Plastic Pipes Conference Association # 2023 Lake Buena Vista
Chris Ampfer, P.E., Harvey Svetlik, P.E.
The current polyethylene (PE) pipe bending limits (longitudinal and circumferential deflection) are based on a circumferential (tangential) strain tolerance limit of approximately 1%. Research in the late 1970’s using constant tensile strain focused on what strain can be imposed on HDPE by bending and not have it progress into SCG. After some rational consideration of the allowable stress intensity on PE materials of the 1980's when 192-hours of ESCR (Environmental Stress Crack Resistance, ASTM D1693) was the standard, industry leaders (Frank Rice, Amos Shriver, and others) picked 1% as the circumferential strain limit. Today, with slow crack growth resistance > 2000 hours (PENT, ASTM F1473), the circumferential strain limit of PE4710 is much higher than 1%. With viscoelastic strain relaxation, PE4710 sheds about 60% of the related stress over time keeping the slow-crack growth stress-intensity well below the crack initiation threshold.
The existing circumferential (ring) deflection limits for buried pressurized PE pipe are lower than the deflection limits for buried non-pressurized pipe. Combined loading from internal pressure and external soil load does not shorten the life of PE pipe but can increase its life. The ring deflection limits should be the same for both pressurized and non-pressurized pipe. The ring deflection limit can be based on limiting strain or based on limiting flow reduction in the pipe. If limiting flow reduction to less than 1% is a concern, a ring deflection limit of 8% should be used for all PE4710 pipe regardless of DR. The PE pipe ring deflection limits in ASTM F714, ASTM F1962, and the PPI Handbook of PE Pipe (Chapters 6 and 12) should be updated to the proposed value of 8% (see Table 12) without differentiation between pressurized and non-pressurized.
The existing long-term longitudinal bending coefficients used to determine minimum bending radii for PE pipe are 3 to 7 times greater than the calculated critical bending coefficients and have longitudinal strains of 2.5% or less and circumferential strains of 1.125% or less. Revised minimum bending coefficients have been proposed that are 2 times greater than the critical bending coefficients and have circumferential strain of less than 4%. These proposed minimum bending coefficients are similar to published short-term bending coefficients. The PPI Handbook of PE Pipe (Chapters 7 and 10) should be updated to have a single minimum bending coefficient for each DR based on those proposed. The terms short-term and long-term in reference to bending coefficients should be removed.
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