Evaluation of fatigue lifetime and elucidation of fatigue mechanism in plasticized poly(vinyl chloride) in terms of dynamic viscoelasticity

The fatigue behavior of plasticized poly(vinyl chloride) was investigated by means of a tension–compression‐type fatigue apparatus. Complex elastic modulus and mechanical loss tangent were obtained continuously with time under the conditions of constant ambient temperature as a function of imposed strain amplitude. Brittle failure was observed under the conditions of low ambient temperatures and small strain amplitudes, or forced convection of air, whereas thermal failure was observed under the conditions of high ambient temperatures, or large strain amplitudes and natural convection of air. In the case of brittle failure, the dynamic storage modulus E ′ exhibited a maximum and the loss tangent tanδ exhibited a minimum on approaching the point of failure. In the case of thermal failure, E ′ decreased and tanδ increased monotonously until the onset of thermal failure. It was found that failure occurs when the effective energy loss reaches a certain magnitude depending on an ambient temperature. The fatigue criterion was represented schematically from a standpoint of self‐heating. When the heat generation rate of the specimen under cyclic staining is larger than that of the heat transfer to the surroundings, thermal failure takes place. In this case, the specimen temperature increases up to a limiting constant temperature corresponding to the α‐absorption temperature. When the heat generation rate is nearly equal to that of the heat transfer to the surroundings, the specimen temperature does not change appreciably and brittle failure takes place.