Abstract:The creep behavior of a commercial grade polycarbonate was investigated in this study. 10 different constant stresses ranging from 8 MPa to 50 MPa were applied to the specimen, and the resultant creep strains were measured at room temperature. It was found that the creep could be modeled linearly below 15 MPa, and nonlinearly above 15 MPa. Different nonlinear viscoelastic models have been briefly reviewed and used to fit the test data. It is shown that the Findley model is a special case of the Schapery model, and both the Findley model and the simplified multiple integral representation are suitable for properly describing the creep behavior of the polycarbonate investigated in this paper; however, the Findley model fit the data better than the simplified multiple integral with three terms.
Stress induced change in intrinsic time scale was investigated by nonlinear creep tests of polypropylene (PP) at room temperature of 27 ℃. The time-dependent axial elongations of the specimen were measured at 5 different stress levels, from 10.2 to 20 MPa, and modeled according to the time-stress superposition principle. The test duration was only 1 h. The test results show that the creep compliance vs logarithm time curves at different stresses depart from each other, indicating nonlinear viscoelastic behavior, and can be horizontally shifted to overlap onto a smooth master curve up to 51.5 h at the reference stress of 10.2 MPa. It is demonstrated that the time−stress superposition principle provides an accelerated test technique to evaluate the materials' long-term mechanical properties.
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