ECCOFIC

The final objective of the project is to develop a more realistic than today methodology for the evaluation of lifetime of components sensitive to Stress Corrosion Cracking (SCC), in order to apply it for Pressurized Water Reactors (PWR) management. Currently, lifetime of an industrial component prone to SCC is evaluated from two kinds of data (laboratory tests): the initiation time of the first crack and the propagation rate of long cracks. Yet, on a practical point of view, long crack formation, whose behaviour can be predicted by laboratory crack velocity measurements, often results from multiple initiation process, followed by propagation and coalescence of colonies of short cracks, which can constitute a great part of components lifetime. In that case, the actual methodology leads in very under-estimated lifetimes and therefore to non-optimized maintenance costs, which could be reduced by the present project. This problem was widely studied in the case of external corrosion of pipelines and models were built up to forecast short cracks behaviour and their coalescence into long and high propagation rate cracks. This kind of model is specific for each material/medium system and requires a heavy experimental support to acquire basic data (initiation frequency evolution with time, conditions of propagation and propagation rate of short cracks, conditions of their interaction and coalescence). Similar data were obtained for alloy 600 in primary water. They evidenced that the model developed for pipelines cannot be directly applied, which requires the rapid and accurate acquisition of data necessary to build up models of initiation, propagation, interaction and coalescence of SCC short cracks, for metals exposed to aqueous media at high temperature and high pressure, representative of Pressurized Water Reactors conditions. On the experimental point of view, the chosen technique is the Digital Image Correlation (DIC) technique that allows performing cartographies of successive displacements of a surface and to detect cracks by local anomalies of deformation. Beyond the interest of detecting cracks and determining their length, the technique allows also estimating their opening and deducing therefore local mechanical solicitations, which are determining in case of multi-cracking. It will be coupled with classical technique of SCC detection and monitoring such as electrochemical noise (EN) or acoustic emission (AE). Its use will necessitate the installation of windows on autoclaves The experimental process will be accompanied by the 3D mechanical modelling of cracks interaction, which must led to the simulation of risky components behaviour. The main interest of this approach for PWR is the study of SCC in primary water of thick components made of alloy 600 (bottom head penetration, M support block, etc..), and also stainless steel cracking in nominal and contaminated (by dissolved oxygen and impurities) primary water. The final objective is the modelling of initiation, propagation, interaction and coalescence of colonies of short cracks, with a view to a more realistic (i.e. reliable but less conservative) prediction of lifetime for components prone to SCC or FC. Such a modelling should contribute to optimize non destructive evaluation operations and to reduce maintenance costs of risky components.