Page 156 - Plastics Engineering

Mechanical Behaviour of Plastics 139
In amorphous polymers it is possible that cracks may develop in the voids
which are formed during viscous flow.
Moulded plastics will also have crack initiation sites created by moulding
defects such as weld lines, gates, etc and by filler particles such as pigments,
stabilisers, etc. And, of course, stress concentrations caused by sharp geomet-
rical discontinuities will be a major source of fatigue cracks. Fig. 2.72 shows a
typical fatigue fracture in which the crack has propagated from a surface flaw.

Free
/
&-- surface

Fig. 2.72 Qpical fatigue fracture surface

There are a number of additional features which make polymer fatigue a
complex subject and not one which lends itself to simple analysis. The very
nature of the loading means that stress, strain and time are all varying simul-
taneously. The viscoelastic behaviour of the material means that strain rate
(or frequency) is an important factor. There are also special variables peculiar
to this type of testing such as the type of control (whether controlled load
or controlled deformation), the level of the mean load or mean deformation
and the shape of the cyclic waveform. To add to the complexity, the inherent
damping and low thermal conductivity of plastics causes a temperature rise
during fatigue. This may bring about a deterioration in the mechanical prop-
erties of the material or cause it to soften so much that it becomes useless in
any load bearing application.
Another important aspect of the fatigue of all materials is the statistical
nature of the failure process and the scatter which this can cause in the results.
In a particular sample of plastic there is a random distribution of microcracks,
internal flaws and localised residual stresses. These defects may arise due to
structural imperfections (for example, molecular weight variations) or as a result
of the fabrication method used for the material. There is no doubt that failure