INFLUENCE OF DEFORMATION DEFECTS ON THE DEVELOPMENT OF STRAIN GRADIENTS DURING THE TENSILE DEFORMATION OF POLYETHYLENE

The influence of initial deformation defects (localized prestrain) on the subsequent tensile behaviour of cylindrical bars of high-density polyethylene was investigated experimentally and theoretically. It was observed that neck nucleation occurs at the defect for moderate prestrains and outside the defect for larger prestrains. In the former case, the growth of the neck is limited and a secondary neck forms on the border of the predeformed zone. In all instances after the rod is fully drawn, a hard nodule remains at the defect location, the diameter of which increases with the prestrain. A simplified long-wavelength analysis proves that the above phenomena are controlled by the effect of the prestrain on the constitutive behaviour of the material. It is shown in particular that (i) prestraining softens the anelastic response of the polyethylene and (ii) prestraining hardens the material in the steady-state plastic regime. The complete stretching evolution of defect-containing samples can be modelled precisely by means of a finite-difference computer analysis.