Experiments and modeling of high-crystalline polyethylene yielding under different stress states

The mechanical response of high-density polyethylene (HDPE) was examined under different stress states. The biaxial yielding of HDPE material was investigated from a series of biaxial shear/tension and shear/compression tests using butterfly-shaped specimens deformed with an Arcan apparatus equipped with a digital image correlation (DIC) system for local strain measurements. In order to investigate a wider range of stress states, notched round bar specimens with different curvature radii were also tested using a video-controlled tensile testing apparatus. More conventional mechanical loading paths (uniaxial tension/compression and simple shear tests) were also examined to provide better insights on the stress state effects. The present investigation is more particularly focused on the yield envelope determination of HDPE material. A combined DIC and analytical approach was proposed to measure the yield strengths of butterfly-shaped specimens in the region where the yielding occurs. The relevance of classical yield criteria, exhibiting dependence on both the deviatoric and hydrostatic stresses, is verified. Considering HDPE as a heterogeneous medium consisting of a percolated crystalline matrix and a discrete amorphous phase, a micromechanics-based yield locus is tested. The experimental biaxial yield data are found to support this theoretical yield criterion and thus the suggested morphological representation for high-crystalline polymers.