Double Step Strain and Relaxation of Polymer Melts

A possible description of the viscoelastic behaviour of polymer melts in the non-linear r’ange which has been recently considered makes use of stress equations of the form $$ \mathop{\sigma }\limits_{ \sim } (t) = \int_{{ - \infty }}^{t} {\dot{\mu }(t - t')\mathop{Q}\limits_{{ \sim t}} } (t')dt' $$ (1) where the tensor \( \mathop{Q}\limits_{{\sim t}} (t') \) is determined by the deformation occurred between t’ and t and the memory kernel \( \dot{\mu } \) is a function of time only.