Study On Life Prediction Of Polyethylene Pipe By Cyclic Loading Method

Polyethylene(PE)pipe has been widely used in urban gas and water supply pipelines because of its excellent corrosion resistance,long service life and environmental protection.The design life of polyethylene pressure pipes is generally 50 years.At present,the buried polyethylene pipes in China have been used for more than 40 years,which is the potential peak period of failure.Failure accidents may cause serious damage to infrastructure and even endanger personal safety,so it is of great practical significance to evaluate the life of polyethylene gas pipes accurately.The main reason for the failure of PE pipe in long-term service is the slow crack growth(SCG)of pipe.The traditional lifetime method for pipe failure caused by SCG is the pipe pressure test.Due to the improvement of the chemical structure of PE,the SCG resistance of the PE pipe has been significantly improved.Using this method for life assessment needs at least one year and it costs a lot,which does not meet the practical needs.So it’s urgent to develop a new life prediction method.In this paper,a new method-cyclic load crack round bar(CRB)test method is studied,which can quickly evaluate the life of PE pipes,and developing the corresponding rules of the test.The main research work and results are as follows:(1)The life prediction of PE pipe by the CRB method under cyclic load was studied.The CRB specimens of three different grades of PE pipes were made,and the cyclic load CRB tests were conducted under different load ratios(R)and the quantitative grade evaluation was worked out.Through the analysis of the cross-section of the CRB specimen,the average crack growth rate was calculated,and the crack growth kinetic curve of the material was obtained.Then the dynamic curve of crack propagation under static loading was extrapolated,the fracture mechanics parameters of the material were derived,and the lifetime is predicted according to the specific pipe model.The experimental results show that the cyclic loading method can evaluate the grade and life of PE materials quickly.For PE80 pipes,the longest testing time is 2 months,and the longest testing time for PE100 pipes is 4 months.(2)The classical full notch creep test(FNCT)method and CRB test method was compared and analyzed.The full-notched specimens of three different grades of PE pipes were made,and FNCT was conducted at different temperatures and the quantitative grade evaluation was worked out.Through the test,the relationship between the crack opening displacement(COD)and the failure time can be got,and the relationship between the crack growth rate and the test temperature was obtained by fitting,from which the crack growth rate at room temperature was derived.Furthermore,the fracture mechanics parameters of the material were calculated and predicting pipes’ life based on the specific pipe model.The experimental results show that the SCG resistance of the PE pipe evaluated by FNCT is consistent with that of the CRB test,and the service life obtained by the FNCT method is close to that of the cyclic load CRB test.(3)The influence of microstructure parameters of PE pipe on its SCG resistance was studied.The results show that the molecular weight,molecular weight distribution,crystallinity,and lamellar thickness had a good positive correlation with the SCG resistance of the material.Therefore,the polymer parameters that affect the SCG resistance of PE pipes,can be used as index parameters to evaluate the SCG resistance of different grades of PE pipes.