Page 118 - Plastics Engineering

Mechanical Behaviour of Plastics 101

then for K2 = -0.2 MN/m2 s, T = 100 seconds and u2 = 125 seconds

~(125) = 0.094%
(c) After the time T', the change in stress is given by
Change in stress, a(t) = K2(t - T')

dm)
=
- K2
dt
Hence,

and from this, for u3 = 200 seconds and Kz = -0.2 MNlm2 s

~(200) = 0.075%
This will in fact be constant for all values of u3 because the Maxwell Model
cannot predict changes in strain if there is no stress. The overall variation in
strain is shown in Fig. 2.46.
Example 2.15 In the previous Example, what would be the strain after
125 seconds if (a) the stress remained constant at 10 MN/m2 after 100 seconds
and (b) the stress was reduced to zero after 100 seconds.
Solution
(a) If the stress was kept constant at 10 MN/m2 after 100 seconds as shown
in Fig. 2.47 then the effective change in stress would be given by
change in stress, a(t) = -Kl(t - T)

d4)
=
- -K1
dt
so

KIU~T KIT KlT2
+---
E(U2) = -
17 e 217
~(125) = 0.125%