Lifetime Analysis-Modelling # 1995 Edinburgh
Once a grade of polymer has been specified for pipe production, the properties of the finished pipe depend on the processing conditions. By properties we mean here those attributes that will influence the in-service performance, and these can be grouped under three headings:
- Pipe geometry. To meet specifications and avoid wastage of material, pipe wall thickness must be sufficiently uniform. This beaimes increasingly di&cult to achieve as pipe size and wall thickness increase, due to grmity induced flow (or sag) of polymer which remains molten for long periods near the bore, whilst the pipe is wold from the outside.
- Thermal stresses. Density changes accompanying cooling induce thamal messes - compressive on the outer and tensile on the inner surface. Axial and hoop stress components are of similar magnitude - around a few MPa. Hoop stresses can be significant compared with in-service values resulting from intend pressure. Axial messes are responsible for the turn-in at a cut end, which can be inconvenient in jointing. Tensile suesses gendy can result in crack propagation - Wi & (11 state that for flaws of order 10 4 propagation can occur for stresses of 1 MPa
- Crystallinity and morphology. For a given polymer gade, the details of the cooling process determine (for semi-crystalline materials) the crystallinity and mim-mcture through the pipe wall, and these in turn are related to mechanical properties. Whilst the qualitative relationships between cooling rate and, say, spherulitc size are clean, qmtitatkve pdctions are not, at present, possible.