Temperature and strain rate independence of critical strains in polyethylene and poly(ethylene-co-vinyl acetate)

True stress-strain curves measured during uniaxial tensile deformations of polyethylene generally show four characteristic points where the differential compliance and the recovery properties change. They can be associated with the onset of isolated inter- and intralamellar slip processes (point A), a change into a collective activity of slips (B), the beginning of crystallite fragmentation (C), and chain disentanglement (D). Experiments were carried out for high density polyethylene, low density polyethylene, and an ethylene-vinyl acetate copolymer, with variation of the temperature and the strain rate. While the stresses at the critical points change remarkably, the related strains turn out as constant. The only exception is point D, which shifts to lower strains on heating. Observations correspond with the previously found independence of the critical strains on the crystallinity.