An Analysis On Failure Mechanism Of Polyethylene Pipe Butt Fusion Joints With Process Defect

With the advancement of society and technology, because of superiorperformance of polyethylene (PE) pipe, the traditional metal pipe has beengradually replaced by PE pipe. With more and more widely application in thefield of water and gas transportation, PE pipe has become an essential pipingsystem to maintain a normal life of urban residents. Connecting methods for PEpipes mainly includes butt fusion (BF) welding and electric fusion (EF) welding,but BF welding is most widely applied. In the process of BF welding, processdefects cause of wrong welding parameters are offen neglected， leaving asignificant risk to PE pipe system. Therefore, a study on mechanical properties ofBF joints with process defects and analysis on damage mechanism caneffectively avoid process defects, ensure security and stability of piping system.In this paper, by summarizing and comparing mechanical property testmethods for PE pipe, and combining with test target and conditions, tensile test isused to determine the performance of BF joints. Before tensile test, the types ofwelding defects were summarized, process defect was divided into threecategories: cold welding defect, over welding defect and too short weld defect.According to GB/T8804and GB/T19810, the form of tensile specimen wasmodified to achieve a new specimen which is easy processing and with moredirect access to tensile properties, then redefine the stretching rate. The result ofvariance analysis of BF jointsˊtensile yield strength showed that heatingtemperature and welding pressure have a significant influence on tensile property,while the other three welding parameters (heating time, changeover time, coolingtime under joining pressure) have minimal impact. A heating temperature of210℃and welding pressure of0.2MPa showed the best tensile property.Starting with failure fracture of BF joints, the failure fracture of BF joints with process defect was microscopically observed by SEM, the testedcrystallinity of BF joints by XRD. With extensive comparison, micro-morphology of failure fracture was obtained. Failure fracture is divided intocraze zone and black lamellar zone, and there is obviously demarcation betweenthem. Then the cause of formation of the micro-morphology was analyzed andgot. Finally, combining with the tensile yield strength of BF joints, we analyzedthe relationship between micro appearance and crystallinity of failure fractureand tensile property, then obtained the failure mechanism of process defectscaused of wrong heating temperature and welding pressure. The most importantinfluencing factor is the number and orientation of tie molecules in polyethyleneamorphous interlayer of BF joints under stress.