The influence of process history on the ductile failure of polyethylene pipes subject to continuous hydrostatic pressure

Failure times of plastic and metal pipes subjected to hydrostatic pressure at various levels assist pipe manufacturers to not only design pipes for certain applications, but also to give them an indication of the useful service lifetimes of these pipes. In this work, a medium density polyethylene was converted into various pipes by varying extrusion parameters. These pipes displayed low levels of lamellar orientation and a skin‐core gradient in morphology. The hydrostatic pressure performance (ductile failure regime at 23°C) of the pipes was observed to depend on morphology, which was inferred by the systematic dependence of the failure times on the tensile behavior of the pipes. It was also found that pipes annealed at 80°C prior to testing displayed 400% longer failure times; annealing was found to increase the crystallinity of the pipe only marginally. Consequently, this performance improvement was attributed to relaxation of some of the frozen‐in residual stresses in the pipe. © 2005 Wiley Periodicals, Inc. Adv Polym Techn 24: 226–232, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/adv.20044