Fatigue Crack Propagation

The use of polymeric material such as polyethylene (PE) and polyamides (PA) made it possible to achieve significant profits in design construction times and installation costs. The objective of this study is to highlight the various mechanisms of rupture of polyethylene pipes in service and in laboratory conditions under fatigue and creep loadings. It is known is some cases 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 one side. On the other side, the manufacturing process of the test specimens influences considerably the obtained performance for viscoelastic materials subjected to working conditions and environmental effects. Brittle-to-ductile transition is studied under fatigue crack propagation mode using an energy criterion. 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. The assessment of polyethylene pipe and polyamide parts 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. Recent results from environmental stress cracking of PE pipe and exposed polyamide PA66 to detergent will be presented and correlated to mechanical properties degradation.