Experimental Investigation On Mechanical Properties Of Polyethylene Pipe Butt Fusion Joints With Process Defects

Currently,China is vigorously promoting the process of urbanization,and as one of the most widely used energy,the gas demand is growing rapidly.Because of the excellent mechanical properties,polyethylene(PE) pipe has been widely used for the transportion of gas in the cities.And as one of the most popular connecting methods for PE pipes,butt fusion(BF) joint becomes to be one of the key factors of pipeline safety.However,the process defect in BF joint because of wrong operation is very common on the work site,and has become to be one of the most serious risks of pipeline system.Comhesive researches on mechanical properties are important for the studies of failure mode and damage mechanism of polyethylene pipe BF joints with process defect,and provide data support to the optimization of welding process and the safety assessment of joints.Funded by the "National Science and Technology Support Program" (2006BAK02B01) and the "National High Technology Research and Development Program"(2009AA044801),researches on mechanical properties of polyethylene pipe BF joints with process defect are carried out,and the main conclusions are as follows:(1) A comprehensive classification method for defects in BF joints is proposed.In this method,the defects in BF joints are divided into four categories:crack,holes, inclusion on fusion interface,process defect,and the defects are characterizated though their features.(2) The tensile testing method in use for BF joints has been improved,and the stability of testing results increases significantly.In this improvement the width and arc number of smples are reduced,and the efficiency of sample preparation has been increased obviously.Then the temperature control accuracy is improved from 23±2℃to 23±0.5℃,and the relative error of tensile strength caused by temperature fluctuations is reduced from 6.4%to 1.6%.(3) In the common range of process defect(heating plate temperature:150-270℃, welding pressure:0.1-0.5MPa),the tensile properties of BF joints with process defect have been researched carefully,and the relationship between tensile failure mode and welding parameters has been revealed.Furthermore,the Vickers hardness of BF joints with process defect has been tested,and it is found that,the Vickers hardness at the middle of welding interface is greater than that around the pipe wall.