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Khelif, R.
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article
Critical Energy Analysis of Fatigue Brittle-To-Ductile Transition in Polyethylene Gas Pipe Materials
Abstract
Plastic pipes used for gas and water industries are retaining more and more attention through the multitude of studies that are concerning various behavioral aspects. Recent statistics indicate that more than 90% of newly installed piping gas systems throughout the world are made of Polyethylene (PE) because of its ease of installation and relatively low cost. Today, it is well established from laboratory work that extruded plastic pipes fail in a ductile manner as applied loads are sufficiently high and failure zone is characterized by large deformations around the damaged area. Despite such favorable ductility, PE pipes are also found to undergo brittle-like fracture when subjected to low stresses for long service periods. Such conditions usually favor Slow Crack Growth (SCG) fracture mode and as a result, constant load tests exhibit two general crack propagation mechanisms, ductile failure which is dominated by large scale homogeneous deformations in the bulk and brittle failure that starts at stress concentration points. This study is aimed to investigate fatigue brittle-to-ductile transition in Polyethylene pipes through a correlation between crack growth rate and the amount of irreversible work expended on viscoelastic processes in the bulk. Fatigue crack propagation tests carried out at ambient temperature show that two important damage mechanisms are competing while crack is running. These are brittle and ductile failure mechanisms. The proposed method is based on the measurement of two fatigue parameters: the rate of surface crack growth, obtained at different loads levels and the rate of irreversible work which is calculated from fatigue instantaneous hysterisis loops. The obtained correlations, for maximum fatigue loads between 20 and 35 % of the yield stress, show average critical energies of 211 and 695 J/m² respectively for brittle and ductile regimes.