Page 111 - Plastics Engineering

94 Mechanical Behaviour of Plastics

Stress
=0
-I. Time

d

w

&I

1
Time
Fig. 2.42 Response of standard linear solid

(iii) Recovery
If the stress is at a value of 0’ and then completely removed, the governing
equation becomes
1)3(e1 + e2)& + e1e2& = 0

The solution of this differential equation may be obtained using the boundary
condition that when the stress is removed, the strain is given by

E’ = a’/(e1 + (2)

(2.52)
This predicts an instantaneous recovery of strain followed by an exponential
decay.
It may be observed that the governing equation of the standard linear solid
has the form
+ U,O = bli + bo&