It has previously been highlighted (see The FESI Bulletin, Autumn/Winter 2019) that various key ‘themes’ have been identified as part of a more structured approach going forward on the way we go about organising FESI-related events for the increased benefit of our members and for the structural integrity community in general.
During a FESI Council meeting held on February 27, 2020 at the University of Leicester, the ‘champions’ of four of the themes presented initial information they had each compiled on providing an introduction of their theme and potential planned events to be held over the next year or so aimed at progressing the theme. We will publish their conclusions as each theme progresses over the coming months.
Below, Dr Alan Turnbull FRS of NPL outlines the FESI Theme of “Environmentally Assisted Cracking”.
Background
Environmentally assisted cracking (EAC) of metals encompasses corrosion fatigue, stress corrosion cracking, hydrogen embrittlement and liquid metal embrittlement. It is a potential failure mechanism in every major industrial sector, including nuclear and conventional power generation, oil and gas, and aerospace. The incidence of failures is declining through improved understanding and awareness, but the occasional catastrophic nature of the failures is a challenge to the materials and engineering community to improve prediction of the likelihood of cracking and the impact on structural integrity. Such predictive capability, allied with informed system management, is also a necessary requirement in sustaining and extending the life of high capital cost and safety critical infrastructure, such as nuclear plant.
EAC is intrinsically a complex process and sensitive to a multiplicity of interacting environmental, mechanical and material variables that can be quite different at a local level from the nominal conditions. Characterising the early stages of crack development in environmentally assisted cracking is a particular goal. For example, corrosion pits can both respond to and create an aggressive chemistry and will localise stress and strain; mechanical preparation of the surface, or welding, may introduce changes in local mechanical and microstructural properties of the material. The material may age (because of irradiation for example), and transient variation in stress, temperature and environmental chemistry may occur from scheduled excursions or from unintentional fluctuation in system control.
Despite the daunting nature of the complexity, remarkable progress in understanding has been made with the advent of advanced measurement tools such as X-Ray computed tomography, focused ion beam-SEM imaging, atom probe tomography and a range of surface analytical techniques, coupled with multi-scale modelling.
Impressive though these advances may be, there is a basic question that needs to be addressed and that is how to improve the linkage between short term laboratory tests in simulated service environments and the prediction of the onset of cracking, its rate of development, and the projected life of the component or structure in often more complex service environmental conditions.
The EAC activity in FESI provides a bridge between researchers and engineers to promote awareness of advances in technological development and best practice in testing, design, and material selection, with the goal of optimally managing EAC in service.
The specific EAC themes from 2020 onwards reflect current and future challenges.
Seminars/workshops
(1) EAC in Nuclear Plant (R Clark, 2020)
(2) EAC Non-Nuclear, With Session on Atmospheric SCC (A Bannister, 2020/2021?)
(3) EAC General: ESIA (A Turnbull, 2021)
(4) Corrosion Pits and EAC (N Larrosa and A Turnbull, 2021/2022)
(5) SCC of Aluminium Alloys (T Burnett and H Holroyd, 2022?)
(6) EAC in Low Carbon Technologies (topic leader tbc, 2023/2024)
Alan Turnball
Senior NPL Fellow in Electrochemistry
National Physics Laboratory