Tensile deformation of semicrystalline polymers follows a common scheme with changes in the mechanism at critical strains. Choosing a poly(ethylene-co-12% vinyl acetate) (PEVA12) as an example, we measured true stress-strain relationships at constant strain rates, determined the elastic and plastic part of imposed strain in step-cycle experiments, and followed the stress relaxation at fixed strains. On the basis of the general observations, a model was constructed and then used for a description of the properties of PEVA12. The model treats the stress as arising from three contributions: quasi-static stresses originating from the stretched network of entangled chains in the fluid regions and from the force-transmitting skeleton of crystallites, plus the viscous forces described by Eyring's equation. Adjustment of the measured data to the model provides a decomposition of the stress in the three parts. With increasing strain the dominance shifts from the crystal-to network-transmitted stress, while the viscous forces increase continuously. Stress relaxation can be treated by an analytical solution of a differential equation that reproduces the results of the measurements.