Investigation of slow crack growth initiation in polyethylene pipe grades with accelerated cyclic tests

Abstract

A model for crack initiation in CRB specimens for a high density polyethylene (PE-HD) pipe grade was developed for possible implementation in a lifetime assessment procedure. Additional, an analysis of the crack tip process zone was performed in order to get a better understanding for the mechanisms of the initiation of slow crack growth. Using concepts of linear elastic fracture mechanics (LEFMs), cyclic tests on Cracked Round Bar (CRB) specimens at a temperature of 23°C were carried out at different R-ratios. The tests, which were performed at two different Stress Intensity Factors (KI,max), were stopped after different cycle numbers prior crack initiation. Using a hot melt adhesive, the crack tips of the CRB specimens were frozen in a loaded condition, in order to prevent a collapse of craze fibrils in the process zone. After a suitable specimen preparation with microtome, optical microscopy and scanning electron microscopy (SEM) were used to observe the geometry of the process zone at the crack tip and to measure the dimensions of the process zone deformation. Depending on the cycle numbers the results showed an increase of the length of the process zone. The measured process zone geometries were extrapolated to an R-ratio of 1 which represents static loading conditions and compared to the Dugdale-Model, which predicts the process zone size at a crack tip.