Page 148 - Failure Analysis Case Studies II

133

Bilge strake
shell Bulb
plate / bar
propagation i
up port shell
plate

Crack propagated
from ground bar
butt weld via
ground bar to
shell weld into
bilge strake plate
Brittle crack
propagation
running into
bottom shell

Fig. 20. Crack initiation in ground bar weld and subsequent propagation into shell-port bilge keel detail.

3.3. Propagation
Once the crack had initiated in the ground bar weld, it was able to propagate into both the bulb
bar and the shell, since the dynamic toughness of the shell to ground bar fillet weld, and the bulb
bar to ground bar intermittent lap weld were insufficient to arrest a running crack at the sea-water
temperature (- 1 "C), as evidenced by the Charpy data obtained on representative samples (Fig.
21). In addition, there was no crack arrest hole in the ground bar butt weld, the presence of which
may have prevented the crack from propagating into the hull. Furthermore, the dynamic toughness
of the Grade A plate used in the hull and the bulb bar was also inadequate to arrest a running
crack, as discussed in [I].
Although the oil cargo was believed to be heated to around 60 "C, it is surmised that the shell
plate below the water-line was close to the sea temperature of - 1 "C, since the results of the Pellini
drop weight tests indicate that the nil ductility transition (NDT) temperature was around 0 "C
(Table I). Had the shell plate been at a higher temperature, it is likely that shear lips would have
been observed on the shell plate fracture surface, and no such evidence for ductile fracture was
observed .
Once the crack had entered the shell, it propagated in two directions:
(a) Up the port side until it arrested at an indeterminate point at least 3m above the bilge keel.
Due to extensive mechanical damage to the fracture surface, the precise point of arrest was not
evident.
(b) The crack also ran across the entire breadth of the bottom shell plate and up the starboard side

IVU'
- 90.
80-
6 70-
x
60.
al
5 50- Shell plate (1C)
$ 40. --- -- Bilge bar (IA)
30. WLld metal (6W)
IO -
20. 27 J level
0- '