Damage zone analysis and brittle cracking under cyclic loading in polyethylene gas pipes

Plastic pipes used for water and gas transmission continue to be the subject of many studies that treat various aspects of material's behaviour. Recent statistics indicate that more than 90% of gas distribution systems lately installed in the whole world are made exclusively of polyethylene (PE) because of its ease of installation and relatively low costs. Constant load tests show two general mechanisms of crack propagation: a ductile rupture, which is dominated by homogeneous deformations on a large scale in the volume and a brittle fracture that starts at stress concentration points. This work aims at studying the brittle-to-ductile transition from fatigue in polyethylene tubes and the characterization of the associated damage zone. The suggested method is based on the measurements of two fatigue parameters: the crack growth rate, obtained at various load levels and the irreversible work spent on damage and crack growth, which is calculated using instantaneous hysterisis loops. The obtained correlations, for maximum fatigue loads between 20% and 35% of the yield stress, give average critical energy release rates of 211 J.m-2 and 695 J.m-2 respectively for brittle and ductile regimes.