Page 360 - Failure Analysis Case Studies II

Failure Analysis Case Studies II
D.R.H. Jones (Editor)
0 2001 Elsevier Science Ltd. All rights reserved 345

SUSTAINED LOAD CRACK GROWTH LEADING TO
FAILURE IN ALUMINIUM GAS CYLINDERS IN TRAFFIC

J. W. H. PRICE*, R. N. IBRAHIM and D. ISCHENKO
Mechanical Engineering Department, Monash University, 900 Dandenong Road, East Caulfield,
Victoria 3145, Australia
(Received 26 June 1997)

Abstract-Some common portable aluminium gas cylinders have shown a liability to develop cracking. This
cracking has in some cases led to leaks and on occasions to violent and sometimes fatal failures. There are a
number of features of this cracking which have not been properly explained. Previous modelling of the growth
of these cracks under sustained load has been developed from specimen testing. As is shown in this paper these
data produce results which appear to produce values of crack growth which are too slow by a factor of the
order of lo* to explain the observed phenomenon. It also appears that crack growth can be rapid even in
cylinders with low levels of lead. This paper presents a numerical simulation of the growth of these defects
based on the local stresses in the vicinity of the crack edge. This information is related to cracks actually found
in cylinders which have leaked or failed in service. From this procedure an equation for the crack growth rate
is developed, This also leads to an explanation as to why “leak before break” is not always observed in these
cylinders. 0 1997 Elsevier Science Ltd.
Kejwords: Emhrittlement, pressure-vessel failures, residual stress, slow crack growth, sports equipment,
failures.

1. INTRODUCTION
Portable aluminium cylinders are in common use in the world for purposes such as self-contained
underwater breathing apparatus (SCUBA), respirators for fire and medical use and other uses. In
Australia about 1,700,000 of these cylinders are in circulation, and large numbers exist in all
developed countries. There has been a history of cracking developing in some of these cylinders in
the position shown in Fig. 1.

1.1. The nature of the cracking
The cracking tends to grow from notches created during the forming process for the top end of
the cylinders and is driven by stress not only from the pressure contained in the cylinder, but also
residual stresses from their manufacture. Understanding the cracking and estimating the rate of
cracking growth is an objective which has interested a number of researchers in order to achieve a
basis for assessing acceptable defect sizes [I, 21.
The crack growth has in many quarters been stated to involve a phenomenon called solid metal
induced embrittlement (SMIE) where crack growth is aided by surface diffusion of certain elements,
the most important of which is lead. The cracks grow under constant load, so it is also described as
“sustained load cracking”. Since diffusion of elements is involved, there are some similarities to
creep crack growth and this terminology has also been used. The fundamentals of this process are
described elsewhere [2, 31.

~~~ ~
*Author to whom correspondence should be addressed.
Reprinted from Engineering Failure Analysis 4 (4), 259-270 (1997)