## 5 Prevention of Fatigue Characteristic Degradation in Hydrogen Environment
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Categories** & **Applicability** & **Complexity** & **Advantages** & **Limitations** \\ \hline \multirow{3}{*}{Empirical models} & Perform a quick assessment of steel with a few known parameters. & Fitting parameters to experimental data are required. & Simple, low cost, easy to implement in design standard. & Valid only within tested parameter space. \\ \hline \multirow{3}{*}{Physical models} & Situations where micro-mechanisms (HELP/HEDE) must be captured. & Needs microstructural parameters and crack-tip stress analysis. & Offers mechanistic insight. & Microscopic parameters are difficult to obtain. \\ \hline \multirow{3}{*}{Numerical simulations} & Simulation of hydrogen-coupled crack growth in arbitrary structures or components. & Dedicated software and discretization of the finite element mesh are required. & High accuracy, Capable of capturing detailed crack path evolution. & The computational cost for computers is high. \\ \hline \multirow{2}{*}{
\begin{tabular}{c} Machine learning methods \\ \end{tabular} } & Fast and high-throughput prediction & Requires large datasets. & Once the model is trained, fast predictions can be achieved. & More sensitive to training data and less interpretable. \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparative summary of fatigue life modelling and prediction for hydrogen environments. susceptible to hydrogen embrittlement than mild steels, underscoring the importance of careful material selection. Proper alloy design [114] can enhance hydrogen embrittlement resistance by adjusting alloy composition and microstructure, which helps to slow the rate of fatigue crack propagation. For instance, high-strength, corrosion-resistant alloys or composites may be chosen and optimised for enhanced hydrogen resistance. Surface treatments also play a vital role. Effective surface treatments can slow hydrogen diffusion and aggregation within the material, improving resistance to hydrogen embrittlement and extending fatigue life. Techniques such as surface coatings, electrochemical treatments, or specialised coatings reduce hydrogen diffusion and adsorption, thereby minimising hydrogen-induced fatigue degradation. In addition, efforts have been made to control hydrogen diffusion and adsorption [115; 116; 117]. Controlling these processes can reduce hydrogen accumulation and damage within the material, achieved by altering the microstructure, optimising hydrogen loading conditions, or adjusting the hydrogen concentration in the environment. If hydrogen atoms are already present within the material, hydrogen removal techniques, such as heat treatment, electrochemical methods, or gas-phase treatment, can be applied during the manufacturing process. These techniques reduce hydrogen content in metals, mitigating hydrogen's impact on mechanical properties and slowing fatigue crack growth. Stress control and design improvements [118; 119; 120] also prove effective. Sound structural design can reduce stress levels in hydrogen environments, slowing fatigue property degradation. Methods may include reducing stress concentration areas [121; 122], optimising structural shape [123; 124], or altering loading methods to minimise embrittlement risks [125; 126]. Finally, regular monitoring and maintenance are crucial for timely detection and management of hydrogen-induced damage. Non-destructive testing, surface inspection, and real-time monitoring techniques can help identify damage early, enabling necessary maintenance. Additionally, environmental control during usage--adjusting parameters such as temperature, pressure, and humidity--can further mitigate hydrogen's effects on material properties, extending the material's lifespan. Protective measures, working environment adjustments, or process optimizations may all contribute to this goal.