13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Review of Hydrogen Diffusion Models for the Analysis of Hydrogen Embrittlement of Materials Jesús Toribio1,*, Viktor Kharin1 1 University of Salamanca, Campus Viriato, 49022 Zamora, Spain * Corresponding author: toribio@usal.es Abstract Hydrogen-assisted fracture (HAF) of metals is the problem of major concern in structural integrity. HAF is rate limited by hydrogen accumulation in fracture sites in metals. Diffusion is a relevant stage of transport that supplies hydrogen to fracture process zones. Addressing the purposes of HAF analysis, modelling of hydrogen diffusion with account for physical-mechanical fields and factors of material microstructure (hydrogen trapping) is revisited. Generalised system of equations of trapping-affected hydrogen diffusion is derived from the kinetics principles. For circumstances, which are relevant to plenty of engineering HAF cases, specialised models of “microstructure informed” stress-strain-assisted hydrogen diffusion in metals are retrieved and collated under the prism of their suitability for HAF analyses. Keywords Hydrogen assisted fracture, Hydrogen diffusion, Modelling 1. Introduction Hydrogen is often the key factor in environmentally assisted fracture of metals, as far as it may be present per se in the environment or discharge in cracks due to favourable local electrochemistry [1]. Harmful effects of hydrogen on metals (“hydrogen embrittlement”) form the long-standing problem of the mechanics of materials. Hydrogen transport to damage sites is ubiquitous component of every hydrogen assisted fracture (HAF) process, which is rate-limited by hydrogen delivery to meet requirements of operating fracture mechanism. A series of kinetic processes involved in HAF have been identified, among which hydrogen diffusion has been substantiated as the governing mode of hydrogen supply to fracture nuclei [1-3]. Continuum modelling of hydrogen diffusion has been focused as the key issue of HAF studies and their engineering applications [1,3-6]. Concerning hydrogen behaviour in metals, important disconformities [2,7,8] were witnessed between experience and the Fick’s diffusion laws. Various issues have been pointed out as potential causes of these inconsistencies, and several analyses have been performed culminating in continuum equations of diffusion built up from microscopic or phenomenological considerations [1,4-11]. However, comprehensive accounting for a variety of potentially influencing factors is complicated undertaking, and numerous specialised diffusion models, which attended to different microstrictural features, have been raised [8]. An outlook of hydrogen diffusion modelling for the purposes of HAF analysis is here presented. 2. Background Theory of HAF [1,5] HAF is considered a result of synergic action of stress, strain and hydrogen amount in material. Fracture event takes place in a locus identified by position vector x, when hydrogen concentration C accumulated there over time t reaches the critical level Ccr dependent on stress-strain state: ( ) ( , ), ( , ) ( , ) t t C t C p cr x ε x σ x = , (1)
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