Oksana Hembara

An up-to-date scientific and applied problem of assessing the integrity of metallic structu­res in the context of the development of hydrogen energy is considered. The wide imple­mentation of hydrogen power engineering, transport, metallurgy, chemical industry, and municipal services imposes increasingly strict requirements on the reliability of pipelines, high-pressure vessels, cylinders, compressors, and hydrogen refuelling station equipment. Current understanding of the mechanisms of hydrogen embrittlement of metals, in particular HELP, HEDE, and AIDE, which are considered as interrelated components of a multi-scale material degradation process, is sum­marized. Engineering integrity criteria which account for the influence of hydrogen are analyzed, including empirical indicators (RRA, Ni-equivalent, limiting values of strength and hardness), fracture toughness para­meters (KIH, KISCC, J-integral, fracture energy Wс(CH)), as well as fatigue life and hydro­gen-assisted cracking criteria. Special attention is paid to modern models for integrity assessment: diffusion models with trapping, coupled physicomechanical damage models, cohesive zone models, and phase-field approaches, as well as their role in developing sim­plified engineering methods for service life prediction of hydrogen infrastructure compo­nents. The main scientific and technical challenges and promising directions for further research, including the unification of integrity criteria, multi-scale inte­gration of models, and the implementation of probabilistic and digital approaches to durability assessment, are formulated.

The mechanisms of hydrogen-induced failure and the processes of hydrogen transport in metals are considered. A mathematical model of hydrogen diffusion is proposed, which accounts for the interaction between local elastoplastic deformations and hydrogen diffusion. Using the finite element method, the distribution of hydrogen in the vicinity of crack-like defects on the inner surface of steel pipelines under internal pressure is analyzed. Analytical dependences are derived to assess the local hydrogen concentration near defects of various geometric shapes. It is established that with increasing internal pressure, the concentration of both diffusible and trapped hydrogen significantly increases, contributing to hydrogen embrittlement of the material. The obtained results can be used to predict service life of pipelines in hydrogen power applications.

The problem of hydrogen diffusion in a metal with pores is formulated. Pores are sources of absorption (traps) of hydrogen. Traps were considered to be point-like, distributed uni­formly over the entire volume of the metal. Solutions to the problem of diffusion in a defective body have been obtained. In particular, expressions for determining the concen­tration of both diffusion-moving and trapped hydrogen are obtained for the plate. We also obtained a ratio for determining the amount of hydrogen sorbed by a defective plate at an arbitrary moment of time during its hydrogenation. The amount of hydrogen sorbed by a defect-free plate and a plate with uniformly distributed traps was compared. A 2-3 time excess of hydrogen content in the defective plate compared to the defect-free plate was established.

A theoretical-experimental approach for predicting the kinetics of fatigue crack growth and determining the lifetime of the responsible elements of structures in hydrogen is proposed. Based on the created calculation model of fatigue crack growth and experimen­tally constructed kinetic diagrams of fatigue failure of 35KhN3MFA steel, the residual life of the rotor shaft of the steam generator, weakened by a surface semi-elliptical crack in air and in gaseous hydrogen environment, is determined. It is established that hydrogen reduces the residual life of the rotor shaft by two orders of magnitude in hydrogen, compa­red to its residual lifee in air.

To ensure the reliability and durability of structural elements, modern approaches require data characterizing the local stress-strain state of the material in risk zones. In order to predict the fracture resistance of structural elements, a theoretical-experimental method based on the damage accumulation model using the energy approach is proposed. One of the unique characteristics of the proposed approach is that it uses local parameters of the stress-strain state, which are determined using a highly accurate and easy-to-use noncontact method of optical–digital image correlation (ODIC). This can be used both in laboratory conditions and for structural elements under real operating conditions. The proposed method of plotting stress–strain curves makes it possible to determine the true stresses near the concentrators in structural elements under a complex load. Using these diagrams in calculations and the finite element method (FEM), a study of local strain not only of the surface, but also of the internal volumes of the material was carried out. The damage parameter is introduced as the ratio of the elastoplastic strain energy of the local volume to its critical value. It is confirmed that the damage of the sample material starts from its centre. It was established that the damage parameter in the centre of the sample is 25–35% higher than its value on the surface of the sample.

In the paper, experimental studies of the hydrogen and pre-strained effect on fracture resistance of steam generator steel 22K were carried out. Special cylindrical samples were loaded up to fracture under a uniaxial tensile test with different pre-strained degrees and hydrogen charged times of the material. Stress-strain curves «S_i–e_i» were plotted. The true strain «e_i» in the local volume was determined using the method of optical-digital image correlation (ODIC). The results showed that the hydrogen influence is practically absent in the elastic area of strain. The fracture energy of steel 22K decreases under the hydrogen influence and pre-strain in all investigated cases. It is shown that during six months of air exposure, with the 0% pre-strained samples release almost all hydrogen. In pre-strained samples, the hydrogen concentration decreased by 1–3% compared to the initial values. This indicates that they have trapped hydrogen that cannot escape on its own. Hydrogen embrittlement (HE) indexes for 0% pre-strained samples at different levels of hydrogen-charging calculated by the strain and energy approaches are equal to each other. There is a difference in the values of the HE index depending on the hydrogen-charge time for pre-strained samples. This indicates that both strain and strength characteristics of the material, which are integrally taken into account in the energy approach, are sensitive to HE.