Creep and physical ageing of polypropylene: a comparison of models

Tensile creep data obtained from testpieces of polypropylene at various ages and a number of different stresses have been circulated to Ave laboratories as part of a round-robin exercise. The aim of this exercise was to compare functions that are currently being used to model the effects of physical ageing and stress on the creep behaviour of plastics, with a view to producing an international standard in this area. All of the functions studied were able to successfully model the effects of physical ageing on the creep of plastics over a limited timescale, where the age of the specimen under test remained effectively constant. However, the fits to longer duration tests where further ageing of the testpiece occured during the creep were more discriminating, revealing differences in the ability of some of the models to accurately describe the data. Additional disparities became apparent in modelling long-term data obtained at high stresses which appeared to change the apparent age of the material, causing higher than expected creep rates. Further work is proposed to study the origin of some of the discrepancies before the preparation of a draft standard can be undertaken. Crown copyright (C) 1997 Published by Elsevier Science Ltd.