## 1 Introduction  
Hydrogen has become one of the most popular energy carriers these years. Its high-energy storage and low pollution stand out from various energy carriers. After observing hydrogen embrittlement explosions during hydrogen transport, researchers have turned their research towards hydrogen embrittlement. Hydrogen embrittlement is an important problem that has long plagued the field of metallic materials engineering and refers to the overall degradation of the mechanical properties of metals in a hydrogen environment [1, 2, 3]. Although most metallic materials exhibit excellent toughness and plasticity at ambient temperatures and pressures, once exposed to hydrogen-enriched environments, their mechanical properties and durability tend to deteriorate drastically and even undergo severe brittle damage. The study of hydrogen embrittlement involves not only the field of materials science and engineering, but also chemistry, physics, and other disciplines, and has attracted much attention because of its wide range of applications and far-reaching impact [4; 5; 6]. In the past decades, scientists have conducted extensive research on hydrogen embrittlement and achieved some important results [7; 8; 9; 10]. However, the mechanism of hydrogen embrittlement is extremely complex and still not fully understood, and there are some limitations in the methods of preventing and mitigating hydrogen embrittlement. The classical hydrogen embrittlement mechanism also fails to fully express the hydrogen embrittlement phenomenon. Therefore, in-depth understanding of the mechanisms, influencing factors, and preventive measures of hydrogen embrittlement is of great significance for improving the reliability and durability of metallic materials. By revealing the intrinsic mechanism of hydrogen embrittlement, we can design new materials that are more hydrogen-resistant and take targeted measures to mitigate the effects of hydrogen embrittlement [11; 12; 13]. Meanwhile, by exploring new testing methods and evaluation techniques, we can more accurately assess the performance of materials in hydrogen environments and identify potential safety hazards in a timely manner. Current research considers hydrogen embrittlement to involve several aspects, such as the interaction between hydrogen atoms and metal atoms, changes in lattice structure, and crack formation and extension. Hydrogen atoms enter the metal through adsorption, diffusion, and absorption and accumulate at defects such as grain boundaries, dislocations, or internal cavities, leading to an increase in material embrittlement [14; 15; 16; 17]. The most noteworthy of the many hydrogen embrittlement phenomena is that the increase in brittleness not only manifests itself in the fracture strength and toughness of the material, but may also lead to a sharp decrease in the fatigue life of the material. Hydrogen embrittlement fatigue is also a material degradation phenomenon involving the fracture and damage of metals or alloys due to fatigue loading in the presence of hydrogen. This phenomenon has a significant impact on the safety and endurance of engineering structures, especially in the oil and gas, aerospace, and nuclear industries [18; 19]. In recent years, scientists have developed numerous prediction models in order to predict the fatigue life of materials in hydrogen environment, such as empirical models [20; 21; 22; 23; 24; 25], physical models [26; 27; 28; 29; 30; 31], machine learning models [32; 33], and finite element models [34; 35; 36; 37; 38; 39]. These models have solved the problem of fatigue life prediction for some materials to a certain extent, but the hydrogen embrittlement mechanism is still not uniformly explained, which leads to a certain degree of failure to reach a consensus among different models. In order to further investigate the internal causes of hydrogen embrittlement, scientists have summarised the influencing factors of fatigue crack growth in hydrogen environment.