Similar thermal expansion instabilities, consisting of isothermal, time‐dependent changes in thermal expansion after a rapid change in temperature, were observed in epoxy resins with different degrees of cross‐linking. Creep experiments performed at different stages of expansion show decreases in tensile creep rate with decreased expansion. The role of changes in moisture content as a possible cause of the dimensional instability is examined for epoxy resins and a highly cross‐linked polyurethane. Results indicate that the state of expansion is the primary cause of changes in creep rate rather than temperature or moisture content.
A nonlinear creep experiment under combined stresses and time-varying temperature is reported. By using the constitutive equations for combined stressing at various temperatures and a time-temperature superposition principle, the creep of polyurethane under constant combined tension and torsion and varying temperature was computed. Results were best described by assuming no temperature-history dependence on increasing temperature and temperature-history dependence on decreasing temperature.
Nonlinear creep under combinations of constant tensile and torsional stresses and recovery are reported for various constant temperatures ranging from 75 to 145°F. Results of the creep tests were describable by a power function of time whose exponent is independent of stress and temperature. These results, in conjunction with previous results from pure tension and pure torsion creep experiments on polyurethane, permitted using the multiple-integral representation to describe the creep and recovery behavior of this material under different combinations of constant tensile stress, torsional stress, and temperature. Agreement with the creep-test data is good, and prediction of creep under other states of stress and recovery is satisfactory. Most of the nonlinearity was found in the coefficient of the time-dependent term.
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