Page 114 - Plastics Engineering

Mechanical Behaviour of Plastics 97

Time (s)
0.5

0.4

3 0.3
v
9 0.2

0.1

0
0 100 200 300 5 K)
Time (s)
Fig. 2.43 Strain predictions using superposition theory

(iii) at 350 seconds; 01, 02, a3 as above, a4 = 10 MN/m2 at r4 = 300 s
SO,
~(350) = 0.003 = 0.3%

(iv) and in the same way
~(450) = 0.004 = 0.4%

The predicted strain variation is shown in Fig. 2.43(b). The constant strain
rates predicted in this diagram are a result of the Maxwell model used in
this example to illustrate the use of the superposition principle. Of course
superposition is not restricted to this simple model. It can be applied to any
type of model or directly to the creep curves. The method also lends itself to a
graphical solution as follows. If a stress 01 is applied at zero time, then the creep
curve will be the time dependent strain response predicted by equation (2.54).
When a second stress, a2 is added then the new creep curve will be obtained
by adding the creep due to a2 to the anticipated creep if stress a1 had remained