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Title: Residual stress effects on the performance of pressurised thermoplastic pipe
Author: Guevara Morales, Andrea
ISNI:       0000 0004 2707 3701
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2011
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Nowadays most of the newly installed pipe for water and gas distribution systems is extruded from PE. Once a PE grade has been selected for pipe production, processing conditions control the properties and performance of the finished pipe. The aim of this study is to investigate the development of residual stresses during post-extrusion cooling, and the effect they have on the pipe performance. A comprehensive simulation of the cooling process considering temperature dependent thermal properties is presented. The development of crystallinity is predicted by coupling the heat transfer analysis to a kinetic model of crystallisation. Using the residual temperature field concept, the residual stress distribution through the pipe wall is found. Different techniques to measure residual stresses such as ring slitting and layer removal are compared experimentally, and a method is proposed to resolve a very localised region of high compressive stresses at the outer surface of the pipe. Shell theory is used to predict barreling: the tapering of the pipe wall near the cut ends due to the release of residual stresses. It was found that these variations in diameter could increase the gap between the pipe and the electrofusion fitting, which could in turn reduce the strength of the joint and the integrity of the pipeline. The effect of residual stresses on Rapid Crack Propagation (RCP) is also investigated. Because thermal history determines not only residual stresses but also crystallinity, two experimental techniques were used to heat treat the pipes to modify one variable at a time and investigate its effect on RCP separately. It was found that the effect of crystallinity is not as signi cant as that of residual stresses. Lower critical temperatures were found for pipes with lower residual stresses. A qualitative explanation for this transition is offered in terms of crack shape.
Supervisor: Leevers, Patrick Sponsor: CONACyT-Mexico ; Radius Systems-UK
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral