Page 74 - Plastics Engineering

Mechanical Behaviour of Plastics 57

For some plastics, particularly nylon, the moisture content can have a signifi-
cant effect on the creep behaviour. For such plastics, creep curves are normally
available in the wet and dry states and a knowledge of the service conditions
enables the appropriate data to be used.
For convenience so far we have referred generally to creep curves in the
above examples. It has been assumed that one will be using the correct curves
for the particular loading configuration. In practice, creep curves obtained under
tensile and flexural loading conditions are quite widely available. Obviously it is
important to use the creep curves which are appropriate to the particular loading
situation. Occasionally it is possible to obtain creep curves for compressive or
shear loading but these are less common.
If only one type of data is available (e.g. tensile creep curves) then it is
possible to make conversions to the other test modes. It should always be
remembered, however, that these may not always be absolutely accurate for
plastics under all situations.
Generally there is a stiffening effect in compression compared to tension. As
a first approximation one could assume that tension and compression behaviour
are the same. Thomas has shown that typically for PVC, the compression
modulus is about 10% greater than the tensile modulus. However, one needs to
be careful when comparing the experimental data because normally no account
is taken of the changes in cross-sectional area during testing. In tension, the
area will decrease so that the true stress will increase whereas in compression
the opposite effect will occur.
The classical relationship between moduli in tension, compression and
flexure is

(2.14)

where
MR = Ec/ET

It may be seen that if E, = ET then Eflex = ET. However, if E, = 1.1E~
(for example), then Eflex = 1.05E~.
The classical relationship between the shear modulus G, and the tensile
modulus, E, for an isotropic material is

(2.15)

where v = Poissons ratio.
Finally, although it is less commonly used for plastics, the bulk modulus, K,
is given by
I7
c
K= (2.16)
3(1 - 2~)