SEPTEMBER 1 966
SHIP STRUCTURES LABORATORY
TECHNOLOGICAL UNIVERSITY
- DELFI
COMMISSION XIII
- I.I.S./I.I.W.
DOCUMENT XIII-409-65
PARTIAL FRACTURE OF BILGE- AND
BOTTOMPLATING OF AN OIL-TANKER
Partial fracture of bilge- and bottomplating of an oil-tanker.
le General.
Mainly welded oil tanker, deadweight: 18.200 ts, built in 1955.
2e Circumstances of fracture.
Fractured partially in January 1964, sea-temperature about +4°C.
The total length of the crack was only 1,5 m. As the Robertson-arrest-tempe-rature of the material at the end of the crack was much higher than 4°C, this short crack-length might be due to the nominal stresses being low; the maximum might be about 1000 kg/cm2.
3e Details of fracture.
The fracture started at an interruption of the flat bar of a bilge-keel where a transverse butt weld in the bilge plating passed. (See figure 1).
The evidence is large that the origin of the brittle crack was a small fatigue crack of about 20 mm long and 5 mm deep.
The contour of this crack was rather well visible on the rusted fracture surface, as can be seen in figures 2a and 2b.
At the other end of the interruption in the bilge keel a small crack has
been found 15 mm in length and about 2 mm deep. (See figure 1). It is believed that this one is also a fatigue crack.
At the higherend
the brittle fracture did not run far.ft
was arrested in a weld. This can be attributed to 3 factors:a the small energy available as a consequence of the small distance between the origin of fracture and the weld.
b the presence of a favourable (residual) stress-field which led the crack into the weld.
e good Charpy-V characteristics of the weld. (See fig. 5 and 6).
4e Properties of the material in the vicinity of the fracture.
The micro-structure, chemical composition and grain size are given in and below fig. 3 and 4. The results of Charpy-V; Drop-weight and Robertson tests are given in and below fig. 5 and 6. They apply respectively to the plate in which the fracture started (E8), the plate in which the fracture ended (D9), for the material of the butt-weld in which the other end of the crack had been arrested and for plate E7 without fracture.
Fig.1
BiLge
knee
Brittle
Position of CharpyV specimens.
fracture
butt - we Ldsmall fatjue crack
Section IAA
- -
-Crack
(total Length 1.5m
Bilge plate
(19mm)
Tha nsve r sebutt weLd
22r ivet-7d
Bottom
plating
(19.6 mm)
w&D9
Fig. 2a
of fracture.
Fig. 2b
Fig.4
Longitudinal section
Chemical composition
C :0.19 °/P :0.059%
Si :0.026%
S :0.053%
Mn.:0.62 °/
Grain-size: 6-7 (,A.S.T.M. designation 19-46)
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Fig.3
Transverse section
STRUCTURE:
Ferrite +PerliteFerrite bands including sLag.
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Fig.5
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O 80 60 40 20 ORESULTS OF CHARPYV IMPACT TEST
60
-'-MATERIAL OF LONG.
AND TRANSVERSE
BUTTWELD
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RESULTS OF
N.D.T. Temp.
Isotherm Arrest Temp.
DROPWEIGHT
E.7+30°C
E.7AND ROBERTSON
E.8... +30°C E.8+40 °C
TESTS D.9... +20°C D.9
35 °C
_400
20°
0020°
+40°
+600
40°
_200
°
20°
40°
+ 60°
TEST TEMPERATURE (CENTIGRADE)
50
40 30 Vt -a20
15 105e An electron-microscope investigation clearly revealed the fatigue-character
of the crack at which the brittle fracture started. It seemed to be a normal high-cycle-low--stress crack. (See appendix).
Conclusions.
This is a normal case of fracture because:
a The design detail was inferior.
b A concentration of weidments was present.
C The material was really bad.
d A (corrosion-)fatigue crack had developed.
7e Comments.
A similar brittle fracture formed two years before in a ship of the same
type. The material at the origin of the fracture was of the same quality. The fracture was larger in length and had been arrested at one end in a bottomplate of Grade D quality and at the other end in a side-shell plate of Grade B quality.
A photograph of the origin of the fracture is included. (Fig. 7, below fig. 2).
The fracturesurface was badly corroded so that in this particular case nothing can be said with respect to the influence of fatigue.
Appendix
Visual and Electron-microscope investigation of fracture surface.
- fatigue fracture started at arrow in figure 8;
- startingpoint was situated in transition from weld to plate;
- there was no indication of a "hot crack";
- fracture surface was smooth with resting lines;
Microscopical investigation.
Carried out by Metal Institute T.N.O.
Visual investigation.- no deformation was observed along the fatigue fracture;
- the fatigue-crack had an extension which did not coincide with the final brittle fracture surface. (Figure 9).
Fig.8
V: lix
Fig.9
V: 50x
Electronmicroscopical investigation.
It is known that low stress fatigue gives rise to dislocation-bands while high stress fatigue is accompanied by cells.
Examples of dislocationstructure due to high-stress fatigue are given in figures 10 to 13. Cell-structure and dislocation loops are clearly visible.
The dislocationstructure due to brittle fracture is quite different. It is characterised by long dislocationlines orientated in two main directions and a low dislocation-density at temperatures close to 0°C.
Results of investigation.
Figures 14 to 20 show results for the fatigue-part of the fractured plate and figures 21 to 24 for the brittle fracture part. In figures 14 to 20 a high dislocation density is visible which is a proof of prolonged deformation. No pronounced cell-structure exists which suggests relatively low cyclic
stresses. Clusters of band structure and many loops also point to normal fatigue. Figure 20 which applies to the material close to the borderline between the
