A method for mechanical property assessment across butt fusion welded polyethylene pipes
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The use of high-density polyethylene pipes in gas and water distribution networks is steadily growing worldwide. If the resistance of plain pipes is at present time well established using appropriately designed standards, welding issues continue to be globally approached equally in terms of structure and mechanical properties. Consequently, further practical investigations should be aimed at studying mechanical properties in the weld region which includes the melt zone and its heat-affected zones. This work presents a method based on removing layers in order to assess localized variances in mechanical properties throughout the weld seam in both radial and circumferential directions. An experimental plan based on specific machining operations allowed testing 39 standard specimens representing the weld volume matter in three concentric layers for given pipe dimensions and their counterpart standard unwelded ones. The typical stress–strain behavior of semi-crystalline materials is preserved in welded and unwelded specimens but with different characteristic limits. At the weld inner layers, properties such as elastic modulus, yield, and failure stresses displayed lower values, whereas in welded outer layers, the tendency is inversed. The cold drawing extend remained approximately steady for unwelded and welded cases across the pipe wall. This property is less affected by the presence of the weld as it described a constant material flow which is mostly a function of available material quantity for yielding. The approach developed in this study gives consistent indications on welding quality around the pipe weld and across the thickness. Accordingly, outermost and innermost welded layers may exhibit lower or even bad-quality welds as imperfections can concentrate stresses at the joint interface because of cold weld problems. Such method enabled detecting 23% of failures at the weld seam from outer and inner layers while the middle layer did not reveal any failure at the weld. The causes of this behavior are approached using crystallinity evolution in welded and unwelded pipes.
KeywordsHDPE pipe Butt fusion welding Mechanical properties Radial direction Circumferential direction Structural variances
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The authors would like to express their gratitude to those who provided assistance to this research work: SARL Z.A.—Annaba (Water Construction Network Co.), Mechanical Eng. Dept. (Hall Technologique) of Guelma University (8 May 1945 U.), R&D Dept., POLYMED Co., Skikda, Unité de Recherche Matériaux, Procédés et Environnement (URMPE) of Boumerdes University (UMBB). Fruitful discussions with members of LR3MI of UBM Annaba are also greatly appreciated. Part of this work was supported by Algerian Ministry of Higher Education & Scientific Research, CNEPRU Research Project “Study of thermo-mechanical behavior of butt welded joints in HDPE pipes,” Project Code A11N01UN23012014122 (2014).
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