Application of time-stress equivalence to nonlinear creep of physical aging poly(methyl methacrylate)

The effects of physical aging and stress on creep of poly(methyl methacrylate) were investigated, and the time-aging time superposition principle (TASP) and the time-stress superposition principle (TSSP) were applied to construct a unified master creep curve. After annealing above glass transition temperature, the specimens were quenched and aged at aging temperature 50 degrees C for different aging times, and then the short-term creep strains were measured under different stress levels at ambient temperature. The results show that the creep rate increases with stress, but decreases with aging time. It is also shown that the aging time shift factors vary with the stresses at which the shifts are applied. For linear viscoelastic material, the aging shift rate is independent on imposed stress, while for nonlinear viscoelastic material, the aging shift rate decreases with increasing stress. The unified master creep curve is constructed by shifting the creep curves horizontally along the logarithmic time axis to overlap each other. It is demonstrated that TSSP, associated with TASP, can be used to assess the long-term mechanical behaviors of physical aging polymers.