Research On Slow Crack Growth Of Electrofusion Joint Of Polyethylene Pipe

With the rapid development of the domestic economy,oil and gas pipeline,as the lifeline of the national economy,grows with a speed of more than 10%per year.For its high performance in corrosion resistance,welding,and economical and environmental protection and long service life,Polyethylene(PE)pipe has been widely used in oil and gas distribution and also other fields.According to the statistics,PE pipes take up over 90%of the newly layed low and mediun urban gas pipeline,most of them are connected with electrofusion joints.Survey results suggest that they are weak links of the whole pipeline system,and whose safety is the key to the safety operation of the pipeline.Slow crack ggrowth(SCG),which extends through the fitting of electrofusion of polyethylene,is one of three main failure modes of the electrofusion.SCG is a cracking that may last more than several decades at maximum,far exceeding the 1000h specified in the standard.In consequence,this type of failure mode can not be effectively detected by the existing standards.At this stage,the assessment of resistance ability of SCG is mainly depended on test results of standard specimen.The cracking failure time will be obtained for the special cracked specimen under tensile creep condition.However,due to the difference of the structure and loading mode,the existing SCG standard tests can not determine the relationship between service life of electrofusion and cracking propagation.At present,no inspection and assessment mothod for SCG failure of electrofusion joint is reported,it must be studied in depth,which would be a previous technical preparation for the proposal of design and detection method based on this failure mode,and it will improve the security capability of PE pipelines.With the financial support from the Youth Foundation of National Natural Foundation of China"Formation Mechanism of Eigen-Line in Ultrasonic Inspection of Electrofusion Joint for Connecting Polyethylene Pipes"(Project No.51305394)and the General Program of National Natural Foundation of China"Study on the Mechanism of Slow Crack Growth in Polyethylene Pipe Based on the Viscoelastic Damaged Model"(Project No.51575480),numerical method for the study of SCG in the electrofusion of polyethylene pipe is developd in this thesis,by the combination of theoretical model,experimental study and numerical computation.The main content and conclusions are shown as follows:(1)Relaxation tests were conducted under various temperature,and the Prony series of linear viscoelatic constitutive model of polyethylene were obtained under the temperature of 25?and 80?,based on the time-temperature superposition principle(TTSP).A damage model was proposed to represent the growth of crack,the specification of getting parameters of this damage model was made,through creep experiments under two different stress levels,and the relevant parameters under 25?and 80 ? were preliminarily given.Based on the concept of effective stress,the damage model and linear viscoelastic constitutive model were combined,and a viscoelastic damaged model was developed,which is the theoretical basis of the numerial simulation of slow crack growth.(2)Numerical discretization was conducted for linear viscoelastic equation in the form of 1D.On the basis of this process,the incremental equations of viscoelastic damage model in the form of 3D were given,and update of the damage model and stress was decoupled,which will improve the convergence of numerical method.Based on the proposed theoretical model,and following the programming rules of UMAT,the secondary development of the damaged constitutive model was realized.A sliding semi-automatic notch preparation device for Pennsylvania Notch Test(PENT)was designed.For the research of SCG,PENTs were conducted under 25?,and the experimental process was recorded by non-contact video extensometer called VIC-2D.A finite element analysis model for the simulation of PENTs was proposed by using the programmed UMAT,with damaged elements representing the growth of crack.The numerical results predict the curves of tensile and opening displacement vs time well,which proves the effectiveness of the proposed numerical method.On the basis of aboved fact,the reason of slow growth of the crack was explained.Influence rules of parameters of the damaged model to the cracking propagation were dicussed.The numerical method developed in this chapter would be an effective tool for the simulation of SCG in realistic component when the loading conditions were involved.(3)SCG which extended through the fitting of electrofusion of polyethylene pipe containing defects in the process of hydrostatic test under 80? was analyzed and discussed in detail.Linear elastic finite element method and fracture mechanics were used to analyze the phenomenon that the angle between the crack path and the axial direction of the pipe is about 75°.The developed numerical method which uses damage to represent the crack growth was utilized to realize the procress of cracking through the fitting of electrofusion.The failure time was available,and the transfer law of the stress in space-time during the cracking was discussed.On the basis of the numerical results,the influences of thickness of fitting of electrofusion,thickness of pipe,length of inner cold welding zone and length of fusion zone to the SCG were studied.Meanwhile,the curve of operating pressure vs SCG failure time was drawn,and the corresponding equation was given.