Page 120 - Plastics Engineering

Mechanical Behaviour of Plastics 103

Stress (MNlm2)

*
0
loo up Time (s)

Fig. 2.48 Stress history: Example 2.15(b)
so
u2 u2
E(U3) = / rl + - rl + 0 - &2 (-Kl)dt - AC7
trl Kldt+/ trl E(U2 - T)
11 f2
K,T~
E(U3) = -
211
~(125) = 0.05%
It is apparent therefore that the Superposition Principle is a convenient
method of analysing complex stress systems. However, it should not be
forgotten that the principle is based on the assumption of linear viscoelasticity
which is quite inapplicable at the higher stress levels and the accuracy
of the predictions will reflect the accuracy with which the equation for
modulus (equation (2.33)) fits the experimental creep data for the material.
In Examples (2.13) and (2.14) a simple equation for modulus was selected in
order to illustrate the method of solution. More accurate predictions could have
been made if the modulus equation for the combined MaxwellKelvin model
or the Standard Linear Solid had been used.

2.12.2 Empirical Approach
As mentioned earlier, it is not feasible to generate test data for all possible
combinations of load variations. However, there have been a number of ex-
perimental investigations of the problem and these have resulted in some very