At present, the fiber optic ducts are connected by a mechanical type of joint. In this method, two ducts cut in the right angle are pushed in from both sides of the joint, and takes approximately one second to joint ducts together. The problem with the existing joint technology is that if there is water inside of the joint, it will be damaged when the water freezes into ice, and then may cause leakage. There is a risk of explosion when compressed air to blow the fiber. Thus, a joint protection device (silicone rubber sleeve) was developed to seal the joint for protection utterly. However, this will cause the larger size of the entire joint and limit the number of single-duct joints next to each other in a multi-duct joint. Fiber optic ducts are made of High-Density Polyethylene, which is the best plastic for remelting and can be welded by using the electro-fusion welding method. Based on the thermoplasticity of this material, this thesis developed a plastic joint with a built-in conductive metal wire inside. The applied voltage will heat the wire, then remelt the duct surfaces to weld them together through the joint. The welding system uses a portable battery operating system, so there is no need to connect it to the grid. To prevent the battery from being damaged by supplying too much current, a capacitor bank is used to store high energy for the preheating joints. The system uses a microcontroller to control and monitor current and voltage to ensure uniform heating of the metal wire. Theemphasisof this thesis isplacedon the implementation of basic experiments to run the welding system. Multiple welding experiments show that the welding system can manually set parameters to control the welding current of different joints, thereby ensuring the welding quality. Using a 2.5Ω joint to weld ducts will approximately consume 120J from the battery, so a fully charged 42V, 4.4AH rechargeable battery can perform almost 5600 times of welding. The suitable range of joint resistance will decrease as the required energy consumption increases/ the welding time decreases.

Le soudage par électrofusion est la technique majoritairement utilisée pour assembler les tubes et les accessoires en polyéthylène utilisés sur le réseau gazier. Ses très bons résultats initiaux ont été ternis ces dernières années par un certain nombre de dysfonctionnements relevés tant sur le terrain qu'en laboratoire. Ils se traduisent par des soudures de qualité médiocre du fait de la présence de zones de très faible cohésion. L'objectif de ce travail est de développer des outils experts tant expérimentaux que numériques permettant d'optimiser les conditions de soudage par électrofusion. L'originalité de notre étude a été de développer un modèle éléments finis tridimensionnel prenant en compte les différents mécanismes et phénomènes physiques sous-jacents, responsables de la formation de la soudure. Le modèle permet de calculer le taux de transformation de la matière, de prendre en compte les enthalpies de fusion et de cristallisation, de calculer le taux d'interdiffusion des macromolécules à l'interface entre les différents corps à souder pour finalement prédire la qualité de la soudure en fonction des conditions de chauffage imposées. Pour valider le modèle numérique, tout en facilitant l'accès aux grandeurs expérimentales, nous avons conçu et réalisé un dispositif expérimental se présentant sous la forme d'une géométrie relativement simple et plane mais respectant les caractéristiques d'un accessoire réel. La confrontation entre résultats numériques et expérimentaux a permis de démontrer les capacités du modèle numérique à reproduire fidèlement la réalité. Les différents outils ont alors été utilisés pour étudier l'influence des conditions de soudage sur la soudure et pour étudier le soudage de géométries plus complexes telles que celles rencontrées dans les pièces industrielles. / The Electrofusion welding process is widely used to join polyethylene components in gas distribution networks. Even trusty as a technique, the field feedback points out some divergences whose influence on the long term performance of the weld. One of the well-known consequences of these divergences is the “sticking” (aka “cold weld”) that is the result of an uncompleted or even inexistent interdiffusion of the macromolecules of the materials to join. Most numerical simulations are two-dimensional whereas the process is usually three-dimensional both in terms of heat transfer and mechanical aspects. The main objective of the work was to develop a 3D finite element model and to validate it by comparing to a real situation the temperature evolution and the thermally affected areas in a simple planar welding geometry with the same dimensional characteristics as a real fitting chosen to make easier the instrumentation. The numerical model takes into account the fitting parameters such as polyethylene thermal and mechanical properties (i.e. melting and crystallization kinetics, phase transition, thermal expansion) and the electrical and geometrical settings. It computes a criterion based on the macromolecular interdiffusion theory able to determine whether a good welding occurred or not at the end of the welding cycle. The computed results (temperature, melted and cold areas and fracture surfaces) were compared with experimental data and gave very good agreement in terms of temperature, liquid phase fraction distribution and fracture surfaces. Finally the numerical model and the experimental process were used to study the influence of welding conditions on the weld itself and to study the welding of complex geometries such as the industrial fittings.

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