Practical fracture of bilge- and bottomplating of an oil-tanker, Commission XIII I.I.S./I.I.W., Document XIII-409-65, 1965

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ARCHIEF

REPORT No.

SEPTEMBER 1965

SSL 102

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

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s

Commission XIII, IXD I.I.S./I.I.W.

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:

SSL 102

Fractured partially in January 1964, no further particulars submitted. As the total length of the crack was only 1,5 m, the nominal stress at

fracture will not have been excessive; 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

at end of this communication).

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 lower-end (bottom-end) the brittle fracture did not run far. It was arrested in a weld. This is probably due 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.

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Transverse section

STRUCTURE:Transverse section; Long.section

Ferrite + Perlite+ Ferrite bands including slag.

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Longitudinal section

Chemical composition

C :0.19 %

P :0.059%

Si :0.026%

S :0.053%

Mn.:0.62 %

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RESULTS OF CHARPY-V IMPACT TEST

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60 RESULTS OF

DROP-WEIGHT

AND ROBERTSON

TESTS O

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MATERIAL

BUTT-WELD

OF

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N.D.T. Temp.

Isotherm Arrest Temp.

E.7

+30°C

E.7

35 t

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+40 °C

D.9... +20°C D.9

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MATERIAL OF LONG.,

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5e Conclusions.

4e Material in the region of fracture. (See page 2).

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.

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. (Figure 3). The fracturesurface was badly corroded so that in this particular case nothing can be said with respect to the influence of fatigue.