The tensile creep behaviour of cold-drawn low-density polyethylene

Abstract

Sheets of highly anisotropic low-density polyethylene possessing transverse symmetry were prepared by the simple tensile drawing at room temperature of initially isotropic sheets. Classical elasticity theory shows that five constants are necessary to characterize the deformation behaviour of a linear elastic material possessing such symmetry. The time-dependent equivalents of these constants were determined from the simultaneous measurement of longitudinal and lateral strain during tensile creep of specimens cut at 0°, 45° and 90° to the draw direction. Creep measurements were also made on specimens cut at intermediate angles from the drawn sheets and on specimens cut at various angles from the isotropic sheets. A wide range of tensile creep strains, from 0·1% upwards, was used for all specimens to enable the extent of linear behaviour to be examined. For the oriented material, the tensile strain at the onset of non-linear behaviour varied systematically with angle, being lower than 0·2% for the 90° specimen. The isotropic material exhibited non-linear behaviour down to at least 0·1% tensile strain. At low strains it is found that the formalism of classical elasticity theory holds if isochronous creep data are used, and the extension and contraction results are combined to calculate volume changes occurring during tensile creep. It is shown that at angles away from 0° and 90° the deformation behaviour of the oriented material is dominated by an easy-shear mechanism, parallel to the oriented chains, for a wide range of tensile strains.