On the ageing of polymers and the time‐shifting procedure

The time‐shifting procedure, originally developed for time‐temperature equivalence in the modelling of polymers mechanical behaviour, has later on been extended to time‐age equivalence. The present paper discusses both situations in the context of linear viscoelastic behaviour, from the point of view of mathematical representation and the fitting of experimental data. The general equations in the cases of ageing and non‐ageing are introduced and the cases of hardening and softening types of ageing are precisely characterised. Situations in which time shifting can prove effective are delimited and alternative more general expressions are proposed.