Page 37 - Plastics Engineering

20 General Properties of Plastics

material deforms more. Another important aspect to the behaviour of plastics
is the effect of strain rate. If a thermoplastic is subjected to a rapid change in
strain it appears stiffer than if the same maximum strain were applied but at a
slower rate. This is illustrated in Fig. 1.4.

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O O l Z 3 4 5
Strain (“A)
Fig. 1.4 Effect of strain rate on stress-strain behaviour of plastics

It is important to realise also that within the range of grades that exist for a
particular plastic, there can be significant differences in mechanical properties.
For example, with polypropylene for each 1 kg/m3 change in density there is a
corresponding 4% change in modulus. Fig. 1.5 illustrates the typical variation
which occurs for the different grades of ABS. It may be seen that very often a
grade of material selected for some specific desirable feature (e.g. high impact
strength) results in a decrease in some other property of the material (e.g.
tensile strength).
The stiffness of a plastic is expressed in terms of a modulus of elasticity.
Most values of elastic modulus quoted in technical literature represent the slope
of a tangent to the stress-strain curve at the origin (see Fig. 1.6). This is often
referred to as Youngs modulus, E, but it should be remembered that for a plastic
this will not be a constant and, as mentioned earlier, is only useful for quality