Page 362 - Failure Analysis Case Studies II

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2. EXAMINATION OF TWO SEVERELY CRACKED CYLINDERS

Among several defected cylinders and samples the authors have investigated two Australian made
SCUBA cylinders which have large defects in them. Cylinder A leaked during filling and cylinder B
failed catastrophically during hydrotest.
These cylinders do not conform to the model of failure as described by Failure Analysis Associates
[4]. Although the origin of the cracking was in the same location on the neck as the U.S. experience,
neither cylinder has significant neck folds and lead levels are below the limit of measurability using
the standard spectral analysis test, that is, below 10 ppm.

2.1. Cylinder A
Cylinder A was made in 1983 and leaked during filling in 1994. In 1994 the Health and Safety
Organisation (HSO) in Victoria withdrew an aluminium cylinder from traffic and provided it to the
authors for investigation. This cylinder exhibited cracking so large as to cause a leak, making it
impossible to fill. There is a second defect of almost the same size almost opposite the leaking crack
(see Fig. 2). The cylinder is of 8.65 kg water capacity with a test pressure of 32.4 MPa, manufactured
in August 1983. The leaking defect penetrated through to the upper surface of the cylinder outside
the O-ring contact surface and thus caused the leak.
The fracture surfaces have been examined under scanning electron microscope as reported in
Price et al. [9]. Most of the defect surface exhibits the features observed before (for example by
Lewandowski et al.) and thus probably has the SMIE growth mechanism. However there is evidence
of a defect about 3 mm deep by 15 mm long at the neck shoulder which has a different appearance
and is probably a pre-existing defect from which the whole defect grew.

2.2. Cylinder B
The other specimen is part of the cylinder which failed in New Guinea. Discussed by Poole [7],
this was made in August 1987 but otherwise has similar specifications to cylinder A. This cylinder
failed catastrophically during hydrotest in New Guinea on 13 February 1994 and was inside a water-
filled concrete tank which also burst. The failure occurred in four places around the neck and the

Fig. 2. Inside the top of cylinder A. Two large cracks almost opposite each other are marked. This specimen
has been broken open and crack growth was found to proceed under the surface further than detectable using
dye penetrant on the surface. The defect appearance conforms to the intergranular growth mechanism.