Tensile stress and birefringence in both real and model amorphous polymer melts have been measured during constant rate uniaxial elongational flow. We focus on investigations where deviations from the linear stress-optical behavior are pronounced. A rate-dependent contribution to the stress which is not directly related to the intramolecular conformations ("stress offset") is detected for both types of macromolecular fluids. Independent of the flow history, during relaxation a linear stress-optical behavior is revealed. Nonequilibrium molecular dynamics (NEMD) computer simulations on the multibead anharmonic spring model are shown to provide insight into the molecular mechanisms underlying the viscoelastic behavior : during relaxation the intermolecular interactions become dominant in correlation with linear stress-optical behavior; the stress offset is shown to be very similar to the stress arising in the corresponding simple fluid; the total stress can well be approximated by a sum of three parts which are based on single-particle and single-link distribution functions only; the yield point behavior at high elongation rates reflects the transition from affine to nonaffine motion of bonds and is understood without reference to strong inhomogeneities resulting from local plastic strain production [the chemical structure does not influence the qualitative behavior]; distinct microscopic stress contributions under elongation and subsequent relaxation such as inter- and intramolecular, attractive and repulsive, kinetic and potential contributions are resolved.