Effect of Crystalline Relaxation on Fatigue Behavior of the Oriented High-Density Polyethylene Based on Nonlinear Viscoelastic Measurements

Oriented high-density polyethylene (HDPE) specimens were prepared by drawing the HDPE film up to the draw ratio of 5.3 at 353 K corresponding to the crystalline relaxation temperature. Nonlinear viscoelasticity and higher-order structural change of the oriented HDPE during the fatigue process were investigated at various ambient temperatures. Nonlinear viscoelasticity of the oriented HDPE was estimated on the basis of nonlinear viscoelastic parameter (NVP). The magnitude of NVP at the α₁-crystalline relaxation (intermosaic block crystalline relaxation) temperature region was lower than that at room temperature, since deformation in the intermosaic block region preferentially occurred in addition to that in the amorphous region. However, the fatigue strength at the α₁-crystalline relaxation temperature was lower than that at the other ambient temperatures, because deformation in the intermosaic block region was very sensitive to strain or stress concentration. Since both lamellar crystalline and amorphous regions were deformed at the α₂-crystalline relaxation temperature region, the magnitude of NVP became lower than that at the α₁-crystalline relaxation temperature. Also, the fatigue strength at the α₂-crystalline relaxation temperature was greater than that at the α₁-crystalline relaxation temperature due to a more homogeneous deformation of the specimen.