Page 144 - Failure Analysis Case Studies II

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Kurdistan, deduced from the investigations of [I] are presented in chronological order below,
drawing on the test data where necessary to justify the conclusions reached. A summary of the
mechanical test results is given in Table 1.

3.2. The initiation of fracture
The port bilge keel detail extracted in the principal test sample was the primary initiation site.
Crack initiation by a cleavage mechanism occurred from a defect situated in the ground bar butt
weld contained in this sample. This defect was formed by incomplete penetration due, in part, to
the lack of an edge preparation on the ground bar plate (Fig. 15). In addition, during assembly of
this detail the bulb bar plate was almost certainly attached by intermittent welds (Fig. 16) prior to
the completion of the ground bar weld. This procedure resulted in an area of no weld on the outer
lower edge of the ground bar joint, which, together with the lack of penetration defects present in
the double-sided section of the weld, formed the defect shown in Fig. 17. This defect constituted an
effective stress raiser, which was extended in service by fatigue, as illustrated in Figs 18 and 19.
Three separate areas along the length of the defect showed evidence of fatigue damage, as shown in
Fig. 20. This fatigue crack growth had the separate effects of increasing the overall defect size and
enhancing the acuity at the notch tip. The initiation toughness of the weld metal in this region was
believed to be low at the sea temperature at the time of the incident (- 1 "C) at low strain rates, and
very low under medium and high rates. This conclusion was reached from toughness tests carried
out on the corresponding weld detail from the starboard side of the vessel (Table 1). Metallurgical
examination showed that this detail had similar microstructure, composition and hardness to the
ground bar weld metal in which the initiation site was located, and is thus considered to have very
similar toughness properties. The weld detail of the ground bar weld of the starboard sample differed
from the port sample in that it was double-sided over its entire length, although, due to the lack of
edge preparation, it contained a comparable lack of penetration defect. No evidence of any fatigue
crack propagation comparable to that found in the port sample was observed.

Table 1. Summary of mechanical test results
Plate tensile properties (primary sample) at + 20 "C
uy = 243-258 N mm2
au =41748 Nmm2
Elongation = 32-34.5%
Pellini NDT temperatures
Plate No. NDT
1A >O"C
IC -5°C
2A +5"C
3A 0°C
Weld metal Charpy results (starboard bilge keel sample)
Test temperature ("C) CV energy (J) Crystallinity (%)
- 10 8 95
-1 5, 8, 48, 80 95,90, 50, 35
+5 6, 10 95,85
+ 10 14,30 60,60
+ 20 28 70
Weld metal CTOD results (BS 5762) at - 1 "C
Test rate (mms-I) CTOD (mm) Type of result
0.01 0.10 6U
0.11 6m
1 .o 0.04 6C
0.16 6C
0.07 6c
4.50 0.03 6C
0.04 6C