Page 146 - Failure Analysis Case Studies II

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Fig. 17. Close-ups of port bilge keel

in the port sample had been two-sided, stress levels approaching ultimate tensile strength of the
plate material would have had to have been experienced under static conditions to cause failure.
The defect that actually existed in the ground bar detail in the port sample was greater than the
maximum defect size, which it is believed could have been tolerated safely at stresses likely to be
experienced under normal operating conditions at low strain rates (R= 60 N mm-3'2 SKI).
From visual inspection of the fracture surface, it was deduced that initiation occurred from the
fatigue cracked areas situated in the ground bar weld metal (Fig. 19). The intermittent weld and
ground bar to shell plate weld details in this area were not considered to be prime initiation sites
since:
(a) crack tip opening displacement (CTOD) tests have shown that these regions were likely to have
had a significantly higher initiation toughness than the partial penetration butt weld detail;