International audienceThe electrofusion welding process is widely used to join polyethylene components in gas distribution networks. This article provides experimental and 3D-finite element tools capable of reproducing the major phenomena that occur during an electrofusion process. A specific version of the 3D- finite element model simulation software Forge® has been developed to take into account the fitting parameters such as polyethylene thermal properties (i.e., melting and crystallization kinetics, phase transition, and thermal contact resistance) and the electrical and geometrical settings (i.e., welding input parameters). From a numerical point of view, a well refined highly anisotropic mesh adaptation is applied to well capture the contact condition between the heat source and the polyethylene. The computed results (temperature, melted, and cold areas) were compared with experimental data and gave very good agreement in terms of temperature and liquid phase fraction distribution

SynopsisThe fusion welding behavior of a medium density polyethylene resin has been studied for a wide range of heating rates using a recently developed test methodology. With this method, the thermal and physical phenomena occurring at the interface of two thin polyethylene pieces assembled by fusion can be studied. It consists of a thermal welding phase and a phase of mechanical separation of the welded assembly. For the mechanical phase, an adaptation of the T-peel test was used. These conditions make it possible to determine the thermal welding parameters (temperature, time) for optimal mechanical quality of the joint, according to a criterion established by optimization of the peel test used. The variations in minimum temperature required for an optimum weld, as a function of heating rate, can be simulated with a numerical model based on the concept of macromolecular interdiffusion. Consistent with the experimental behavior, the numerical model involves two parameters characteristic of the diffusion behavior of the polyethylene resin. Thus, these parameters characterize the weldability of the polyethylene resin under study.

PE (polyethylene) is a widespread material for the manufacturing of gas and water distribution pipelines and a large number of PE electrofusion pipe joints can be found over the thousands of kilometers of PE pipelines all over the world, thus generating an increasing interest in checking the quality of both these pipes and joints. Within the last few years it has been proved that-among the available concepts-the PAUT (phased array ultrasonic technique) was one of the most promising candidates for non destructive analysis of PE welds and pipes. No current operational NDE (non destructive examination) Technique to date has been adapted to inspect the small diameter electrofusion joints (OD from 20 to 63 mm) which are frequently encountered in particular on the French gas distribution networks. In the present paper, the acuity of PAUT to detect and size both poor weld quality (e.g. cold weld) and calibrated faults has been evaluated on 63 mm electrofused welds by ENGIE Lab CRIGEN jointly with Institut de Soudure Industrie. Moreover the results of the PAUT evaluation and those of the destructive peel tests according to ISO 13956 standard have been confronted in order to evaluate the degree of correlation between both techniques.

SynopsisThermal history of a polyethylene-welded assembly has been determined by measuring the crystallinity via absorbance ratio (ROD) measurements. Fourier transform infrared microspectrometry has been used because it allows a space-localized analysis within a diameter of 200 pm.A linear relation between ROD and local crystallinity has been obtained using reference samples for calibration. Samples were calibrated by differential thermal analysis and density measurements. This relationship and the simulation of the various thermal treatments by calorimetric analysis allow us to characterize the different phases of the thermal history during the welding experiment.

ABSTRACT:The selective crystallization behavior of a series of commercial mediumand high-density polyethylene resins has been studied by means of an original modified procedure of the stepwise isothermal segregation technique using differential scanning calorimetry. The technique consists of a sequenced multiple-stage stepwise thermal treatment of the materials allowing separation of the macromolecules with respect to their length-to-branching content and distribution. It is assumed that such a separation process gives an image of the proportion of specific crystallizable species, which are in turn responsible for slow crack growth resistance of the resins under study. A drift molecular parameter is calculated from a combination of the crystallization data at 119°C and 114°C. This molecular parameter is capable of revealing some divergences regarding the fabrication of a commercial polyethylene resin within a decade. Such divergences are well correlated with premature brittle failures under hydrostatic pressure testing of a series of correctly extruded pipes. Furthermore the drift molecular parameter allows a ranking of different medium-to high-density polyethylene resinsthe lower this parameter the better the slow crack growth resistance of the materials.