An investigation into the fatigue behaviour of polymeric materials

The study of the behaviour of the polymers subject to cyclic stress is very complicated. Many variables affect the material behaviour: material variables mainly composition, molecular weight, orientation, additives etc. as well as external variables mainly the magnitude of the applied stress (stress amplitude, mean stress), the test frequency and the test temperature. The present paper represents an experimental investigation of the effect of loading variables on the behaviour of two thermoplastic materials, polyethylene and Polyvinylchloride, subjected to cyclic load. Tests were carried out on a specially designed and constructed apparatus. In the scope of the ranges of loading parameters studied in this investigation, the obtained results showed that the increase in stress amplitude or in the mean stress caused a shortage in the fatigue lifetime of both the PE and the PVC tested specimens, while the cyclic frequency had a negligible effect on the fatigue behaviour of the PVC and a little increase in the fatigue lifetime of the PE. Three modes of failure: creep, fatigue or thermal resulted depending on the loading variables.