Page 365 - Failure Analysis Case Studies II

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Residual stress is an important when remaining life is to be calculated. The maximum residual
stress is estimated from the tempering regime. Temper T6 involves solution heat treating at 530 "C
and quenching in water. The effect of this is to produce tension residual stress at the inside surface
(Schroder 1121). The value of this stress could be as high as the yield stress of the material at room
temperature in an unaged condition (called temper T4) and this is listed as 155 MPa in the material
specification. In the case of temper T6, these residual stresses will be reduced, but not completely
removed, by an artificial aging treatment for 18h at 160°C. Thus 155MPa is an upper bound
estimate of residual stress.
If a residual stress of 155 MPa is included (such as is proposed in Price et ai. [l]) then K, = 22.3 MPa
Jm. The calculated growth rates predicted by Eqns (1) to (4) above are as shown in Table 1.
From Table 1 it is seen that Eqns (1) to (4) predict that with no residual stress there would be
either no growth or virtually no growth.
If maximum residual stress is included, three of the predictions suggest growth could be rapid,
though the prediction which relates specifically to specimens removed from Australian cylinders is
still too slow. While residual stresses can produce a reasonable growth it must be remembered that
residual stress can only effect growth for the first one or two millimetres. After that stage the residual
stresses will quickly drop to the value predicted by the models with low residual stress.
Given the fact that in some cases in traffic the crack grows through the walls of the gas cylinder
in a few years, the growth rates predicted by these equations are unsatisfactory. This is discussed
below.

3.4. Leak before break
The argument to substantiate the case of leak before burst requires KI at the time of leak to be
less than the material critical stress intensity factor, KIc. KIC was found during our testing to be
32 MPaJm for A1 6351. The data presented in Table 2 indicates there is a margin between leak and
burst for all the most extreme situations.
Given the above analysis it is not clear why some cylinders, such as cylinder B fail catastrophically
without leaking, while some cylinders such as cylinder A leak prior to failure.
The principal explanation of this probably lies in the fact that the growth of the defect as observed
does not occur in the regular fashion assumed in classical crack growth analysis. If crack growth
can occur in a shape which does not lead to leak, but is nevertheless eventually large enough to
produce rapid failure, then leak before break could occur. This possibility is suggested in both the
cylinders which were examined and is discussed below.

Table 1. Growth rates predicted by various equations for various cracks and stresses
Type of specimen Growth rate
used in tests to Growth rate with Growth rate with with 155 MPa
develop equations zero residual stress zero residual stress residual stress
Defect size 4 mm deep 20 mm deep 4 mm deep
20 mm long 45 mm long 20mm long
Kl 5.54MPa Jrn 9.95 MPa Jrn 22.3 MPa Jrn
Equation mmlyear mlyear mmlyear
(1) Specimens with 0.0033 0.036 28
100 ppm Pb
(2) From Australian 0.00015 0.001 1 0.36
cylinders (< 10 ppm Pb)
(3) 100 ppm Pb (below threshold) (below threshold) 36
(adjusted to 20T)
(4) 30 pprn Pb (below threshold) (below threshold) 18.5
(adjusted to 20°C)