Failure analysis of polyethylene gas pipes

Natural gas transmission and distribution industry is using pipes of different origins to transport hydrocarbons under pressure. The use of polymeric material such as polyethylene (PE) made it possible to achieve significant profits in construction times and installation costs. However, some catastrophic failures took place for various reasons. The objective of this study is to highlight the various mechanisms of rupture of PE pipes in service and in laboratory conditions. It is clear that at least two mechanisms control PE pipe failures based on results cumulated in operating conditions. They are nominally ductile and brittle mechanisms respectively characterizing short and long-term failures. Several laboratory tests are used to extract design data for long-term failure-type prediction based on stress and time to failure relationship. It remains difficult to assess the relation between creep and fatigue loadings on the one side. On the other side, the manufacturing process of the test specimens (extruded pipes and compression molded sheets) influences considerably the obtained performance for viscoelastic materials subjected to working conditions. When analyzing different results, it is found that there is a certain correlation between failure times under both constant and fluctuating loading patterns and moreover, fatigue can be used as an accelerating agent of brittle fracture which normally occurs in the long-term span and under low load levels. Brittle-to-ductile transition is studied under fatigue crack propagation mode using an energy criterion. It is found that the approach leads to critical energy rates of 211 and 695 J/m(2) for brittle and ductile regimes respectively. The brittle fracture damage zone is characterized by a single craze made up by locally drawn fibers and dispersed voids whereas ductile rupture is rather dominated by highly yielded material and significantly transformed matter as observed under polarized-light microscopy. PE mechanical behavior is affected by several factors including the state of residual stresses and the morphological variances induced by the extrusion process and subsequent cooling. As a result, there is a crystallinity increase from inner to outer layers of the pipe. The assessment of PE pipe failure mechanisms is to contribute to a better understanding of effects of other external chemical agents such as solvents in degrading the pipe overall resistance.