## 3 Fatigue Crack Growth in Hydrogen Environment
### Factors Affecting Fatigue Crack Growth  
The effect of hydrogen environment on fatigue crack growth is influenced by a combination of factors. Over the course of many years of research, researchers have identified many factors that affect fatigue crack growth in hydrogen environments. Some of the major factors that have been explored by researchers are listed below [41; 93; 94; 95; 96; 97; 98; 99; 100]: Firstly, hydrogen concentration is one of the most important factors affecting fatigue crack growth. According to the research by [PERSON] et al. [101], it is concluded that the higher the hydrogen concentration, the faster the crack propagation. This is because it increases the opportunities for hydrogen-metal interactions. Stress level is another important factor. Higher stress levels promote the diffusion and aggregation of hydrogen and, as mentioned earlier, hydrogen atoms migrate and aggregate at high stresses, leading to faster crack growth. In actual working conditions, the temperature will vary depending on where the material is installed and the environment in which it is used. Temperature also affects the rate of hydrogen diffusion and the plasticity of the metal. Usually, the higher the temperature, the greater the entropy contained in the hydrogen atom, the faster the activity, and the faster the crack expansion rate, so the high temperature will promote the diffusion of hydrogen atoms and the softening of the metal. In addition to this, we still need to consider that different types of metallic materials have different sensitivities to hydrogen. Certain metallic materials may be more sensitive to hydrogen, resulting in faster crack propagation. There is evidence that different materials are not equally sensitive to hydrogen. In actual working conditions, if the metal is exposed to an environment containing corrosive substances, the corrosion effect will work together with the stress to accelerate the crack growth process. This is similar to the SCC effect mentioned earlier, where hydrogen atoms will act as a catalyst-like agent, further affecting the fatigue life of the material. At the same time, according to the research by [PERSON] et al. [101], changes in loading frequency can also affect the diffusion and concentration of hydrogen in the metal, thereby influencingthe crack propagation rate. A lower loading frequency allows more time for hydrogen to diffuse and accumulate at the crack tip, thereby accelerating crack growth, whereas at higher frequencies the limited diffusion time mitigates hydrogen-assisted damage and the crack-growth behaviour approaches that observed in inert environments. Finally, the microstructure of the metal also plays an important role, e.g., grain size and grain boundary properties, which also affect the behaviour of hydrogen diffusion and crack growth. As mentioned in the previous section on hydrogen embrittlement mechanisms, hydrogen atoms can travel through and accumulate at weak grain boundaries to form internal defects in the metal, which in turn affects the fatigue performance of the material.  
A review of several studies [88; 101; 102; 103; 104; 105; 106; 107] reveals that the factors affecting fatigue crack growth in hydrogen environments are complex and must be considered comprehensively to fully understand the mechanisms of crack propagation and to develop appropriate preventive measures. However, there is still a lack of fundamental understanding of the factors influencing hydrogen embrittlement. A more in-depth exploration of these factors is needed.