Page 115 - Plastics Engineering

98 Mechanical Behaviour of Plastics

alone. This is illustrated in Fig. 2.44(a). Then if all the stress is removed this
is equivalent to removing the creep strain due to 61 and 02 independently as
shown in Fig. 2.44(b). The procedure is repeated in a similar way for any other
stress changes.

------ 1 ------- -
i
I
0 4 tL tsm
Fig. 2.44(a) Stress history

Fig. 2.44(b) Predicted strain response using Boltzmann’s superposition principle

(b) Continuous Changes of Stress
If the change in stress is continuous rather than a step function then equa-
tion (2.55) may be generalis4 further to take into account a continuous loading
cycle. So
t
(2.57)

where a(t) is the expression for the stress variation that begins at time, tl.
The lower limit is taken as minus infinity since it is a consequence of the