Slow Crack Growth Behavior And Numerical Simulation Of PE100 Pipes

High density polyethylene (PE100) pipes are widely used in various fields due to their excellent chemical and mechanical properties.Stress concentration zones caused by cracks are the most dangerous weak-links in service processes under long term hydrostatic load.PE100 pipe usually undergoes slow crack propagation caused by creep damage at crack tip.The pipe ruptures rapidly and results in medium leakage or gas explosion when the crack spreads to the critical dimension that the pipe can stand,endangering people's lives and property.Therefore,it is very important to study the creep and slow crack growth behavior of PE100 pipe.In this paper,the creep crack kinetics relationship based on stress intensity factor was used to predict the residual lifetime of PE100 pressured pipe with external axial and circumferential semi-elliptical crack.The nominal outer diameter of pipe ranged from 75 to400 mm for the standard dimension ratio SDR=11,13.6 and 17,and the maximum allowable initial relative crack depth and corresponding stress intensity factor value of pipe were obtained under the condition of service lifetime of 50 years.The results showed that the allowable relative crack depth value a₀/t of the pipe gradually decreased as the outer diameter of the pipe increased,and a₀/t decreased with decreasing SDR.The fatigue crack growth of cracked round bar specimens was simulated based on the extended finite element method(XFEM)in ANSYS.It was found that the simulation data agreed well with the experimental results in the steady creep crack growth stage.This method was applied to simulate the fatigue crack growth of PE100 pipe(SDR=11,D₀=250mm)with external axial and circumferential semi-elliptical crack subjected to internal pressure under different stress ratios R.The law of crack growth rate changing with stress intensity factor under creep condition was got by extrapolation.This law was used to evaluate the residual lifetime of the PE100 pipe with external axial and circumferential semi-elliptical crack.The residual lifetime of the cracked pipe obtained by the CRB extrapolation formula was basically consistent with the simulated value under the condition of the same crack depth.The C* of PE100 pipes with external axial and circumferential crack were calculated by the reference stress method.The creep crack kinetics based on creep fracture parameter C*was obtained by creep and creep cracking tests on smooth specimens and cracked round bar specimens.The residual lifetime of PE100 pipe(SDR=11,D₀=250 mm)with external axial and circumferential semi-elliptical crack was predicted by creep crack kinetics relationship.It was determined that the maximum allowable relative crack depth of the PE100 pipe with external axial semi-elliptical crack was 0.231 corresponding to stress intensity factor value of 0.621 MPa·m^0.5,C*of 2.831x10^-7N/(m·s)under the condition of 50years service life.The maximum allowable relative crack depth of PE100 pipe with external circumferential semi-elliptical crack was 0.588 corresponding to stress intensity factor value of 0.586 MPa·m^0.5,C* of 4.184x10^-7N/(m·s).