Plastic Pipes Conference Association # 2006 Washington DC
Boros, Mamoun
In order to establish a Hydrostatic Design Basis (HDB) for PE materials according to ASTM D 2837, it is required to validate that the ductile/brittle transition is beyond 100,000 hours. This procedure uses elevated temperature hydrostatic stress testing to “validate” that the extrapolation of the ductile stress regression curve will remain linear through this time. With the vast improvements in PE materials over the last 20 years, it has become increasingly difficult to validate these materials in a reasonable period of time. In some cases it can take two years or more. The goal of this research is to develop a new standard test method that can validate these highly stress crack resistant PE materials in a much shorter time. This test can also be used as a resin development or possibly a qualification tool. The Hydrostatic Stress Board (HSB) of the PPI initiated a research project and contracted with the Gas Technology Institute to develop a correlation of an accelerated test method to the well known and understood hydrostatic stress rupture testing of PE pipes at elevated temperatures and result in a slow crack growth failure. This paper is a summary of the Plastics Pipe Institute’s Hydrostatic Stress Board and the Gas Technology Institute’s research report published under contract No. 8494 and GTI Project No. 15196. This project was co-funded by the Gas Research Institute. Six polyethylene materials were chosen to represent a broad spectrum of SCG performance. Not all of the resins used in this study were pressure pipe grade PE materials. The materials were fully categorized for physical properties then subjected to elevated temperature hydrostatic testing per ASTM D1598, PENT testing per ASTM F 1473, then cyclic pressure fatigue testing under various conditions of amplitude, frequency and stress ratio, both with and without an axial notch on the outer surface of the pipe.
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