Page 121 - Plastics Engineering

104 Mechanical Behaviour of Plastics

useful design aids. From the experimental point of view the most straightfor-
ward situation to analyse and one that has considerable practical relevance is
the loadno-load cycle. In this case a constant load is applied for a period and
then completely removed for a period. The background to this approach is as
follows.
For a linear viscoelastic material in which the strain recovery may be
regarded as the reversal of creep then the material behaviour may be
represented by Fig. 2.49. Thus the time-dependent residual strain, E,(?), may
be expressed as
E,(?) = E,(?) - Ec(? - T) (2.59)

Fig. 2.49 wical creep and recovery behaviour of a plastic

where E, is the creep strain during the specified period denoted by (t) or (t - T).
Since there can be an infinite number of combinations of creep and recovery
periods it has been found convenient to express this behaviour in terms of two
dimensionless variables. The first is called the Fractional Recovery, defined as

Strain recovered -
E,(T) - E&)
Fractional recovery, F, = - (2.60)
Max. creep strain &,(TI
where E,(T) is the creep strain at the end of creep period and E,(?) is the
residual strain at any selected time during the recovery period.
The second dimensionless variable is called the Reduced Time, t~,
defined as
Recovery time
Reduced time, t~ = (2.61)
Creep time
Extensive tests have shown that if the final creep strain is not large then a
graph of Fractional Recovery against Reduced Time is a master curve which