Research On Slow Crack Growth Tests And Law Of Viscoelastic Fracture Of Polyethylene Pipe Materials

Slow crack growth(SCG)of Polyethylene(PE)pipe is considered to be the main reason of the brittle failure which leads to leakage under the long-term internal pressure.As the carrier of gas transportation,the leakage of PE pipes will directly affect people’s daily life,and it’s more likely to result in catastrophic consequences.The performance evaluation tests,theoretical applications and numerical simulation of SCG behavior and viscoelastic fracture law of PE pipe materials are studied in this thesis.The main contents are shown as follows:(1)Experimental study on SCG resistance evaluation of PE pipe materials.The strain hardening modulus of different PE specimens was obtained by strain hardening test method,and the validity of strain hardening test method was verified by cone test.The correlation between strain hardening modulus and microstructure parameters such as lamellar thickness,weight average molecular weight,molecular weight distribution and comonomer content was respectively analyzed.The rapid,comprehensive and reliable evaluation of the SCG resistance of PE pipe materials was carried out by combining the macroscopic and microscopic test methods.(2)Quantitative study on the viscoelastic fracture behavior of PE pipe materials.Based on the time-temperature equivalence principle,the Prony series expression of the relaxation modulus of PE pipe materials was obtained,the viscoelastic fracture behavior of crazing growth,fracture,crack initiation,growth and fracture failure of PENT specimen was studied quantitatively by combining with model of stress relaxation in the craze zone around crack tip and craze-crack propagation mechanism of the bridge type,the growth law of the length of the craze zone was analyzed,the expressions of four key parameters of the viscoelastic fracture behavior,crack initiation time,the opening displacement of the craze fracture surface,the crack growth rate and the fracture failure time of the specimen were obtained.The relationship between the expected life of the PE pipe and the critical crack depth on the outer surface of the pipe was obtained by using the failure time of the PENT specimen,which provides a theoretical reference for the lifetime prediction of pipeline under SCG failure mode.(3)Numerical simulation of viscoelastic fracture law of PE pipe materials The finite element model of PENT specimen was built by finite element software ABAQUS,and the viscoelastic fracture behavior of PENT specimen was simulated by using extended finite element method(XFEM).The rationality and validity of the viscoelastic fracture behavior which simulated by XFEM was verified by comparing the simulation results with theoretical calculations and experimental results.The variation law of crack opening displacement and crack growth length of the PENT sample was studied.The influence of the test stress and the change of test temperature on the PENT was analyzed,and the best experimental condition which ensures the brittle failure mode of PENT specimen and shorter test time was obtained.The numerical simulation technology was used to provide a reliable reference for shortening the time of the PENT under the situation that the test condition stipulated in the PENT standard was no longer applicable to the current PE pipe materials because of the lengthy period of test time.