Page 340 - Practical Ship Design

Structural Design 297

They also have a nuisance value due to leaks that they permit and can necessitate
expensive repairs. Cracks almost invariably start from welded joints and the avoid-
ance of cracks demands that these are given a high standard of detailed design.
Fatigue endurance appears to be independent of the type of steel employed, so
the higher stress levels used with higher tensile steels mean that there is a lower
fatigue life with these steels. As the fatigue endurancehtress relationship is
governed by a cubic law, a small increase in stress can lead to a major reduction in
fatigue life - a fact demonstrated in recent years by the problems found on large
tankers built using a major proportion of higher tensile steel.
The poor fatigue properties of aluminium in a marine environment have already
been mentioned. If the stresses in an aluminium deckhouse are to be so reduced
that it will have an equivalent fatigue life to that of a steel deckhouse, the scantlings
have to be so increased that there is little weight saving.

10.4.3 Brittle fracture
Brittle fracture causes the sudden propagation of a crack which can extend for a
considerable distance and has led to the loss of a number of ships. It is triggered by
the rate of application of stress and is greatly influenced by ambient temperature
and steel thickness.
Ordinary mild steel becomes prone to brittle fracture at temperatures approaching
O’C, and Classification Societies require the use of notch tough steels in areas of
ships which are subject to high stress levels, particularly when these require thick
plating, and a more general use in ships, such as ice-breakers and refrigerated
cargo ships, in which the steel will be subject to sub-zero temperatures.
The quality of steel is usually indicated by its “Charpy” value which is a
measure of the energy required to propagate a crack. Charpy “J” values together
with the temperatures at which these tests are made for different grades of steel are
given in Table 10.1.
In specialist ships such as warships a more general use of notch tough steel may
be wise.

10.5 MINIMISING STEELWEIGHT AND/OR STEELWORK COST

Minimising steelweight is of particular importance in deadweight carriers, in ships
required to have a limited draft, and in fast fine lined ships.
On other ship types it is still desirable to minimise steelweight to reduce
material cost but only when this can be done without increasing labour costs to an
extent that exceeds the saving in material costs. On the other hand, a reduction in
structural labour cost achieved by simplifying construction methods may still be
worthwhile even if this is obtained at the expense of increasing the steelweight.