MECHANISTIC ANALYSIS OF FATIGUE-CRACK INITIATION IN MEDIUM-DENSITY POLYETHYLENE

Kinetics parameters of craze evolution preceding fatigue crack initiation (FCI) in medium-density polyethylene (MDPE) pipe materials were determined and analysed within fracture mechanics theory. A single craze initially preceded the notch tip, a root craze, which subsequently became accompanied by a few side crazes. Crack initiation transpired after the craze-zone growth had reached its fully developed configuration. The length of the root craze of the fully developed zone was found to be equal to the length of the first discontinuous crack band on the fracture surface. The growth of the root-craze length and the crack-tip opening displacement (CTOD) followed a power law over the major portion (94%) of the FCI time. Measurable rupture of the craze material was only noted within the final portion of the FCI time and was associated with exponential increase of the CTOD. The Dugdale/Barenblatt model overestimated the craze length by 30% and underestimated the CTOD by 50% which was hypothesized to be due to multiple crazing at the notch tip.