Plastic Pipes Conference Association # 2010 Vancouver
J.Zhou
Stress intensity in Polyethylene (PE) pipe is determined by applied stress, defect size, pipe diameter and DR (dimension ratio of outside diameter to wall thickness). For the same stress, stress intensity varies as the defect size and the pipe diameter change. Defects may be introduced into the pipe wall after it is extruded due to many reasons such as rock impingement and surface scratching. PE industry has used a 10% scratch depth as the maximum allowable defect size for all pipe diameters as a simple inspection practice. In keeping the scratch depth constant at 10% of the pipe wall thickness, the stress intensity increases as the pipe diameter increases. The greater stress intensity in a larger diameter pipe will produce faster slow crack growth (SCG) failure than in a smaller diameter pipe. This paper investigated the effect of stress intensity on SCG resistance in scratched PE pipe at three design stresses, 512 psi for natural gas distribution, 800 psi and 1000 psi for water pipe. To achieve equivalent SCG performance, the stress intensity should be the same for all pipe diameters. The maximum allowable % scratch depth as a function of pipe diameter for DR 11 pipe thus can be obtained for a given stress intensity such as 0.468 MPa m1/2 which is used by the standard PENT test. The impact of stress intensity at the fixed 10% scratch depth was estimated for various pipe diameters by comparing standard PENT and ISO notched pipe test results. Finally the paper discusses the relationship between the minimum SCG requirement and the maximum allowable scratch for scratched pipe up to 60” diameter. Increasing the SCG requirement and decreasing the maximum allowable scratch depth are two ways to compensate for the stress intensity effect on SCG resistance of scratched pipe with large diameters.
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