Analysis Of Key Structural And Welding Process Parameters For Electrofusion Joints Based On Weld Zone Failure

With the scarcity of traditional energy sources like oil and environmental degradation caused by the use of non-renewable energy,the world’s energy consumption structure is being transformed from traditional energy sources such as oil and coal to efficient and clean energy sources such as natural gas.China responds actively to the energy transition,and the application scalability of town gas is expanding.With its excellent structural characteristics,polyethylene piping has become the first choice materials for gas pipeline networks and more widely used.And as the main method of the pipe jointing,electrofusion welding will also play an increasing significant role.With the increase of the pressure and caliber of the pipeline system to which the polyethylene electrofusion joint applies,the weld zone becomes one of the weakest part of the joint.And it can lead to the failure of the joint,which may cause economic losses and even endanger the public safety.Current design methods for electrofusion joints are generally determined through tests,but in practice,weld zone failures still occur sometimes.Although the existing research outcomes have indicated that the causes of weld zone failure mainly include structural and technological aspects,it is still an urgent problem to be addressed that how to design the welding structure and process specifically to avoid the weld zone failure.This research is supported by the "Eagle Plan" project funding(grant number:CY2022002)and the first batch of key research and development programmes in Shandong Province(major science and technology innovation project)project funding(project number: 2021CXGC010303).In this paper,I studied from the structure and welding process respectively,took the polyethylene electrofusion joints as the object of research and use numerical models as research tools to investigate the relationship between the failure pressure in the weld zone and the joint structure,and the influence of the welding process on the results,etc.The main work accomplished included:(1)explored the influence of different structural parameters and loading on the failure pressure and mode of electrofusion joints.A numerical calculation model for the electrofusion joint failure was established to analyses the failure process of the material in the weld zone as the internal pressure increases: the failure of the weld zone starts from the junction of the internal cold welding zone and the melt zone,and the overall yielding of the material occurs along the weld zone;it also studied the impact of different structural parameters such as the fusion zone length,fitting thickness,the burial depth of the resistance wire and the internal cold welding zone length on the failure pressure and failure mode of the electrofusion joint.Then based on correlation and significance analysis,this paper clarified the main structural factors affecting the failure pressure of the weld zone is the length of the melt zone and the wall thickness of the pipe;it explored the effects of the axial force on the failure pressure of the weld zone under different loadings.(2)explored the different influence patterns of initial resistance values,resistance temperature coefficient and welding voltage on the final welding results.This paper also summarized the existing researches on cold welding and over-welding defects,and proposed that the maximum temperature and the depth of the melt zone should be considered when deciding the welding process.And based on that,give the evaluation function of the welding process to determine the objectives for the process design;establish a welding temperature field model for electrofusion joints to explore the different influence patterns of initial resistance values,resistance temperature coefficients and welding voltage conditions on the welding results.(3)proposed a joint design method based on the weld zone failure of the electrofusion joint.With the design goal of making the strength of the fittings and weld zone stronger than the pipes,this paper proposed the design method of electrofusion joint structure based on the laws given in the numerical analysis of the weld zone failure.Meanwhile,with the goal of non-occurrence of cold welding and over-welding defects,this paper proposed a design method to meet the requirements of the maximum temperature and depth of the melt zone.