Approaches to Fatigue Design in Thermoplastic Pipe

Cyclic loading of plastic pipe is a known risk factor that can lead to fatigue related failure. While the re-rating of plastic pipe based on cyclic loading is addressed by advisory documents on other continents, there is limited guidance for design engineers in North American practice. Domestic polyvinyl chloride (PVC) and polyethylene (PE) standards include surge allowances above rated working pressure based on limiting values of hoop stress, but only C900 and C905 for PVC pipe include a method for addressing fatigue in design and then only as an informational appendix, not as part of the adopted standard. In this paper, the author uses field measured transient data to develop three classes of cyclic loading that are typical of water and wastewater practice. Various standard design approaches are then applied to the selection of the appropriate pressure class of plastic pipe for each classified transient. Determination of the corresponding dimension ratios allow for the analysis of the transient data in terms of hoop stress. Given sufficient knowledge of the transient conditions in an existing or proposed piping system, the design engineer can not only design for a total number of lifetime cycles, but also reduce risk of fatigue failure by managing the magnitude of repeated stress cycling.