Page 81 - Ship Construction.DJ Eyres 6Ed

Ch08-H8070.fm Page 70 Wednesday, October 18, 2006 7:29 AM
70 Ship Construction
included. Grade D and Grade E steels which have higher notch ductility are
employed where thick plate is used and in way of higher stressed regions, as
will be seen when the ship structural details are considered later.
In association with the problem of brittle fracture it was not uncommon
at one time to hear reference to the term ‘crack arrester’. The term related
to the now outdated practice of introducing riveted seams in cargo ships to
subdivide the vessel into welded substructures so that any possible crack
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propagation was limited to the substructure. In particular such a ‘crack
arrester’ was usually specified in the sheerstrake/stringer plate area of
larger ships. Today strakes of higher notch toughness steel are required to
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
be fitted in such areas. Lloyds Register, for example, require the mild steel
sheerstrake and stringer plate at the strength deck over the midship por-
tion of vessels of more than 250 metres length to be Grade D if less than
15 mm thick and Grade E if of greater thickness (see Chapter 17).

Fatigue Failures

Unlike brittle fracture, fatigue fracture occurs very slowly and can in fact
take years to propagate. The greatest danger with fatigue fractures is that
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they occur at low stresses which are applied to a structure repeatedly over a
period of time (Figure 8.5). A fatigue crack once initiated may grow unno-
ticed until the load bearing member is reduced to a cross-sectional area
which is insufficient to carry the applied load. Fatigue failures are associated
with sharp notches or discontinuities in structures, and are especially pre-
valent at ‘hard spots’, i.e. regions of high rigidity in ship structures.
With the growth in size of oil tankers, bulk carriers and container ships
there has been increasing use of higher yield strength steels in their hull
structures. The classification societies have subsequently placed special
emphasis on analysis of the fatigue performance of these larger structures,
usually over a 25 year life cycle, as part of their approval process.

Buckling

With the substantial increase in size of oil tankers, bulk carriers and con-
tainer ships in recent years greater attention has had to be given to the
buckling strength of the stiffened plate panels constituting the shell
Buckling of a structural member loaded in compression may occur at a
stress level that is substantially lower than the material’s yield stress (see
Figure 7.1). The load at which buckling occurs is a function of the structural
members geometry and the materials modulus of elasticity rather than the
materials strength. The most common example of buckling failure is the
collapse of a pillar under a compressive load. A stiffened plate panel in