a function of loading rate, structural thickness, notch acuity and
material microstructure.