Tensile creep data obtained from testpieces of polypropylene at variou
s ages and a number of different stresses have been circulated to Ave
laboratories as part of a round-robin exercise. The aim of this exerci
se was to compare functions that are currently being used to model the
effects of physical ageing and stress on the creep behaviour of plast
ics, with a view to producing an international standard in this area.
All of the functions studied were able to successfully model the effec
ts of physical ageing on the creep of plastics over a limited timescal
e, where the age of the specimen under test remained effectively const
ant. However, the fits to longer duration tests where further ageing o
f the testpiece occured during the creep were more discriminating, rev
ealing 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 rate
s. Further work is proposed to study the origin of some of the discrep
ancies before the preparation of a draft standard can be undertaken. C
rown copyright (C) 1997 Published by Elsevier Science Ltd.