Page 137 - Failure Analysis Case Studies II

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Fig. 7. Side view from port showing fractures to bulkheads, port gunwhale and deck plating.

bone markings which usually characterize brittle fractures could be traced from at least 10 ft (- 3 m)
above the bilge keel detail on both the starboard and port sides, and along the length of the bottom
shell to the port bilge keel. There was no evidence of the crack having arrested at any point along
the bottom shell once the crack had begun running in the shell plate. However, this conclusion must
be conditional since the quantity of oil spilling out of the fractured stern section, and the degree of
corrosion on the parts of the shell free from oil, prevented a detailed examination of the entire shell.
Periodic checks of the crack morphology could only be made by local cleaning. Checks were made
particularly at weld details.
Figures 10 and 11 show close-ups of the bilge keel detail. The major initiation site appears to be
contained in the port bilge keel detail. The bilge keel was formed by two plates joined by intermittent
welding, the longitudinal bilge plate (or ground bar) then being continuously welded to the shell.
There is a lack of penetration defect in the weld connecting the longitudinal bilge plates. At this
particular site, there is an intermittent weld on the upper surface of the ground bar. The chevron
marks on the bilge bar or bulb plate would appear to emanate from this site. The longitudinal bilge
plate or ground bar weld has a lack of penetration defect, and also has a notch sited in a weld detail
(Figs 10 and 11). The chevron markings on the shell plate clearly indicate that fracture emanated
from the region of the ground bar to shell plate weld.
The crack morphology of the bilge keel and shell material indicate that the crack propagated
with virtually no ductility, i.e. there was little evidence of shear lips on the crack surface. On areas
which had been protected from corrosion by oil, the crystalline appearance characteristic of brittle
fracture was evident.
Figures 8 and 9 show the bilge keel and shell on the port side. The bilge bracket detail is also