Polyethylene fatigue data

The data presented in Figure 19.7 were obtained on a Sonntag-Universal machine which flexes a beam in tension and compression. Whereas the acetal resin was subjected to stresses at 1800 cycles per minute at 75°F and at 100% RH, the nylons were cycled at only 1200 cycles per minute and had a moisture content of 2.5%. The polyethylene sample was also flexed at 1200 cycles per minute. Whilst the moisture content has not been found to be a significant factor it has been observed that the geometry of the test piece and, in particular, the presence of notches has a profound effect on the fatigue endurance limit. 

Creep failures such as those illustrated in Fig. 11.12 are often called static fatigue and are not uncommon in practical applications such as pressurized piping applications. Kinloch and Young (1983) gives data on the creep rupture of high-density polyethylene pipe (HDPE) and an excellent discussion of the mechanisms associated with static fatigue. 

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