## Materials and Methods  
### Determination of Stress Corrosion Index though Constant Stress-Rate Tests  
Material strength increases with an increased stress rate generally. This principle is used in the constant stress-rate test to relate the fracture strength, and the stress rate applied.  
Under a uniform remote stress, the generalised expression of the stress intensity factor is  
\[K=Y\sigma\sqrt{a} \tag{3}\]  
where \(Y\) is a geometric factor associated with the crack geometry, \(\sigma\) is the stress applied, and \(a\) is the length of the crack.  
The crack velocity, \(v\), is given by  
\[v=\frac{da}{dt}=\frac{da}{d\sigma}\frac{d\alpha}{dt}=\frac{da}{d\sigma}\dot{\sigma} \tag{4}\]  
where \(\dot{\sigma}\) represents the stress rate.  
Rearranging Equation (4) yields  
\[d\sigma=\frac{\dot{\sigma}}{v}da \tag{5}\]By substituting Equation (2) in Equation (5), we obtain  
\[d\sigma=\frac{\dot{\sigma}}{A\left(\frac{K}{K_{\mathrm{C}}}\right)^{n}}da \tag{6}\]  
Substituting Equation (3) in Equation (6) yields  
\[d\sigma=\frac{\dot{\sigma}}{A\left(\frac{Y_{\sigma}\sqrt{a}}{K_{\mathrm{C}}} \right)^{n}}da=\frac{\dot{\sigma}K_{\mathrm{C}}^{n}}{AY^{n}}\frac{1}{\sigma^{n }}a^{-n/2}da \tag{7}\]  
Rearranging Equation (7) and integrating it yields  
\[\frac{1}{n+1}(\sigma_{f}^{n+1}-\sigma_{o}^{n+1})=\frac{2K_{\mathrm{C}}^{n}}{(n -2)AY^{n}}\dot{\sigma}a_{i}^{\frac{2-n}{2}}\left(1-\frac{1}{\left(\frac{a_{f} }{a_{i}}\right)^{\frac{n-2}{2}}}\right) \tag{8}\]  
where \(\sigma_{o}\) represents the initial stress, \(\sigma_{f}\) represents the fracture stress in a corrosive environment, \(a_{i}\) represents the initial crack length, and \(a_{f}\) represents the final crack length.  
Initial stress is assumed to have zero and that \((a_{f}a_{i})^{(n-2)/2}\) quickly becomes significantly greater than unity, since n is usually a large number. Then, Equation (8) can be reduced to  
\[\sigma_{f}^{n+1}=\frac{2(n+1)K_{\mathrm{C}}^{n}\dot{\sigma}}{(n-2)AY^{n}a_{i} ^{\frac{n-2}{2}}} \tag{9}\]  
The fracture toughness in an inert environment can be expressed as  
\[K_{\mathrm{C}}=Y\sigma_{i}\sqrt{a_{i}} \tag{10}\]  
where \(\sigma_{i}\) represents the inert strength in an inert state in which there is no subcritical crack growth.  
By rearranging Equation (10) with respect to the crack length, we obtain  
\[a_{i}=\left(\frac{K_{\mathrm{C}}}{\sigma_{i}Y}\right)^{2};\sigma_{i}^{\frac{n -2}{2}}=\frac{K_{\mathrm{C}}^{n-2}}{\sigma_{i}^{n-2}Y^{n-2}} \tag{11}\]  
Substituting Equation (11) in Equation (9) yields  
\[\sigma_{f}^{n+1}=\frac{2(n+1)K_{\mathrm{C}}^{2}}{(n-2)AY^{2}}\sigma_{i}^{n-2} \dot{\sigma} \tag{12}\]  
Then, taking the log on both sides of Equation (12) gives  
\[\log\sigma_{f}=\frac{1}{n+1}\log\dot{\sigma}+\log C \tag{13}\]  
where  
\[\log C=\frac{1}{n+1}\log\left(\frac{2K_{\mathrm{C}}^{2}(n+1)}{AY^{2}(n-2)} \sigma_{i}^{n-2}\right) \tag{14}\]  
The stress corrosion index \(n\) is determined on the basis of the slope (\(\alpha\)) of the linear regression using all the log fracture strength values compared with the log stress rate results. The stress corrosion index (\(n\)) is defined  
\[n=\frac{1}{\alpha}-1 \tag{15}\]This research used four specific loading rates of 10\({}^{-2}\), 10\({}^{-1}\), 10\({}^{0}\), and 10 MPa/s. Seven to eight specimens were tested at each loading rate and 28 to 32 specimens in total were utilized in each test procedure.