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Title: The Slow Crack Growth And Constant Tensile Load Test Including The Industry Round-Robin Evaluation
Author: L. E. Hulbert, m. m. Mamoun
Source: American Gas Association 1985
Year Published: 1985
Abstract: Except lor third party damage, field failures in polyethylene (PE) gas pipe have been primarily the result of slow crack growth that may start at or near improperly heat fused joints and fittings, under impinging rocks, or at the site o a squeeze-off. These cracks may then slovvlv grow through the pipe wall driven by action of external bending, axial, or torsional loads on the pipe. Such loads often act in combination with high stress concentrations arising at the notches or geometric disconiinuiics at joint/ pipe interfaces. Long-term service life of polyethylene gas distribution systems eventually could be limited by (He slow, time-dependent growth of such cracks through the pipe wall to cause leaks. While the polyethylene gas pipe systems are designed to last a minimum of 50 years, an occasional early leak may occur in a gas distribution system because of a defect or an excessive load. The higher the load, the larger the defect, or the lower the resistance of the pipe material to crack growth, the quicker the pipe will leak. Measuring the long-term strength of polyethylene gas piping is complicated by the fact that crack growth rates and the mechanism of crack growth depend on many factors such as material properties underground soil conditions, and operating temperatures. Stresses due to secondary loads, when they are enhanced by the stress concentration effects of cross-sectional changes between fittings and pipes, are also particularly important. Failures in polyethylene gas piping (except third party damage) never happen quickly. Even if a flaw in the pipe or an imperfectly bonded joint starts to crack, it will typically take the crack several years to grow through the pipe wall. The occasional polyethylene gas pipe failures that have occurred under normal field service conditions have always exhibited this slow growing type of brittle failure, primarily due 10 secondary external loads and the stress concentrations noted above.

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