Modelling and analysis of creep data for plastics at elevated temperatures

Procedures are considered for determining the long-term creep behaviour of plastics at ambient temperature from experimental data obtained from short-term tests over a range of elevated temperature. Errors in long-term properties derived from the application of time-temperature superposition can arise unless proper account is taken of changes to creep behaviour caused by physical ageing. Short-term changes in age state and, hence, creep behaviour occur when the temperature of a specimen is raised. These changes are caused by the erasing of physical ageing that has occurred at ambient temperature prior to the temperature increase. Subsequent reactivation of physical ageing leads to further changes in creep behaviour with time at the elevated temperature. The results of creep tests at different temperatures have been analysed by theoretical expressions that separate the contributions to creep behaviour from structure and temperature changes.