Influence of Strain Rate on the Crystal Structure of High‐Density Polyethylene during Early Tensile Deformation

ABSTRACT High‐density polyethylene (HDPE) has been widely studied due to its extensive variety of uses. The effect of strain rate during tensile deformation on HDPE crystal structure has not been fully characterized, however, and the underlying reasons for the onset of visible necking have also remained unknown. To address these issues, detailed X‐ray diffraction analyses are presented of HDPE pulled at both fast and slow strain rates to three different extensions centered around the onset of visible necking. Comparisons of texture, unit cell parameters, crystallite size, crystallinity, and phase are made at each extension for each strain rate. The results show that strain rate has a dramatic impact on texture, unit cell parameters, and the martensitic phase transformation from orthorhombic to monoclinic HDPE, as well as the mechanism by which HDPE reverts back to orthorhombic phase during strain recovery. A mechanism is proposed to explain the onset of visible necking, which is related to the interplay between crystalline and amorphous regions, and which also changes with strain rate. Strain rate is thus just as important to consider as overall extension when trying to optimize applications or extend the lifetime of HDPE‐based materials.