Residual Stress Studies in Polyethylene Gas Pipes and Fittings. Final Report January 1, 1983-December 31, 1985

摘要

Methods have been developed to accurately measure residual stresses in polyethylene gas pipes. Accurate and rapid machining methods were developed to remove layers of material by turning or boring rings sliced from pipe in order to determine residual stress distributions through the walls of pipe. Comparisons were made between residual stress distributions determined by the above methods and more simplified methods (slit ring) which determine only maximum surface stresses. Polyethylene pipe, independent of resin type or pipe size, generally has biaxial compressive stresses on the outer surface and biaxial tensile stresses on the inner surface. Molded fittings typically have compressive stresses on both inner and outer surface and tensile stresses in the interior. Residual stresses can be relaxed by thermal annealing. In order to remove all of the stress, temperatures above 100 C are required. Fusion of PE pipe relaxes the residual stress in the vicinity of the fusion. The effect of annealing and/or residual stress on mechanical properties of PE pipe was studied. It was shown that the yield strength of annealed PE pipe was greater than for as-received pipe due to a density increase of the material. The lifetime of a PE pipe in a sustained pressure test at 80 C was increased for pipe annealed at temperatures above 100 C. It was attributed to relief of the tensile residual stress at the interior of the pipe. It was also shown that the crack growth rate in PE pipe was influenced by the residual stress. Compressive residual stresses cause reduced crack growth rates while residual tensile stresses cause increased crack growth rates.