Interpretation of results of full notch creep test and comparison with notched pipe test

Abstract

An investigation of two tests commonly used to determine resistance to slow crack growth in PE pipes and materials is detailed, in order to gain a greater understanding of the mechanisms involved and to resolve differences in results observed. The full notch creep test (FNCT) is carried out on small notched bars machined from sheet or pipe loaded to create high constraint at the notch tip. The notched pipe test (NPT) is a pressure test on pipe containing external machined notches. In this test, it has been observed that the use of more flexible materials allows deformation in the crack tip region and contributes to slow crack growth resistance via crack tip blunting. Good pipe performance can be achieved by selecting materials with high inherent slow crack growth resistance or by combining inherent resistance with blunting mechanisms promoted by a lower density material. It is concluded that the FNCT test, while useful for an indication of inherent slow crack growth resistance, cannot be used to predict pipe performance for a range of materials, and therefore is unsuitable as a reference test for a pipe product specification. The NPT test remains the benchmark test for pipe performance and is referenced by many specifications.