Research On Pull-out Failure Of Polyethylene Electrofusion Joint

Our country’s energy structure is characterized by rich coal,poor oil and few gas.During the period from the 13 th five year plan to the 14 th five year plan,more than100,000 kilometers of oil and gas pipelines were built,which will further expand the coverage of oil and gas pipelines.Polyethylene and its composite pipes are more and more widely used because of their good corrosion resistance,high flexibility and strong designability.As one of the most important connection methods of such pipeline system,electrofusion connection will play a more important role.With the increase of conveying pressure and diameter of the pipeline system,the strength of pipes and electrofusion couplers are improved by reinforcing materials such as fiber and steel plate,which makes the fusion zone a weak position in the joint.As a result,the pull-out failure has become one of the main failure modes in electrofusion joint.The pull-out failure,where the pipe is pulled out from the electrofusion coupler along its axial direction,can cause medium leakage and economic loss.At present,the cause,location and influencing factors of the pull-out failure of polyethylene electrofusion joint are not clear,which seriously affects the safety of electrofusion joints,and also restricts the application and development of relevant pipes.With the financial support from the General Program of National Natural Science Foundation of China,the pull-out failure of polyethylene electrofusion joint were studied.By the combination of theoretical model,experimental study and numerical calculation,the stress and material property distributions in fusion zone,the location and cause of pull-out failure,the failure pressure and influence factors were explored.The main contents and conclusions are shown as follows:(1)The distribution characteristics of stress and material properties in the fusion zone of polyethylene electrofusion joint were studied.A theoretical method for calculating the stress field of electrofusion joint considering bending moment was established based on Lubkin and Reissner model,and a U-shape distribution of the shear stress in the fusion zone was obtained.Based on depth-sensing indentation technique,the distribution of elastic modulus in the joint of polyethylene pipe was also explored.It was found that the difference of modulus between base material and fusion zone material was within 7%.Through the theoretical model and material properties,it was found that the influence of material property difference on shear stress distribution was small.An exponential formal relation between the fusion length and shear stress concentration factor was found,which provided a theoretical basis for the pull-out failure test.(2)The pull-out failure tests of polyethylene electrofusion joint were carried out,and the failure law and mechanism were explored.Polyethylene electrofusion joints with different fusion lengths(5 mm～25 mm)were designed and welded,then shortterm burst tests were carried out.It was found that the pull-out failure happened under short fusion lengths,and the failure pressure increased exponentially with fusion length.Electrofusion coupler strength failure occurred when the fusion length increased to a certain value.The morphologies of failure samples were observed.Combined with the theoretical model,it was found that the pull-out failure occurred between the end of the fusion zone and resistance wires,and ran through the fusion zone along its axial direction.Furthermore,a shear test of electrofusion joint was designed.The shear strain distributions around the fusion interface and resistance wires at critical failure was determined by digital image correlation(DIC)technique,then a failure criterion of pullout was put forward.(3)A numerical analysis model for the pull-out failure of polyethylene electrofusion joint was established.The results of numerical analysis and short-term burst tests were compared.The relative errors of failure pressures in different fusion length joints were between 1.8%～29.2%,while the relative errors of the outer wall of electrofusion coupler were between 0.6%～21.2%,which verified the accuracy of the numerical model.Based on the numerical model,the stress distribution and its variation with internal pressure in the fusion zone during the pull-out failure process were analyzed.Considering the viscoelastic constitutive relation of polyethylene in the form of Prony series,a long-term pull-out failure prediction model was established.It was found that the stress and strain in fusion zone varied rapidly with time at the beginning,and then tended to be stable.The influences of inner cold welding length,resistance wire diameter and wall thickness of electrofusion coupler on pull-out failure pressure were discussed.Based on the numerical model,an electrofusion coupler for high pressure RTP were designed,and the rationality of the design structure was verified through a short-term burst test.