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Mechanical Behaviour of Plastics 61

22
20 MWm2
18

16

14
12
10
8
6
4

Fig. 2.15 Creep curves for acetal (20°C)

Accurately performed relaxation tests in which the strain in the material was
maintained constant and the decaying stress monitored, would give slightly
lower values than those values obtained from the isometric data.
It should also be noted that in this case the material was loaded in compre-
sion whereas the tensile creep curves were used. The vast majority of creep
data which is available is for tensile loading mainly because this is the simplest
and most convenient test method. However, it should not be forgotten that the
material will behave differently under other modes of deformation. In compres-
sion the material deforms less than in tension although the effect is small for
strains up to 0.5%. If no compression data is available then the use of tensile
data is permissible because the lower modulus in the latter case will provide a
conservative design.

2.6 Thermal Stresses and Strains

It is quite common in modem engineering designs, for plastics to be used in
conjunction with other materials, particularly metals. In such cases it is wise to
consider the possibility of thermal stresses being set up due to the differences
in the thermal expansion (or contraction) in each material.
The change in shape of a material when it is subjected to a change in
temperature is determined by the coefficient of thermal expansion, a~. Normally
for isotropic materials the value of CYT will be the same in all directions. For
convenience this is often taken to be the case in plastics but one always needs