Tensile Strain Hardening Behavior And Strength Failure Of Polyethylene Pipes

Substituting steel by plastics is the requirement of national sustainable development strategy.Compared with traditional metal pipe,Polyethylene(PE)pipes have many advantages such as strong chemical corrosion resistance,easy construction and maintenance,long service life.They are widely used in national important lifeline engineering projects,e.g.water supply,natural gas supply as well as oil and gas transportation.The PE gas pipelines are designed to serve more than 50 years during which they are subjected to internal pressure,slow crack growth(SCG)is believed to be the dominant quasi-brittle failure mechanism,and controls the service life of the pipeline.Meanwhile,when the pipes are buried underground,the uneven settlement of foundation due to earthquake and excessive extraction of groundwater may cause additional deformations of the pipes,even lead to pipeline failure,thus endangering people's lives and property safety.Therefore,the correct evaluation of SCG resistance of PE pressure pipe and the effective evaluation of the pipeline service behavior have important practical significance and academic value.The main research contents and conclusions of this thesis are as follows:(1)Measurement of strain hardening modulus and evaluation of SCG resistance of PE pipe materials.The strain hardening modulus method for accelerated evaluation of SCG resistance of PE pipes was described in detail..According to ISO 18488,standard specimens were prepared from four PE100 grade materials and one PE100-RC grade material,and the tensile tests were conducted at different strain rates to the specimens at room temperature.The strain hardening of the specimens was analyzed and consequently the strain hardening modulus was obtained.Based on the measurements of the strain hardening modulus,the SCG resistance of the five PE pipe materials was evaluated,and the effects of temperature and tensile strain rate on the strain hardening modulus were discussed.The results show that the PE100-RC pipe material has better SCG resistance than the PE100 pipe materials.When the tensile strain rate is constant,the strain hardening modulus decreases linearly with the increasing temperature.When the temperature is constant,the strain hardening modulus increases in a logarithmic manner as the strain rate increases.(2)Finite element simulation of strength failure of buried PE pipe due to foundation settlement.The rate-dependent constitutive model and its parameters for PE pipes were determined based on the tensile tests at different strain rates.The additional stress and strain of the buried PE gas pipe caused by foundation settlement were simulated by Abaqus finite element analysis.The variation of stress distribution with settlement displacement was analyzed,and the effect of the length of transition section on the yield failure of the pipe was also discussed.The results show that the maximum Mises stress of PE gas pipe increases with the increase of settlement displacement.The dangerous section for yield failure is located at the junction of the transition section and the settlement zone,or at the junction of the transition section and the non-settlement zone.It is also found that the settlement displacement which causes the pipe failure varies with the length of the transition section: the longer the transition section is,the larger the settlement displacement which causes the failure.With the same settlement displacement,the buried PE gas pipe with long transition section is safer than that with short transition section.