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Analysis of slow crack initiation of old polyethylene resins by means of an elasto-visco-plastic rheological model: experimental and numerical approach [PE]


Plastic Pipes Conference Association # 2012 Barcelona

Boujlal Adil

(Polyethylene, Long term behavior, Rheology, Slow Crack initiation time) Predicting PE failure mechanisms needs suitable models in order to describe accurately both material behavior and damage processes. In this area GDF SUEZ’ Research & Innovation Division carries out an in-depth study to determine on the one hand the rheological parameters of the various PE resins laid on the PE distribution networks and on the other hand their resistance to SCG. Modeling the SCG behavior needs to implement experiments allowing to describe accurately the process. Full Notch Creep Test (FNCT) has been chosen and adapted by GDF SUEZ’ R.& I. Division for this purpose. Cylindrical pre-notched specimens - extracted from both excavated pipes and reference pipes – are tested at 80°C or directly at room temperature without any surfactant. Such a procedure allows one to measure not only the failure time but also the crack initiation time in a very accurate way on the one hand, and to make some differentiations in the SCG behaviour of PE materials thought as very similar on the other hand. Moreover the overall set of FNCT experiments carried out up to now suggests that the initiation time could represent about half of the total time up to failure. Coupling the FNCT results with the rheological ones allows the numerical simulation of the FNCT curves up to the initiation time with a very good accuracy. The fair agreement between the numerical data and the FNCT experiments gives way to the calculation of a damage criterion in the crack tip area at the crack initiation time which represents the trigger for slow crack propagation of a given PE resin. Further integration of such a damage criterion in a real plain pipe should allow describing the Slow Crack initiation process in different loading conditions including over stresses.

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