Models of hydrogen influence on the mechanical properties of metals and alloys

The article yields a survey of the key models of mechanics that are used to describe the effects of hydrogen embrittlement, hydrogen cracking, and hydrogen-induced destruction. The main attention is paid to models which are used to calculate the stress-strain state of metal samples, parts and machine components and have the potential for specific engineering applications. From a mechanical perspective, the effect of hydrogen on the material properties is a classic problem of the influence of a small parameter, since the hydrogen concentrations critical for the strength and ductility of metals are usually small. In the vast majority of models this effect is reduced to the hydrogen redistribution within the material volume and localization of concentrations in the critical fracture zones. The authors identified four main approaches that allow one to take into account the influence of a small parameter: (i) hydrogen-enhanced decohesion (HEDE), (ii) hydrogen-enhanced localized plasticity (HELP), (iii) account for additional internal pressure due to the hydrogen dissolved in metals, and (iv) bi-continuum approach that takes into account the internal hydrogen pressure and weakening of material in the framework of a special model of a solid. The links between the main approaches are established. Systematization of publications was carried out, similarities and differences in the description of the internal transport and accumulation of hydrogen in metals are highlighted. It is indicated that the predominant number of publications is devoted to the HEDE model, but so far there is no published data on the application of this model to real problems of engineering practice; only modeling the results of mechanical tests of cylindrical and prismatic samples were considered. In fact, other less popular approaches have more practical applications. The main unresolved issue in the verification of all models is the local concentration of hydrogen, which is a source of premature destruction of metals under load. All the methods for measuring local concentrations are indirect. Even in the case of applying sophisticated physical methods, mechanical surface preparation is required, which destroys the initial natural concentration of hydrogen. The lack of reliable data on the distribution of hydrogen concentration excludes the possibility of unambiguously determination of all the model parameters. On the one hand, it allows fitting to any experimental data, and on the other hand, it reduces the predictive engineering value of all models, since a qualitative fitting is not sufficient for engineering strength analysis.