Page 60 - Plastics Engineering

Mechanical Behaviour of Plastics 43

0 Strain
Fig. 2.1 Stress-strain behaviour of elastic and viscoelastic materials at two values of elapsed
time, t
applied stress is removed the materials have the ability to recover slowly over
a period of time. These effects can also be observed in metals but the difference
is that in plastics they occur at room temperature whereas in metals they only
occur at very high temperatures.

2.3 Short-Term Testing of Plastics
The simple tensile test is probably the most popular method for character-
ising metals and so it is not surprising that it is also widely used for plastics.
However, for plastics the tensile test needs to be performed very carefully
and the results of the single test should only be used as a means of quality
control - not as design data. This is because, with plastics it is possible to
obtain quite different results from the same material simply by changing the
test conditions. Fig. 2.2 shows that at high extension rates (>1 ds) unplas-
ticised PVC is almost brittle with a relatively high modulus and strength. At
low extension rates (<0.05 mm/s) the same material exhibits a lower modulus
and strength but its ductility is now very high. Therefore a single tensile test
could be quite misleading if the results were used in design formulae but the
test conditions were not similar to the service conditions.