Page 138 - Failure Analysis Case Studies II

Investi 123

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Fig. 8. Port bilge keel and keel showing three longitudinals, bilge bracket and fracture face of hull plating

clearly visible. The fracture in the shell occurred just aft of the forward bilge bracket, but there was
no evidence that the crack was connected in any way with the bracket as the fracture of the shell
appeared to be entirely in the plate material. After propagating from the bilge keel, the crack ran
in a brittle manner up the port side shell plate (as shown in Fig. 8). Unfortunately, the exact point
at which the crack arrested could not be ascertained as the brittle fracture continued into the heavily
buckled and deformed region of the hull where all fracture characteristics had been completely
destroyed. Due to the deformation incurred in this region, however, it would appear reasonable to
assume that this was one of the last parts of the hull to fracture, and thus the initially brittle crack
did arrest in this region.
Figure 12 shows the deck plates. From the crew’s evidence, this was the last region to fracture,
the two halves of the ship hinging on the deck. This would seem to be substantiated by the
deformation of the plate. However, the brittle behaviour of the gunwale plate, sheer strake and port
deck section is also evident. A discontinuity of the fracture was apparent on closer examination of
the port gunwale. The chevron markings above and below this discontinuity point towards an
initiation site in the shell plate port side.
These markings were followed down the port sheer strake until the web-sheer strake detail. At
this joint, there appeared to be an initiation point, since the chevrons below this detail indicated
fracture emanating from this weld, where there appeared to be a lack of penetration defect. However,
as the fracture face was almost horizontal at this point, close inspection was difficult. The fracture
appeared to have propagated down the ship’s hull from this detail for approximately 6-8 ft (2-
2.5m) before running into the heavily deformed region described above, as seen in Fig. 13. As this
possible initiation site is - 12ft (-4m) below the level of the deck, it would appear that the crew’s