Mechanisms of Slow Crack Growth in PE Pipe Grades [PE]

# 2001 Munich

Design of pipe systems for 50 year lifetimes depends on accelerated testing of pipe at different temperatures and standard extrapolation methods for assessment of whether the minimum required strength is achieved. Further acceleration is used in the notched pipe test (NPT), which is a medium term (up to 1000 hour) test designed to assess pipe stress crack resistance (SCR). The full notched creep test (FNCT) has been developed for the rapid assessment of pipe grade material and possibly also sections taken from pipes. In the case of the latter, temperature, severe notching and detergent are used to speed up the test. There is evidence to support the use of these techniques in the evaluation of the long term resistance to cracking of pipe grades and products, but the picture is certainly not complete in terms of confidence that all tests give the same information. In particular, there are significant differences between NPT and FNCT which may lead to conflicting messages from these tests. The FNCT is a highly constrained test geometry which minimises the degree of crack tip blunting that can occur at the notch tip. In contrast, the NPT has a much lower level of constraint which allows notch blunting to occur. A reappraisal of these techniques has been performed on a range of pipe grades by mapping out the behaviour in each test as a function of applied stress and interrupting tests to examine the nature of failure occurring. The implications for long term testing and prediction of pipe lifetimes based on such accelerated test data are discussed.

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