## 2 Hydrogen-Metal Interaction and Fatigue Mechanisms
### Summarising Experimental Approaches  
Experimental methods for hydrogen embrittlement play an important role in providing insights into hydrogen embrittlement phenomena, guiding material design and optimisation, validating theoretical models, and providing guidance for applications [8]. Through these methods, the degree of material embrittlement in hydrogen-containing environments can be quantitatively assessed, the formation mechanism of hydrogen embrittlement can be revealed, material design and optimisation can be informed, the accuracy of theoretical models can be verified, and engineering design and material selection can be guided. Therefore, the experimental method of hydrogen embrittlement is of great significance in solving the hydrogen embrittlement problem. Now the current hydrogen embrittlement experiments are divided into five categories. Figure 4 clearly shows four classical hydrogen embrittlement experiments, which are briefly described. The hydrogen embrittlement loading test, as shown in Figure 4a [74; 75; 76], is designed to assess the embrittlement behaviour of metals in a hydrogen environment. After a metal sample is exposed to a hydrogen-containing environment, the fracture behaviour and changes in mechanical properties of the metal are observed by applying loads such as tension, bending or compression to determine the effect of hydrogen on the properties of the metal. It is generally used to assess the risk of hydrogen embrittlement that a metal may face under actual working conditions and to guide material selection and engineering design. The general steps are to prepare a metal sample, expose it to a hydrogen-containing environment, apply loading and record mechanical property data, and finally analyse the fracture surface. Hydrogen diffusion testing, as shown in Figure 4b [77; 78; 79; 80], is designed to study the diffusion behaviour of hydrogen in metals, including diffusion rates, diffusion paths and hydrogen aggregation behaviour. It is generally used to understand the mechanism of hydrogen diffusion in metals, to predict the risk of hydrogen embrittlement, and to develop preventive strategies. The general procedure is to introduce hydrogen into a metal sample using techniques such as electrochemical hydrogen charging or hydrogen permeability testing, and to assess the diffusion behaviour of hydrogen by monitoring the hydrogen concentration distribution or permeation rate. Hydrogen embrittlement mechanism research [79; 81; 82] is conducted through theoretical simulation and computational chemistry methods, to study the interaction between hydrogen and metal and hydrogen embrittlement mechanism, in order to deeply understand the nature of hydrogen embrittlement phenomenon. Figure 4c schematically summarises this transition: at low hydrogen concentrations and moderate stresses, the dominant mechanism is hydrogen-enhanced local plasticity (HELP/HESIV), whereas at higher concentrations or elevated stresses, it shifts to hydrogen-enhanced decohesion (HEDE/HESIV) [61; 83]. Generally, this technique is used to provide theoretical guidance for engineering practice, to guide the material design and processing technology. The general steps are model building, calculations and analyses, verification of the accuracy of the model, and interpretation and extrapolation of the results. Stress Corrosion Cracking (SCC) Test [18; 84]: simulates the occurrence of hydrogen embrittlement of metals in stressful and corrosive environments and evaluates the stress corrosion susceptibility of metals to hydrogen. It is generally used to assess the durability and safety of metals in environments with corrosive and loaded effects. The key test variables, which includes electrochemical potential, load ratio, solution pH, and dissolved-hydrogen activity, are listed in Figure 4d [85], providing a practical envelope for designing SCC experiments consistent with service conditions.