Plastic deformation of crystalline polymers: The role of cavitation and crystal plasticity

Plane strain compression in a channel die is kinematically very similar to drawing; however, the possibility of void formation is limited due to a compressive component of stress. In drawing, voids were detected by small-angle X-ray scattering (SAXS) and density measurements in poly(methylene oxide) (POM), polypropylene (PP), and high-density polyethylene (HDPE), but no voiding was found in polyamide 6 (PA 6), low-density polyethylenes (LDPEs), and ethylene-octene copolymer (EOC). The slope and shape of the initial elastic part of true stress-true strain curves are similar in tension and in channel die compression. When drawn samples of POM, PP, HDPE, and PA 6 already show yielding, the channel die compressed samples still undergo elastic deformation to a much larger deformation and respond with a much larger stress. Channel die compressed POM, PP, HDPE, and PA 6 exhibit strong and rapid strain hardening up to 400 MPa in contrast to their behavior in tension. The difference in strain hardening is related to preservation of chain entanglement density in channel die compression and disentanglement in tensile drawing. True stress-true strain curves for polymers having crystals with low plastic resistance and not cavitating are very similar in channel die compression and in tension. In tensile drawing there is a competition between cavitation and activation of crystal plasticity. Cavitation occurs in polymers with crystals of higher plastic resistance, while plastic deformation of crystals in polymers with crystals of lower plastic resistance. The necessary conditions for cavitation and for plastic deformation of crystal are defined. They explain why the cavitation is observed in POM, PP, and HDPE but not in LDPEs. In PA 6 negative pressure causes cavitation but the cavities, due to their small sizes and healing action of surface tension, are unstable, close quickly, but leave the traces of a structural damage. A model of plastic deformation of crystalline polymers accounting for cavitation is outlined.