Creep Life Analysis Of Polyethylene Pipe

Polyethylene(PE)materials have excellent mechanical properties and play an important role in the society.PE pipes are widely used in natural gas,petroleum,water supply,and sewerage projects.For PE pressure pipes,the failure mechanism and failure time are closely related to the hoop stress and the environmental medium.It is generally accepted that the long-term failure behavior of pressurized PE pipes can be separated into three typical regions depending on the hoop stress in pipes.In first region for relatively high stress levels,the pipe failure is mainly caused by the ductile deformation,resulting in a large plastic region.In second region for moderate hoop stress levels,the pipe failure is manifested as a quasi-brittle slow crack growth(SCG)which preceded by a crack initiation stage,SCG failures usually happens over a relatively long period of time.In third region,the pipe failure is mainly caused by aging process,polymer degradation and stress corrosion cracking,showing stress independence.For the PE pipeline under actual service conditions,SCG is the main failure mechanism.The SCG process accounts for most of the creep failure time of the pipeline.Therefore,it is of great significance to study the SCG behavior of PE pipe.This article starts from the basic mechanical properties of the material and combines the theoretical and experimental methods to evaluate the creep lifetime of PE pipes.The main research contents are as follows:(1)Preparation of test samples.Considering the actual project,the specimens selected for this study are all taken from the pipeline,processed by mechanical cutters,molded without extrusion and heating,and the measured mechanical performance parameters are close to the actual project.(2)Research on basic mechanical properties of PE pipes.Uniaxial tension at different strain rates,creep tests at different stress levels,and fatigue crack propagation tests at different stress ratios are performed on PE pipe specimens.The basic mechanical properties of the pipe are concequently obtained by appropriate data acquisition and processing.(3)Study on creep lifetime of PE pipes.The creep is regarded as a special case of fatigue when the stress ratio equals 1.Fatigue crack propagation behavior of the PE pipe are measured under different stress ratios.While the creep crack propagation law is then extrapolated from the fatigue crack growth rate.The lifetimes of a specific PE pipe are predicted based on the power law of Paris and Erdogan and compared with the extrapolation in accordance with international standard ISO 9080.