# A Comprehensive Literature Review on Hydrogen Tanks: Storage, Safety, and Structural Integrity
## 5 Fundamental Issues
### Safety and Risks
#### 5.3.3 Embrittlement  
The issue of hydrogen embrittlement presents a significant challenge in the development and maintenance of hydrogen storage tanks [8; 140]. Hydrogen embrittlement refers to the process by which metals become brittle and fracture due to the permeation and diffusion of hydrogen atoms into the material's structure [23; 193]. This phenomenon is particularly critical for high-pressure hydrogen storage systems used in various applications, including FCEVs and industrial storage. Hydrogen embrittlement can compromise the mechanical integrity of storage tanks, leading to catastrophic failures and posing severe safety risks [13]. Recent studies focus on understanding hydrogen embrittlement mechanisms, developing resistant materials, and implementing design strategies to mitigate its effects. For Type IV hydrogen tanks, a steel boss is used for valve mounting daily, and the hydrogen embrittlement process significantly impacts the steel's performance. [PERSON] et al. [62] focused on the threaded part of 316L stainless steel bosses, essential for fastening valves in 70 MPa compressed hydrogen tanks. They used an analytical approach to understand crack propagation with and without the influence of hydrogen embrittlement. A crack propagation model can be written as below, considering both hydrogen diffusion and \(f_{critical}\)[194]:  
\[\left(\frac{da}{dN}\right)_{\text{total}}=\left(\frac{da}{dN}\right)_{\text{ airfatigue}}+\left(\frac{f_{\text{critical}}}{f}\right)^{\gamma}\left(\frac{da}{dN} \right)^{\text{max}}_{\text{HAC}} \tag{4}\]  
In Equation (4), (da/dN)\({}_{\text{airfatigue}}\) is the Fatigue Crack Growth (FCG) in the presence of air, while (da/dN)\({}_{\text{HAC}}\) is the FCG due to hydrogen assisted cracking. For the thickness of the polar boss (h\({}_{0}\)), two failure conditions have been introduced [62]: the first due to bending moment (Equation (5)), and the second due to transverse shear stress (Equation (6)):  
\[h_{0}=h(r=r_{0})\geq 2.3\,r_{f}\sqrt{\frac{P}{\sigma_{y}}\,m_{r}^{o}(r_{f})} \tag{5}\]  
and  
\[h_{0}\geq\frac{3 Pr_{o}}{4\tau_{y}} \tag{6}\]  
where \(P\) is the internal pressure load, \(\sigma_{y}\) is the material strength, \(r_{f}\) is the largest diameter polar boss, \(m_{r}^{o}\) is the normalized bending moment. The expression for the axial force acting on the thread is  
\[F_{\text{axial}}=\frac{\pi p_{w}d_{t}^{2}}{4} \tag{7}\]  
where \(p_{w}\) is the working pressure and \(d_{t}\) is the maximum thread diameter. The shear stresses balance the axial force. It is assumed to be uniformly distributed over the threaded surface of area \(\pi\times d_{t}\times t\), and the shear force acting is  
\[F_{\text{thread}}=\pi d_{t}t\sigma_{u} \tag{8}\]  
where \(\sigma_{u}\) is the ultimate shear strength and \(t\) is the thread engagement length. Giving  
\[F_{\text{thread}}=s_{f}F_{\text{axial}} \tag{9}\]and putting Equations (7) and (8) into Equation (9), the expression for \(t\) is obtained [62]:  
\[t=\frac{p_{w}d_{i}s_{f}}{4\sigma_{u}} \tag{10}\]  
After evaluating the combined stress at the thread and the stress intensity factor, the hydrogen embrittlement model has been applied, particularly the HEDE model [20]. We refer to [62] for compactness and any other detailed information. Their results revealed that critical crack length, the threshold at which a crack becomes unstable, increases with the number of loading cycles and, when accounting for hydrogen embrittlement, the critical crack length was 2% to 4% higher, underscoring the increased susceptibility of the material to crack under high-pressure hydrogen conditions. Therefore, considering the effects of hydrogen embrittlement is crucial in designing and maintaining high-pressure hydrogen tanks to ensure their safety and durability. On this line, especially for high-pressure hydrogen tanks for FCEVs, it is essential to explore metallic materials with lightweight properties, high strength, and excellent resistance to hydrogen embrittlement. In the same context, [PERSON] et al.