An Accelerated Characterization Of The Long-term Creep Behavior Of Polymers

Polymers are typically viscoelastic in nature, their mechanical performances depend on temperature, loading time, loading speed and load amplitude, etc. Among these factors, the influences of time, temperature and stress are especially significant. Under certain condition, they have equivalently effects on the viscoelastic properties of polymers. Usually, the long-term mechanical properties under low stress and temperature are equivalent to the short-term properties under high stress and temperature. In this thesis, the basic mechanical properties and their influences of polymers are briefly reviewed in the first chapter, and then the time-temperature superposition principle (TTSP), time-stress superposition principle (TSSP) and time-temperature-stress superposition principle (TTSSP) are introduced, which can be used as accelerated characterization methods to predict the long-term properties of the materials. In the third chapter these three superposition principles are applied to creep analysis of polypropylene (PP) and polymethyl methacrylate (PMMA). Finally some conclusions are drawn. The concrete research contents and the corresponding conclusions are as follows:1. The short-term creep tests are performed on PP and PMMA under various temperatures and stress levels. It is shown that materials are linear viscoelastic under low stress levels while their behaviors become nonlinear viscoelastic for moderate and high stress levels. The critical stress beyond which nonlinear viscoelastic behavior occurs depends on temperature; the higher the temperature, the lower the critical stress is. Moreover, this critical stress is found to linearly vary with test temperature within the temperature range indicated in this study.2. The TTSP, TSSP and TTSSP are introduced and reviewed. It is pointed out that the TTSSP is the generalized form of the TTSP and the TSSP, in other words, the TTSSP can be reduced to the TTSP under constant stress condition and to the TSSP under isothermal condition.3. Using the short-term creep data of PP under various temperatures and stress levels, the effect of stress on time-temperature equivalence is investigated. It is shown that the time-temperature shift factors depends on stress level at which the shifts are applied; it is also shown that the short-term creep data gotten at 16MPa and 26℃for 1500 seconds can be used to predict the creep property up to 3 days, which is much longer than the test duration.4. The short-term creep tests are performed on PMMA under various temperatures and stress levels. Based on the TTSSP, the long-term creep behavior is acceleratedly characterized by two different stepwise shifting routes. The corresponding shift factors and the possible prediction time scales are obtained. The well agreement of the two stepwise shifting routes shows their applicabilities. Compared to the TTSP and the TSSP, the TTSSP is verified to be...