Long-term creep assessment of viscoelastic polymer by time-temperature-stress superposition

Nonlinear viscoelastic creep properties of poly (methyl methacrylate) at various temperatures and stress levels were measured in short-term tests to check the applicability of time-temperature-stress superposition principle, which is the combined form of time-temperature superposition principle and time-stress superposition principle. A unified master creep compliance curve was constructed from the short-term tests by joint application of time-temperature superposition and time-stress superposition. The unified master curve establishes the creep compliance over two years, which is 4.2 decades longer than the test duration. Moreover, it is verified that in nonlinear viscoelastic cases, the time-temperature shift factors are dependent on stresses at which the shifts are applied, while the time-stress shift factors are dependent on temperatures.