Page 141 - Failure Analysis Case Studies II

126

Fig. 12. View of the deck plate fracture: part of the deck and longitudinal ripped out during final fracture is
also shown.

has been removed: the remainder of the fracture surface has corroded. Figure 6 indicates that the
fracture entered the starboard bilge keel forward of the initiation point on the port. The equivalent
bilge keel detail to the port side fracture was thus intact on the starboard side.
The crack appeared to continue propagating upwards through the starboard shell plate for at
least 10 ft (- 3 m). As on the port side, a region of deformed plate was then evident where the
fracture face had been destroyed by mechanical damage. There was also evidence of some damage
occurring to the starboard side after the complete failure of the ship, involving a rip in the ship's
plate just above the starboard bilge keel.
Inspection of the stringer and starboard shell plate detail revealed another possible initiation site,
and a possible arrest and reinitiation site nearby. The fracture initiation site was situated about 12 ft
(-4m) below the deck level in a stringer-shell plate detail just below the sheer strake. The stringer
itself suffered severe deformation. It was impossible to inspect the fracture surface above this site
as it had been badly damaged. The starboard fracture site was evident at the point just to the right
of the deformed stringer at the bottom of the sheer strake.
As described above, the bottom shell fracture appeared to occur either as one event or at least as
a series of very close events emanating from the port bilge keel detail, and the likelihood is that the
bottom shell was fractured during the first reported incident. As mentioned above, it is difficult to
determine the exact extent of the fracture to the centre girder, longitudinals and longitudinal
bulkheads caused during the initial incident.
The centre keel girder is shown in Figs 5 and 6. As the bottom shell plate fractured, so the crack
ran up the girder in a brittle manner for approximately 1-2in. (25-50mm), after which full 45"
shear fracture occurred. This fracture ran up to the cope hole, the ductile fracture continuing
thereafter. The other longitudinals failed, as given below. A general view of the longitudinals can
be seen in Figs 5 and 6.
Longitudinals starboard of centre girder (numbering from centre girder)
1 Brittle fracture
2 Brittle fracture
3 Brittle fracture
4 Brittle fracture