Page 150 - Failure Analysis Case Studies II

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After the first incident:
The centre girder is likely to have experienced 1-2 in. brittle fracture
which arrested leaving the girder essentially intact.
The starboard bulkhead experienced 3 in. brittle fracture which then arrested.
Longitudinals 1-7 starboard of the centre girder are thought to have fractured.
Longitudinals 1 and 2 (port) may have seen crack arrest in the drain holes.
Port longitudinals 3,5,6 and 7 fractured. No. 4 remained intact.
Brittle fracture was apparently continuous along the length of the bottom shell.

brittle
fracture

Continuous brittle fracture
Fig. 24. Crack propagation during the first incident-bottom shell fracture.

After the first incident:
apart from a 3in. brittle crack
Brittle crack propagation
running into region where extending from the bottom shell
fracture face was destroyed the longitudinal bulkhead
remained intact.
crack is thought to have 1 I Longitudinals IO and I I are
by mechanical damage -
arrested before water- thought to have fractured but 8,
9, 12 and 13 probably experienced
arrest of the brittle crack
as it entered the drain holes and
thus remained partially intact.
Brittle fracture was apparently
continuous along the bottom
shell and through the starboard

Continuous
brittle fracture
Brittle crack
propagated through
starboard bilge keel
Fig. 25. Crack propagation during the first incident-starboard bilge keel region.

situated in a low-stress region. Thus, it is considered that these were secondary, and not primary
initiation sites. The extent of propagation from these cracks could not be determined, but cracks
initiating from these sites were thought at the time of the investigation likely to correspond with the
location of oil leaks witnessed by the crew.
After the first incident, it would appear that the vessel was held intact by partially fractured
longitudinal bulkheads and bottom longitudinals, the upper regions of the hull sides, and the deck
plate and its associated longitudinals. The bottom shell plate crack could have arrested either due
to a rise in temperature, and hence toughness of the steel, or to its running out of driving stress.

3.4. Events leading toJinal fracture of vessel
Due to the extensive damage to the shell structure which existed after the initial incident,
separation of the two sections of the vessel is considered to have been inevitable, and thus the events
leading to the final fracture of the vessel must be considered to be of secondary importance. Due to