Measurement and Analysis of Elastic and Plastic Properties of Planar‐Oriented <scp>HDPE</scp>

ABSTRACT The elastic and yielding behavior of oriented unimodal, bimodal, and trimodal high‐density polyethylene (HDPE) was investigated to assess the influence of molecular weight distribution on anisotropic deformation. Planar stretching at 130°C to a 4 × 1 draw ratio produced highly oriented specimens, and tensile tests at multiple loading angles (0° to 90°) were used to determine orthotropic elastic constants and yield stresses. Image‐based strain analysis enabled full orthotropic characterization, and Hill's anisotropic yield criterion provided excellent agreement with measured yield strengths for all three grades. The ranking of stiffness and yield behavior followed crystallinity levels, with the unimodal grade, which had the highest crystallinity, also exhibiting the greatest long‐period shift in small‐angle X‐ray scattering (SAXS). Finite element (FE) simulations incorporating orthotropic elasticity and Hill yield accurately reproduced deformation and strain localization at larger loading angles, although post‐yield behavior at small angles was not captured due to strain‐hardening effects. The results demonstrate that Hill's elastic–plastic model is suitable for small‐scale yielding in oriented HDPE while also highlighting its limitations in cases where strain hardening dominates.