Electrofusion joining is now an essential and widely used method to assist in the creation of polyethylene pressure pipe systems. The process of electrofusion joining is reviewed by examining the experimental and some computer simulation literature relating to the temperature and melt pressure changes during the fusion process, and on how varying fusion time and pipe/fitting gap influences the strength of electrofusion joints. From this literature review, four key stages in the joining process are identified. First, an incubation period where the joint has no strength. Second, a joint formation and consolidation stage where an increasing joint temperature aids molecular diffusion to both increase the joint strength and promote a more ductile mode of failure. A plateau region then follows where the joint strength, and ductility, remain reasonably constant despite the fusion time increasing. This plateau is thought to allow some welding variables, such as gap, to have only a small influence on joint strength (for gap maintained within reasonable limits). Finally there is a cooling stage where the joint bridging “tie molecules” become locked into either side of the joint. It is these tie molecules that give the joint its ductility and strength. The concluding section of the review notes some of the important on‐site practices that, if followed, allow electrofusion joints to acquire their good strength properties, and hence give polyethylene pressure pipe systems of a high integrity.
A narrow size distribution of irregular aluminium particles was blended into power cable insulation grade polyethylene. Some batches of the resulting material were then melt-filtered to reduce the size of particles present and narrow the distributions further. The failure statistics of the loaded polymers were then determined under AC ramped stress. density of defect can clearly be identified. In addition, for the filtered material, a minimum breakdown field can be associated with a given filter size: a result of commercial importance. Some indications exist to suggest that different modes of failure operate at high and low fields. Candidates for these modes are analysed and discussed in terms of the distributions of defects present. Local field enhancement due to the included flaws were calculated using finite-element techniques.The results are compared with a percolation model of breakdown. Predictions are found to quantify accurately the reduction in the characteristic strength of the material over the narrow range of defect concentrations examined.
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