Study On The Creep Crack Growth Property Of PE100 Pipe And Butt-fusion Joint

Because of its many excellent properties,Polyethylene(PE)pipe is widely used in municipal and building drainage,gas supply,heating supply and industrial sewage,etc.It is generally believed that the typical failure mode of PE pipeline under long-term static load is the slow crack growth(SCG)caused by creep crack initiation(CCI)and creep crack growth(CCG),which is brittle failure.Large diameter PE pipe is generally connected with butt fusion joint.Defects and scratches which produced in the processes of welding,transportation and installation can cause creep crack initiation and growth,under static load.It lead to affect the safety of the pipeline system and long service life.Therefore,it is very important to study the creep crack growth property of PE materials and butt fusion joints to ensure the safe use of PE pipes.According to GB/T11546.1-2008,the tensile creep tests were carried out on PE100 parent material and butt fusion joint specimens under different stress conditions at normal temperature.The creep law in the steady creep stage was investigated to analyze the welding influence on PE material creep performance.The creep rate in the steady creep stage of the butt fusion joint specimens is bigger than that of PE 100 parent material,indicating that the creep deformation ability of the fusion joint is lower than PE 100 parent material.The creep crack propagation tests based on crack round bar(CRB)specimens were carried out on PE 100 parent material and butt fusion joint under different static loads at normal temperature.The fracture surfaces of creep crack propagation tests of PE 100 parent material and butt fusion joint were analyzed by scanning electron microscope(SEM).In the steady creep crack growth stage,the Crack opening displacement rate of the CRB specimen of butt fusion joint is bigger than that of PE 100 parent material.It indicates that the creep crack growth resistance of the butt fusion joint is lower than that of PE 100 parent material.The maximum stress produced crazing of PE100 parent material and butt fusion joint under tensile stress is equal to 15.05MPa.If the stress is more than the critical value,the crack is no longer the creep crack growth,and the failure mode turns to toughness failure.Because of Tie-molecule were failed to fully spread in the welding process,the stretched length of crazing fibrils of welded joint reduced.lt made that the creep crack growth resistance of butt fusion joints was decreased.And the micropore which produced in the welding process accelerated the creep crack growth rate of butt fusion joints.Through the compliance calibration tests,the function relation of compliance C and crack depth a of CRB specimens was obtained.the creep crack depth a of CRB specimens PE 100 parent material and butt fusion joint was measured indirectly.The relationship between the creep fracture parameter C*and the crack growth rate a of PE 100 and butt fusion joint is obtained under different static loads.