焊接缺陷对聚乙烯管道焊接接头强度的影响

聚乙烯燃气管道由于其耐腐蚀、柔性好、施工效率高等优良特性，已逐渐在城镇燃气运输中取代钢制管道，成为主要的输送方式。聚乙烯管道的安装主要采用电熔和热熔焊接方式，存在的主要焊接缺陷有过焊、冷焊、氧化皮未去除、电阻丝错位、孔洞等，文中研究了不同焊接缺陷对聚乙烯焊接接头强度的影响程度及影响机理。采用拉伸、压扁、SEM和EDS测试方法对带有人为缺陷的焊接接头进行分析测试。拉伸、压扁力学性能结果表明，影响聚乙烯管道焊接接头强度的主要缺陷为冷焊、过焊、氧化皮未去除、电阻丝错位，其中氧化皮未去除对接头强度的影响最大，为正常焊接断裂应力的0.4%；SEM和EDS测试结果表明，含有氧化皮的管材分子链易交联、断链，流动性差，热氧老化的硬质斑块和机械划伤阻碍了聚乙烯母材大分子链的扩散，同时在紫外线、光、热等作用下，氧与聚乙烯表面分子链发生反应，形成大量羟基自由基团，使得聚乙烯分子链发生热氧老化和降解，大大降低了聚乙烯接头强度。

Influence of welding defect on strength of welded joint of polyethylene pipeline

Due to its excellent characteristics of corrosion resistance, good flexibility and high construction efficiency, polyethylene gas pipelines have gradually replaced steel pipelines in urban gas transportation and become the main transportation method.Installation of polyethylene pipes mainly adopts electrofusion and hot fusion welding.The main welding defects are over welding, cold welding, unremoved oxide scale, dislocation of resistance wire, holes, and so on.Effect of different welding defects on strength of polyethylene welded joints and influence mechanism was studied.Welded joints with artificial defects were analyzed and tested by tensile, flattening, SEM and EDS test methods.The results of tensile and flattening mechanical properties showed that cold welding, over welding, unremoved oxide scale, and dislocation of resistance wire were main defects affecting strength of welded joints of polyethylene pipes, and unremoved oxide scale had the greatest impact on strength of welded joints, which was 0.4% of fracture stress of normal welded joints.SEM and EDS test results showed that molecular chain of pipe containing oxide skin was prone to crosslinking, chain scission, and poor fluidity.Diffusion of macromolecular chains of polyethylene base metal was hindered by hard plaques by thermal and oxygen aging and mechanical scratches.Under action of ultraviolet rays, light, heat, and so on, oxygen reacts with the molecular chain on surface of polyethylene to form a large number of hydroxyl radical groups, which made polyethylene molecular chain undergo thermo-oxidative aging and degradation, and greatly reduced strength of polyethylene joint.