Page 159 - Plastics Engineering

142 Mechanical Behaviour of Plastics

Log cycles to failure
Fig. 2.75 Typical fatigue behaviour of acetal at several frequencies

tendency towards thermal failures whereas with polypropylene there is a
marked propensity in this direction. Thermosets, of course, are very thermally
stable and only exhibit brittle fatigue failures.

2.21.2 Effect of Waveform
Assuming that the cyclic waveform used in the previous section was sinusoidal
then the effect of using a square wave is to reduce, at any frequency, the level
of stress amplitude at which thermal softening failures start to occur. This is
because there is a greater energy dissipation per cycle when a square wave is
used. If a ramp waveform is applied, then there is less energy dissipation per
cycle and so higher stresses are possible before thermal runaway occurs.

2.21.3 Effect of Testing Control Mode
During cyclic loading of a material the energy dissipated is proportional to the
product of the stress and strain. If the loading on a plastic is such that the stress
amplitude is maintained constant, then any temperature rise in the material will
lead to an increase in strain since the modulus decreases with temperature. The
increase in strain means that more energy is dissipated, leading to a further drop
in modulus and a further increase in strain. It is this type of chain reaction which
leads to the thermal softening failures if the heat transfer to the surroundings
is insufficient.
The alternative mode of testing is to control the strain amplitude. In this case
an increase in temperature again causes a drop in modulus but this leads to a