## 3 Fatigue Crack Growth in Hydrogen Environment
### Mechanisms of Fatigue Crack Initiation and Growth
This can help researchers to quickly understand the development of this field. It can be seen from the figure that in the last 10 years, researchers havecarried out research and prediction of the fatigue life of materials around the hydrogen embrittlement mechanism. Studies have shown that hydrogen penetration in metals is a key factor in crack growth. Hydrogen atoms can penetrate into the crack tip and cause hydrogenation reactions, which leads to an accelerated rate of crack growth. In addition, stress corrosion cracking (SCC) in a hydrogen environment is also closely related to crack growth. The crack growth rate is affected by various factors such as stress level, material properties and environmental conditions. The mechanism of fatigue crack generation and propagation in metallic materials in a hydrogen environment involves complex physico-chemical processes, mainly related to the adsorption and diffusion of hydrogen atoms and their interaction with the metal crystal structure. The following section will discuss the existing mechanisms of fatigue crack growth in hydrogen environment. As mentioned earlier, hydrogen atoms can be absorbed on the metal surface and diffuse in the metal crystal structure [11; 12; 88]. This process can be affected by stress and temperature. In the stress field of a metal, hydrogen atoms are more likely to be absorbed and diffuse into crack tips or high stress regions. At crack tips or high stress regions [13; 45], hydrogen atoms may accumulate and cause changes in the local stress field. These stress concentration regions can exacerbate the stress state at the crack tip [19; 89], thus promoting further crack growth. [PERSON] et al. [88] conducted fatigue crack growth tests on two pipeline steel alloys, API 5L X52 and X100. Baseline tests were conducted in air, and  
Figure 5: Visualisation of the literature on fatigue crack growth in hydrogen environments in recent years: (**a**) trends in the number of publications and keywords on fatigue crack growth in hydrogen environments in Wos (2014–2024) produced via CiteSpace software (version 6.3. R1), by year; (**b**) keyword time line mapping of fatigue crack growth in hydrogen environments in Wos (2014–2024) produced by CiteSpace software (version 6.3. R1). these results were compared to tests conducted in pressurised hydrogen. They found that the fatigue crack growth rates of X100 and X52 were significantly higher in the pressurised hydrogen environment than in air. It can be seen from the a-plot of Figure 6 that the rate of crack propagation in the material is also significantly enhanced with the increase of hydrogen concentration. [PERSON] et al. [11] investigated the fatigue crack growth behaviour and its characteristics of AISI-304 stainless steel after tensile overload in three different atmospheres, i.e., dry argon, humid air, and hydrogen. After their experimental study, it was found that hydrogen promotes negative effect on the fatigue life of the material, but the humidity of the air shows little effect on the fatigue life. The researchers also analysed the extension mechanism of the material's microscopic grain boundaries. The diffusion of hydrogen atoms at metal grain boundaries may lead to grain boundary embrittlement and crack formation. Grain boundaries are the conduits for crack propagation, and the presence of hydrogen atoms makes the grain boundaries more susceptible to cracking and propagation. This promotes the propagation of fatigue cracks in metals. The interaction of hydrogen atoms with the lattice structure of the metal may lead to deformation of the crystals or the creation of microscopic defects, thus reducing the ductility and toughness of the metal and making it more susceptible to crack propagation and fracture. For example, [PERSON] et al. [19] conducted two experiments in order to investigate the mechanism of fatigue crack growth along the crystal of mild steel in a low-pressure hydrogen environment. One investigated the effect of cyclic pre-strain on crack growth behaviour.