Long-Term Testing of Plastics

Since the tensile test has disadvantages when used for plastics, creep tests have evolved as the best method of measuring the deformation behaviour of 

Occasionally creep curves are plotted as log (strain) against log (time). This is convenient because quite often this results in straight line plots suggesting that the creep behaviour can be described by an equation of the type 

To be strictly accurate, the right hand side should have the form (Aq -I- At ) to allow for the instantaneous strain at zero time. However, for long creep times, sufficient accuracy can be obtained by ignoring Aq. 

A variety of other creep-strain equations have been proposed by Sterrett et aL These take the form 

Plastics also have the ability to recover when the applied stress is removed and to a first approximation this can often be considered as a reversal of creep. This was illustrated in Fig. 1.8 and will be studied again in Section 2.7. At present it is proposed to consider the design methods for plastics subjected to steady forces. 

---

The key property in the long-term testing of plastic piping materials is the retention of ductility. This... 

---