The paper concerns the time dependent response of polymer systems subjected to superimposed static and cyclic loads. An experimental study of two materials, Nylon 6/6 and a piezoelectric polyvinylidene fluoride (PVDF)-based composite, has been conducted with the objective to determine the degree of cyclic creep effects depending on the mean stress, cyclic stress amplitude and frequency. Creep acceleration due to cyclic loading effects has been observed in both materials even in the range of stresses well below their respective viscoelastic linearity limits. It is clear that these effects are essentially nonlinear, as the response of the materials to cyclic loading conditions does not represent a simple superposition of the responses to static and fully reversed cyclic loads applied separately. An increase of creep rates in both polymers has been observed consistently as frequencies and amplitudes of vibration tended to increase. However, the relation between cyclic creep effects and the mean stress appears to vary depending on the polymer type and loading conditions. Nylon 6/6 has demonstrated material hardening as a result of cycling. Considerable crack development in Nylon 6/6 has been detected only in the range of stresses approaching yield conditions.

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