Stress-strain behavior and corresponding crystalline structures of four types of polyethylene under a wide range of strain rates

Stress-strain behavior of four types of polyethylene (PE), i.e. low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), medium-density polyethylene (MDPE) and high-density polyethylene (HDPE), under a wide strain rate ranging from 0.001 to 1000 s⁻¹, and the crystalline structures of corresponding fractured PEs were investigated. Results demonstrated that when the strain rate was less than 1 s⁻¹, PE exhibited ductile fracture with a rougher section. While at strain rates larger than 1 s⁻¹, the damage was manifested as brittle fracture with a smooth fracture surface. For all PE specimens, the yield strength increased with strain rate, but the increasing dependency of yield stress on strain rate was diverse in various strain rate ranges. Under the same strain rate, due to the largest molecular weight and lowest branching degree, HDPE showed the highest values in yield strength, crystallinity, crystal orientation degree, and crystallite size. At the appropriate stain rate, i.e. 10 s⁻¹, HDPE with less branched structure is more likely to obtain higher crystallinity and molecular chain orientation degree.