## 3 Fatigue Crack Growth in Hydrogen Environment  
### Mechanisms of Fatigue Crack Initiation and Growth  
In the previous section, we have sorted out the classical mechanism of hydrogen embrittlement as well as the experimental methods. Next, this paper will focus on the existing study of fatigue crack growth in hydrogen environment. The relationship between fatigue crack growth and hydrogen embrittlement in hydrogen environment is intricate and significant. As mentioned earlier, in a hydrogen environment, hydrogen atoms penetrate into the metal, leading to hydrogen embrittlement, which makes the material more susceptible to crack growth. This phenomenon is closely related to SCC, in which crack growth is accelerated in a hydrogen environment due to the stress corrosion sensitivity of the material. When hydrogen embrittlement occurs, hydrogen atoms penetrate into the metal lattice, leading to lattice expansion and stress concentration, which reduces the material's ductility and increases its brittleness. This makes the material more susceptible to crack propagation and fracture, especially in hydrogen environments, where stress concentrations at the crack tip can exacerbate embrittlement. Hydrogen embrittlement encompasses a range of performance degradation phenomena of materials in hydrogen environments, so that fatigue crack propagation in hydrogen environments and hydrogen embrittlement phenomena is in an encompassed relationship. Figure 5 presents an overview of the articles published about fatigue crack growth in hydrogen environment that can be retrieved on Web of Science Core Collection (SCI-EXPANDED + SSCI) between 2014 and 2024. The CiteSpace software is used to achieve the presentation of the top ten keywords of researchers' papers in the field of fatigue crack growth in hydrogen environment in the last decade. CiteSpace (version 6.3.R1) was used for bibliometric visualisation. The parameters set are time slicing = 1 year, node type = keywords, and Pathfinder network pruning. Wos was selected in this study, but different databases and other clustering algorithms may have some impact on the output results. It is worth noting that researchers have been enthusiastic about the field of hydrogen embrittlement in the last decade. In particular, the fatigue behaviour in hydrogen environments tops the research list. This equally proves that fatigue cracking in hydrogen environment seriously affects the progress of engineering in practical engineering applications. Included in Figure 5a, it can also be seen that researchers have also explored a lot about the prediction of fatigue life of materials in hydrogen environment. In order to further explore the trend of research directions in recent years, keyword clustering on these papers was performed. This helps researchers to catch the hotspots of research. When evaluating the effectiveness of clustering [87], Q and S are two commonly used metrics that provide an assessment of different aspects of the quality of the clustering results. The Q value is usually used to assess the tightness within the clusters or the quality of the clusters, which measures the degree of similarity between the samples within the same cluster, while the S value is used to assess the dissimilarity of the clusters from the overall data, i.e., how much the samples within the clusters are separated from the overall data. Ideally, one would like to have a high Q value and a high S value, which indicates that the samples within the clusters in the clustering result are sufficiently similar and there is less dissimilarity between the clusters and the data as a whole, which helps to explain the structure of the data and extract useful information. The clustering result of the localisation is Q = 0.6578 and S = 0.879. This shows that the clustering structure is relatively clear, and the internal consistency of the clusters is high, proving the reliability of the keyword grouping. In Figure 5b, the literature keyword timeline spectrogram by using CiteSpace software is presented.