Miscellaneous
Plastic Pipes Conference Association # 1988 Bath
Marshall, Birch-Kisbenyi, Youd
A complete analysis of the Charpy impact system has been made to relate measurements of impact toughness to the geometry of samples. Using data obtained on pipe grade polyethylenes, Gc values at 20°C and 3°C have been found to correlate wel! with measurements of resistance to fast fracture in pressurised pipe tests.
There has been much work carried out in recent years to determine the resistance of polyethylene pressure pipe materials to unstable failure (e.g. refs. 1, 2 and 3). Such a mode of failure cannot be tolerated in a welded gas pipe system and is a highly undesirable prospect for pipes carrying potable water.
A conservative design philosophy nas been adopted by British wes ict ot the Pressure stresses in large pipe sizes (above 250mm diamcter). The anal used invoncs @ simple energy balance hypothesis which postulates that the st wali increases in proportion to the diameter and the only way to reduce the energy which is available to propagate a crack is to reduce the pressure.
Following a series of large scale pressure tests to investigate unstable cracking In gas-grade polyethylenes (ref. 1), the Water Research Centre commissioned British Gas to Carry out similar research into the unstable failure resistance of blue MDPE. It was found that unstable cracking could not be sustained in pipes pressurised with water alone although fast fracture could occur if pipes contained 10% free N2 (ref. 4). Asa consequence WRc have followed the British Gas design policy and advise engineers to reduce wall! stresses in large diameter MDPE pipes. The consequence of the de-rating process has been to encourage design engineers to use pipes with large wall thicknesses. However, PE pipes with wall sections of 40-60mm are not only difficult to produce but are also more difficult to handle, weld and instal] on site and are clearly far more expensive. Although installation problems could increase the risk of initiating a failure (damage, imprecise welding, etc.), there is also a fear that increasing the wall thickness to the extrusion limit ( 65mm) couid engender a change in failure behaviour via a plane stress/plane strain transition in the stress state.
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