Assessment of the stepped isostress method in the prediction of long term creep of thermoplastics

To predict long term creep of thermoplastics, methods based on the time-temperature superposition principle (TTSP) or on the time-stress superposition principle (TSSP) are commonly used. These methods enable the construction of a creep master curve without a lengthy experimental program. Recently, a new accelerated creep testing method, termed the stepped isostress method (SSM), was proposed and used to predict long term creep of technical yarns. This paper focuses on the processing aspects of the SSM test data and its validity in the creep prediction of thick thermoplastic specimens. Excellent correlation is obtained between the master curves constructed by the classical TSSP method and those constructed by the SSM method. The variation of the SSM testing parameters has no significant effect on the obtained master curves, which constitutes proof of the SSM robustness. Further, the trend of the SSM shift factors in terms of the creep stress obeys the Eyring equation.