Page 158 - Plastics Engineering

Mechanical Behaviour of Plastics 141

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Fig. 2.74 Qpical fatigue behaviour of acetal at 5 Hz

failure as shown in Fig. 2.74. Higher stress amplitudes in subsequent tests will
repeat this pattern until a point is reached when the temperature rise no longer
stabilises. Instead the temperature continues to rise and results in a short term
thermal softening failure in the material. Stress amplitudes above this cross-
over stress level will cause thermal failures in an even shorter time. The nett
result of this is that the fatigue curve in Fig. 2.74 has two distinct regimes.
One for the relatively short-term thermal failures and one for the long-term
conventional fatigue failures.
If the frequency of cycling is reduced then stress amplitudes which would
have produced thermal softening failures at the previous frequency, now result
in stable temperature rises and eventually fatigue failures. Normally it is found
that these fatigue failures fall on the extrapolated curve from the fatigue failures
at the previous frequency. Even at the lower frequency, however, thermal soft-
ening failures will occur at high stress levels. If fatigue failures are to occur at
these high stresses, then the frequency must be reduced still further. The overall
picture which develops therefore is shown in Fig. 2.75. In some plastics the
fatigue failure curve becomes almost horizontal at large values of N. The stress
level at which this occurs is clearly important for design purposes and is known
as thefatigue limit. For plastics in which fatigue failures continue to occur even
at relatively low stress levels it is necessary to define an endurance limit i.e.
the stress level which would not cause fatigue failure until an acceptably large
number of stress cycles.
The occurrence of thermal failures in a plastic depends not only on the
cyclic frequency and applied stress level but also on the thermal and damping
characteristics of the material. For example, polycarbonate has very little