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Viscoelastic behavior of the non-Hooke deformation of amorphous PET film before yield was investigated in thetemperature region 74--80.5℃ around the glass transition temperature. The film specimen was drawn to yield point followedby unloading to zero stress, then the residual deformation was held constant, while the subsequent evolution of the stress wasrecorded. An induction period was found in the course of stress evolution fol1owed by a stress step-increase. The inductionperiod decreases with increasing drawing temperature with an activation energy of 1.10 MJ/mol·K, which is attributed tothe time needed for the relaxation of rubbery deformation through cooperative internal rotations. At temperatures lower than74℃, there is no stress increase or the induction period becomes too long to be observed. Thus the nature of anelasticity inthe non-Hooke region before yielding is attributed to stress induced rubbery deformation. The experimental results areinterpreted in terms of Perez’ rheological model of a series connected Hooke spring and a Voigt element consisting of aparallel connected elastic spring and a dashpot.
Viscoelastic behavior of the non-Hooke deformation of amorphous PET film before yield was investigated in the temperature region 74--80.5 ° C around the glass transition temperature. The film specimen was drawn to yield point followed by unloading to zero stress, then the residual deformation was held constant, while the subsequent evolution of the stress wasrecorded. An induction period was found in the course of stress evolution folowed by a stress step-increase. The induction period was decreased with increasing drawing temperature with an activation energy of 1.10 MJ / mol · K, which is attributed tothe time needed for the relaxation of rubbery deformation through cooperative internal rotations. At temperatures lower than 74 ° C, there is no stress increase or the induction period becomes too long to be observed. Thus the nature of anelasticity inthe non-Hooke region before yielding is attributed to stress induced rubber deformation. The experimental results areinterpreted in terms of Perez ’ rheological model of a series connected Hooke spring and a Voigt element consisting of a parallel connected elastic spring and a dashpot.