Page 361 - Failure Analysis Case Studies II

346
I '0 ring seating surface

Notches and cracks
tend to be found in this area

-
14 mm
185 mm
Fig. 1. The top of an aluminium gas cylinder with the area of crack initiation and growth indicated.

1.2. Faiiure experience
In 1983, two fatal ruptures of hoop-wrapped aluminium cylinders occurred in the United States.
A significant report made of those incidents at the time was prepared by Failure Analysis Associates
[4]. This report considered and compared three categories of cylinders which were provided by the
manufacturers. This and other work Icd to the identification of risk factors associated with the
aluminium cylinders in traffic in the U.S. These were:
0 cracking originating at the neck shoulder region as shown on Fig. 1;
0 folds in the neck region; and
0 lead levels of 100 ppm or higher in the aluminium alloy.
There have been recent incidents in the US., namely the death of a fire fighter filling a cylinder in
1993 [5] and an injury in a Miami dive shop in June 1994 [6].
In Australia, most cylinders in circulation are made locally. These cylinders do not have high lead
contents and are not made with recycled scrap, which were two of the factors featured in the early
US. studies. Nevertheless, there has been some failure experience with Australian cylinders, though
no serious injuries. Poole [7] describes a cylinder made to Australian standards which failed cata-
strophically in 1994 in New Guinea during hydrotest in a shop.
One key area of protection of the public is a requirement that all cylinders be inspected regularly.
For scuba tanks the frequency in Australia is annual; for other aluminium cylinders, the required
frequency is four-yearly. This inspection is visual and carried out with lights and dental mirrors.
A number of testing stations in Australia have reported that a significant number of locally made
cylinders experience cracking from the neck. The authors have themselves investigated a number of
cracked cylinders. For some years of cylinder manufacture in the mid-l980s, the frequency of
detection of cracking at the necks in 1995 was as high as 10% at some test stations. Because of the
method of inspection, it is not clear how serious these detections are, and the cracking can often be
shallow. Since 1995, all cylinders with visible cracking must be condemned.
The alloy in use in Australia and other countries was changed from 6351 in T6 temper to 6061
T6 during the last few years. The 6061 alloy is believed by the industry to have less susceptibility to
cracking, though the authors have one cylinder in this alloy which has a crack in the neck area.
Stark and Ibrahim [8] present data comparing the two grades that do not indicate a significant
difference in either K,, or crack growth rates.