Page 103 - Plastics Engineering

86 Mechanical Behaviour of Plastics

From equations (2.27), (2.28) and (2.30)

(2.31)

This is the governing equation of the Maxwell Model. It is interesting
to consider the response that this model predicts under three common time-
dependent modes of deformation.

(i) creep
If a constant stress, a,, is applied then equation (2.31) becomes
. 1
€=-*a, (2.32)
r)
which indicates a constant rate of increase of strain with time.
From Fig. 2.35 it may be seen that for the Maxwell model, the strain at any
time, t, after the application of a constant stress, a,, is given by
a0
00
€(t) = - + -t
5 r )
w t

i
'E
;i

-

0 tl t2 Time

b t

Fig. 2.35 Response of Maxwell model