Characterization of volume strain at large deformation under uniaxial tension in high-density polyethylene

The intensity and origin of volume changes under uniaxial tension is investigated at room temperature in high-density polyethylene samples with a large initial degree of crystallinity. At the macroscopic scale, volume strain is defined as the trace of the finite strain tensor whose components are recorded in situ by means of a 2D video extensometer within a representative volume element situated at the center of the neck. At the microscopic scale, volume strain is ascribed to the competition of cohesive mechanisms and non-cohesive mechanisms. The former are associated both to the packing of oriented chains in the amorphous phase (compaction) and to the decrease of crystallinity (dilatation), as characterized by wide-angle X-ray diffraction analysis. The latter are due to the development of crazes and voids (dilatation) while the spherulitic morphology is progressively transformed into a highly fibrillated structure, as revealed by scanning electron microscopy. Detailed evaluation of the relative importance of these two classes of mechanisms shows that they compensate nearly exactly at moderate strains, so that volume strain is very small in the first stage of the tensile tests. By contrast, the effect of cavitation becomes prominent at large deformation, so that the overall dilatation reaches more than 30% before rupture. It is demonstrated that volume strain measurements obtained from mechanical testing and from microstructural investigation agree fairly satisfactorily.