PSI - Issue 48

Štěpán Major / Procedia Structural Integrity 48 (2023) 230 – 237 Major / Structural Integrity Procedia 00 (2019) 000 – 000

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the chemical structure of the compound or in the microporous structures of materials. When storing hydrogen in containers, we have progressed from steel containers to significantly more durable composite containers, however, the entire working set of hydrogen operation still contains many metal elements. These components, which are often under, are made of steel and even if they are exposed to the influence of hydrogen only secondarily, their service life is reduced by it´s chemical activity, Okonkwo (2023). For this reason, it is desirable to study the effect of hydrogen on the fatigue life of welded steel components used in equipment working with hydrogen.

Nomenclature A

accumulation modulus during plastic flow

crack length

a

hydrogen concentration, subscription L indicate lattice sites and T trap.

C L,T

critical crack tip opening displacement diffusion coefficient in lattice

COD

D L ∆ E

trap binding energy hydrogen flux vector trap equilibrium constant stress intensity factor

J

K T ΔK M L N L,T

mobility of hydrogen atoms in lattice

number of sites per unit volume, subscription L indicate lattice sites and T trap. N i,Tot number of cycles, subscription i indicates the initial phase or tot total number of cycles to fracture N Hard hardening coefficient Y L,T occupancy of sites, subscription L indicate lattice sites and T trap. ε P equivalent plastic strain Δγ max shear strain increment in the plane of its maximum value equivalent plastic shear strain rate φ dimensionless internal variable μ L chemical potential of the hydrogen in the lattice σ H,H hydrostatic stress of hydrogen σ y yield strength σ ij stress tensor and its components σ M mean normal stress ‡“ equivalent shear stress Hydrogen diffuses very easily through metallic materials, which is due to the fact that its atoms are very small. This hydrogen accumulates in local pockets, which results in the creation of internal stress that leads to a local decrease in the strength of the material, see Jiaxing (2023). We refer to this effect as hydrogen embrittlement, see Robertson (2015) and Lynch (2011). In this article three different approaches to modeling of hydrogen embrittlement are discussed. All these models were already presented in literature and now are applied to the specific case of a weld whose mechanical resistance has been reduced due to hydrogen embrittlement. The first model discussed in the article works with hydrogen diffusion described using Fick's law, then looks for a relationship between time, hydrogen concentration and the decrease in mechanical properties, see Major (2022). In this case, the model works with increasing hydrogen concentration (and pressure induced by it, which causes a local growth of strain) in the microcavity around a large non-metallic inclusion and uses a model proposed in fracture mechanics to describe fracture formation. In the second model we work with the nucleation of nanovoids. This approach can be justified by observation of nanoscale voids on fracture surface. And also, by the facts (I) that the plastic deformation leads to production of vacancies and (II) vacancies are stabilized by the presence of hydrogen atoms and (III) stabilized vacancies create hydrogen-vacancies complexes. The third approach is based on the model proposed by Anand, who proposed a model in which, when a certain internal variable is exceeded, inelastic stretching occurs in the direction of maximum principal stress. The article discusses the advantages and disadvantages of these models, which are compared with the experiment. The first