Page 236 - Buried Pipe Design

210 Chapter Four

2. Thus, these strengths (life funds) are separate and independent of
one another.
3. The cyclic pressure life may be a critical parameter if the water sys-
tem is operating “out of control” and the amplitude and frequency
of the cyclic pressures are high and continuous.

Terms to describe cyclic stress. There are different approaches to
reporting fluctuating stresses in fatigue testing. Figure 4.8 illus-
trates the meaning of peak stress ( peak ), stress amplitude ( amp ),
mean or average stress ( mean ), and stress range ( range ) used here.
Authors typically report their results in one or more of these
terms.
Fatigue failure in metals has been well studied and design proce-
dures are available. Typically, one refers to S-N curves (stress vs. num-
ber of cycles to failure) to estimate fatigue life. Also, it is well
documented that both mean and alternating stresses should be con-
sidered in predicting fatigue life.

22
Previous cyclic theories for PVC pipe. Vinson published a method to pre-
dict fatigue failure in PVC pipes. Vinson ran tests on 6-in (152 mm),
CIOD SDR 17 and 6-in (152 mm), IPS OD SDR 26 PVC pipe. The pipes
were cycled from a base hoop stress between 400 and 500 psi (2.76 and
3.45 MPa) up to various peak hoop stresses. Fitting a straight line

150

120 s amp

Stress 90 s peak s range

60

30
mean
s
0
0 2 4 6 8 10
Time (s)
Figure 4.8 Illustration of stress terms.