Time Dependent Failure of Polymers

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. 

At this juncture it is appropriate to recall the failure envelope given in Chapter 2, Fig. 2.21 that displays a comparison of failure stresses for both metals and polymers to the three failure theories mentioned therein. The data provided for both polymers and metals were developed without regard to possible rate and/or viscoelastic effects. In Fig. 11.11 and in Fig. 11.12 it has been demonstrated that yielding of polycarbonate is both rate and time dependent. The same is true for most ductile polymers and, as a result, the yield (or failure) surface for polymers should be understood to 

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