## 6 Challenges and Future Directions  
### Limitations and Challenges  
The study of the multiple challenges and difficulties of fatigue crack growth in metals in hydrogen embrittlement environments is of great significance in improving the safety, reliability, and durability of engineering structures. This is not only related to the safety of metallic materials in engineering applications, but also has a direct role in guiding the design of more durable materials and the development of suitable engineering applications, as well as promoting scientific research and technological innovation in related fields.  
Some challenges are summarised in the section.  
Firstly, the mechanism of influence in this field has not been fully revealed, and there are knowledge gaps and deficiencies. There is still a lack of comprehensive understanding of the specific mechanisms of fatigue crack growth of metals in hydrogen embrittlement environments, which hinders our in-depth understanding and effective control of the phenomenon. As mentioned earlier, although there are now several recognised mechanisms for hydrogen embrittlement, such as the HEDE, AIDE, HELP, etc., these mechanisms are not contradictory to each other, but only the direction and angle of the consideration is different. People still need to combine them to form a mechanism more in line with human cognition.  
Secondly, the complexity of modelling and controlling experimental conditions is also a major challenge. This involves a combination of multiple factors, such as the effects of pressure, temperature, hydrogen concentration, and other environmental factors. The interaction of these factors increases the difficulty of the study and requires careful design and strict control of the experimental conditions in order to obtain reliable and accurate data. Section 3 provides a systematic description of the factors affecting fatigue crack growth in hydrogen environments. It can be seen that different factors have different effects on the final fatigue crack growth rate, and considering these factors more completely is crucial for future research. In addition, long-term performance and stability studies require significant time and resources. In order to validate and assess the fatigue crack growth behaviour of metals in hydrogen embrittlement environments, continuous monitoring and validation is required to ensure the reliability and reproducibility of experimental results. One can only deal with problems quickly once complete surveillance has been carried out. The complexity of data acquisition and analysis is also an issue that should not be underestimated. An in-depth understanding of the mechanisms of fatigue crack growth in metallic materials in hydrogen embrittlement environments requires a large amount of experimental data and advanced data analysis techniques, which take a lot of time and effort to acquire and process.  
Finally, effectively predicting and managing the fatigue performance of metallic materials in hydrogen embrittlement environments is equally challenging. This requires close integration of theoretical research with engineering practice to develop reliable and practical prediction models and management strategies to cope with complex and variable environmental conditions and material properties.  
In summary, the study of fatigue crack growth of metals in hydrogen embrittlement environments faces many complex challenges and requires interdisciplinary co-operation and the integrated use of advanced technological tools and methodologies in order to deepen the understanding of the laws and mechanisms and to provide better support and guidance for scientific research and engineering applications in related fields.