Creep damage mechanisms in polyethylene gas pipes

This study deals with the creep damaging processes of two polyethylene (PE) resins. One is a ductile material while the other is a brittle one. Scanning electron microscopic (SEM) observations as well as chemical analysis are used to identify elementary process involved in the crack initiation and propagation. Of the two considered resins, only the second exhibits a lifetime controlled by slow crack growth (SCG). It is shown that catalytic residues act as initiating agents for the damage. Nevertheless, the presence of such particles in an extruded resin does not lead to a creep damage in every case. Intraspherulitic micro-cracks (mirror zones) can then propagate. Propagation takes place between the lamellae, seemingly through a disentanglement of tie molecules connecting the lamellae. The orientation of the micro-cracks is perpendicular to the largest principal stress. Cryogenic fracture surfaces indicate that the mirror zone gives rise to discontinuous crack growth bands (DCGB). In a pre-cracked specimen, only the DCGB stage takes place in the crack tip.