Based on the observation that high stress results in increasing creep rate of polymeric material, which is analogous to the
time-temperature equivalence, where high temperature accelerates the process of creep or relaxation of polymer, the
time-stress equivalence is investigated. The changes of intrinsic time in polymer induced by temperature and stress are
studied using the free volume theory, and a clock model based on the time-temperature and time-stress equivalence is
constructed to predict the long-term creep behavior of polymer. Polypropylene is used for this work. The specimens with
shape of dumbbell are formed via injection molding. The short-term creep tests under various stress levels are carried out
at ambient temperature. The creep strains of specimens are modeled according to the concept of time-stress equivalence,
and the corresponding stress shift factors are calculated. A master creep curve is built by the clock model. The result
indicates that the time-stress superposition principle provides an accelerated characterization method in the laboratory.
Finally, the time-dependent axial elongations at sustained stress levels, whose values are close to the tensile strength of
polypropylene, are measured. The three phases of creep, i.e., the transient, steady state and accelerated creep phases, are
studied, and the application and limitation of the time-stress superposition principle are discussed.