On time-temperature-dependent viscoelastic behavior of an amorphous polyimide

Development of new polyimide formulas has led to an increase of the use of polymer matrix composites by the aerospace industry in critical applications such as aircraft engines. In such an extreme environment, the polymer matrix is likely to exhibit a viscoelastic behavior due to the elevated temperatures. Furthermore, physical ageing is likely to appear during the service life of the material. In order for engineers to design with such materials, constitutive theories able to take into account those effects, must be developed. This paper deals with the mechanical behavior of a new polyimide matrix tested under complex thermo-mechanical histories while accounting for physical ageing. The experimental data obtained was modeled with a newly developed constitutive theory stemming from Schapery's thermodynamical framework. The model parameters were obtained from creep-recovery data under various temperatures, ageing times, and loads. The model was subsequently validated against independent complex thermo-mechanical histories. An excellent agreement between the experiments and the predictions has been obtained.