be large enough to melt the plastic. To avoid this, fatigue tests are normally
carried out with lower-frequency alternating stresses than is usual with metals.
The results of such tests, however, are not entirely valid if the alternating
stresses experienced by the plastic component in service are higher than those
used for the test.
Cu
o
M
60
r
E
| 40
1
20
"a
I °
S 10
2
10
3
10
4
10
5
10
6
10
7
^ Number of cycles
Figure 7.14 S/N graph for unplasticized PVC, alternating stress being a square
waveform at frequency 0.5 Hz
Figure 7.14 shows an S/N graph for a plastic, unplasticized PVC. The
alternating stresses were applied with a squar
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