Page 36 - Plastics Engineering

General Properties of Plastics 19

In many respects the stress-strain graph for a plastic is similar to that for a
metal (see Fig. 1.2).
At low strains there is an elastic region whereas at high strains there is a non-
linear relationship between stress and strain and there is a permanent element
to the strain. In the absence of any specific information for a particular plastic,
design strains should normally be limited to 1%. Lower values (-0.5%) are
recommended for the more brittle thermoplastics such as acrylic, polystyrene
and values of 0.2-0.3% should be used for thermosets.
The effect of material temperature is illustrated in Fig. 1.3. As temperature
is increased the material becomes more flexible and so for a given stress the

Fig. 1.2 Qpical stress-strain graph for plastics

-20°C

20°C
f
50°C

70°C

Strain (7")
Fig. 1.3 Effect of material temperature on stress-strain behaviour of plastics