Page 79 - Ship Construction.DJ Eyres 6Ed

Ch08-H8070.fm Page 68 Wednesday, October 18, 2006 7:29 AM
68 Ship Construction
Second World War the incidence of this phenomenon was high amongst
tonnage hastily constructed, whilst little was known about the mechanics of
brittle fracture. Although instances of brittle fracture were recorded in
riveted ships, the consequences were more disastrous in the welded vessels
because of the continuity of metal provided by the welded joint as opposed
to the riveted lap which tended to limit the propagating crack.
Brittle fracture occurs when an otherwise elastic material fractures with-
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out any apparent sign or little evidence of material deformation prior to
failure. Fracture occurs instantaneously with little warning and the vessel’s
overall structure need not be subject to a high stress at the time. Mild steel
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
used extensively in ship construction is particularly prone to brittle-fracture
given the conditions necessary to trigger it off. The subject is too complex to
be dealt with in detail in this text, but it is known that the following factors
influence the possibility of brittle fracture and are taken into consideration
in the design and material selection of modern ships.

(a) A sharp notch is present in the structure from which the fracture
initiates.
(b) A tensile stress is present.
(c) There is a temperature above which brittle fracture will not occur.
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(d) The metallurgical properties of the steel plate.
(e) Thick plate is more prone.

A brittle fracture is distinguishable from a ductile failure by the lack of
deformation at the edge of the tear, and its bright granular appearance.
A ductile failure has a dull grey appearance. The brittle fracture is also
distinguished by the apparent chevron marking, which aids location of the
fracture initiation point since these tend to point in that direction.
The factors which are known to exist where a brittle fracture may occur
must be considered if this to be avoided. Firstly the design of individual
items of ship structure must be such that sharp notches where cracks may
be initiated are avoided. With welded structures as large as a ship the com-
plete elimination of crack initiation is not entirely possible owing to the
existence of small faults in the welds, a complete weld examination not
being practicable. Steel specified for the hull construction should therefore
have good ‘notch ductility’ at the service temperatures particularly where
thick plate is used. Provision of steel having good ‘notch ductility’ prop-
erties has the effect of making it difficult for a crack to propagate. Notch
ductility is a measure of the relative toughness of the steel, which has
already been seen to be determined by an impact test. Steels specified for
ship construction have elements added (particularly manganese with a carbon
limit), and may also be subjected to a controlled heat treatment, which will
enhance the notch tough properties. To illustrate the improved notch ductility
of a manganese/carbon steel against a plain carbon steel Figure 8.5 is