Plastic Pipes Conference Association # 1998 Gothenburg
Davis, Greenshields, Dabas, Leevers
The study of Rapid Crack Propagation (RCP) in pressurised pipelines is of particular importancr to th? g a industry, where the possibility of such a failure must he minimised. Polyethylene (PE) currently dominates the market for plastic gas pipelines, and as the demands for larger diameters .and higher operating pressures increase, a high resistance to RCP must be maintained. However, whilst the mechanics of RCP in PE pipelines is well understood, its apparent sensitivity to processing conditions is not. Recent work suggests that RCP resistance. is in part 'processed-in' during pipe manufacture. and strongly dependent on postextrusion cooling rates.. For PE-80, which is typically brittle at 0 OC. processing effects are reflected in the critical pressure obtained from the small scale steady state (S4) test for RCP. A 30% increase in critical pressure is observed in single-surface cooled pipe compared to dual-surface cooled. The superior performance of single-surface cooled pipe is explained in part, by the release of residual stresses during fast, brittle fracture. RCP i n PE-100 pipe is characterised by a brittle-tough transition temperature, which again is sensitive to processing. This problem is more complex, since it is difficult to attribute differences i n this behaviour to a single mechanical property. A new method has been developcd which determines a similar brittle-tough transition using small plaque specimens, which can he prlduced conveniently under a ranze of cooling conditions. The effect of processiproperty interactions on the brittle-tough transition in PE-100 pipe also appears to depend on the raw material itself. A cross-section of comniercial PE- 100s ;re characterised using an isothermal segregation technique, in conjunction with conventional differential scanning calorimetry. The results are then correlated RCP perforn~ance.
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