Study On Viscoelatic Behavior During Slow Crack Growth Of Polyethylene Pipes

Polyethylene pipes have more advantages than metal pipes, so they are widely used in gas field. Consequently, safety of PE pipes in service is quite important. However Slow Crack Growth (SCG) is recognized as one of the major concerns for polyethylene pipes. Welding joint and scratch are not avoided in service. So research on resistance to SCG of PE pipes and predict pipe lifetime is one of most effective methods to pretend catastrophic failure.In order to reduce test time and observe the crack growth conveniently, Full Notched Crack Test is choosed. The preparation of sample and the procedure is based on ISO16770 .Also previous data required from Pennsylvania Notched Test is used to help obtain the creep parameters which is based on ASTM F1473. Creep fracture is micromechanism of SCG, so creep performance of non-notched speciment is tested at required temperature. Based on basic creep performance, SCG of welding joint and parent material is analyzed qualitatively according to viscoelastic theory. Also crack growth rate is deduced according to local energy criterion. It shows that crack of joint grows faster than parent material. A new methodology developed by Norman is used to predict the lifetime of in-servive PE pipes from short-time laboratory test.The result of FNCT demonstrates that stress crack resistance of parent material is greater than joint. The primary creep accounts for 40 percent of the whole creep process, so craze damage and viscoelastic behavior during primary creep are the dominant factors of predicting and evaluating material characteristic. Based on power law constitutive equation, finite element method(FEM) is used to analyze the stress and strain distribution of crack tip that is helpful to understand the motivation of crack propagation. Also the stress-dependent factor and fracture parameters are obtained by FEM analysis. The result indicates that performance of PE80 is dependent on stress. creep compliance is different at different stress. Most of all, COD primary behavior of FNCT is predicted.