Page 106 - Plastics Engineering

Mechanical Behaviour of Plastics 89

Fig. 2.37 Response of KelvinNoigt model

(ii) Relaxation
If the strain is held constant then equation (2.38) becomes

U=C*E
That is, the stress is constant and supported by the spring element so that the
predicted response is that of an elastic material, Le. no relaxation (see Fig. 2.37)

(iii) Recovery
If the stress is removed, then equation (2.38) becomes

0 = C.E+ r)€

Solving this differential equation with the initial condition E = E’ at the time
of stress removal, then
i!
=
~(t) de- (2.4)
This represents an exponential recovery of strain which is a reversal of the
predicted creep.

(c) More Complex Models
It may be seen that the simple Kelvin model gives an acceptable first approx-
imation to creep and recovery behaviour but does not account for relaxation.
The Maxwell model can account for relaxation but was poor in relation to creep