Page 63 - Plastics Engineering

46 Mechanical Behaviour of Plastics

Applied load
IW

Fig. 2.3 Three-point bending

polymeric materials. In these tests a constant load is applied to the material and
the variation of strain with time is recorded as shown in Fig. 2.4(a). Normally
a logarithmic time scale is used as shown in Fig. 2.4(b) so that the time depen-
dence after long periods can be included and as an aid to extrapolation. This
figure shows that there is typically an almost instantaneous strain followed by
a gradual increase. If a material is linearly viscoelastic then at any selected
time each line in a family of creep curves should be offset along the strain
axis by the same amount. Although this type of behaviour may be observed
for plastics at low strains and short times, in the majority of cases the response
is non-linear as indicated in Fig. 2.5.
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

E(t) = At" (2.8)
where 'n' is a material constant and 'A' is a constant which depends on the
stress level.
To be strictly accurate, the right hand side should have the form (A0 +At")
to allow for the instantaneous strain at zero time. However, for long creep
times, sufficient accuracy can be obtained by ignoring Ao.