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The overall cyclic response of ductile polymers is independent of the micromechanistic deformation phenomena and is always manifest as cyclic softening. Cyclic hardening does not occur irrespective of temperature and strain rate. Varying the deformation mechanisms by microstructural modification—amorphous versus crystalline versus composite—does not affect the response in kind, only in degree. In a similar manner, changing testing variables such as strain rate and temperature only results in a change in the magnitude of the softening. At sufficiently high strain rates, self-heating of the specimen can occur, resulting in failure due to thermal softening rather than true cyclic fatigue failure. The interaction of creep and fatigue in polymers is complex and although, in general, damage accumulation is accelerated, under certain conditions the combination is not deleterious.
fatigue polymers, cyclic softening, cyclic hardening, cyclic stress-strain curve, hysteresis, creep component, thermal effects
Engineering and research staff, Ford Motor Co., Dearborn, Mich.