## 3 Results
### StrainLife  
StrainLife is a STEP for the fast and cost-effective evaluation of S-N curves, which is considered here to be adapted and validated to nuclear engineering applications. The value of this method is, as mentioned before that only one STF and two total strain-controlled CATs are required for the calculation of a complete total strain S-N curve. The input variables for the StrainLife method include the measurements of \(\sigma_{a}\), \(\varepsilon_{a,p}\), \(\Delta T\), \(\Delta R\), \(E_{OCP}\) and \(\Delta M_{tang}\), respectively, with their development over the fatigue life to be considered as an additional source of information, as shown for the different experimental results presented before. In the following, the procedure is described for the calculation based on electrical resistance values. Figure 6: Total strain-controlled constant amplitude tests performed in BWR conditions with the courses of the stress amplitude and the open-circuit potential for (**a**) a total strain amplitude of 0.3 \(\times\) 10\({}^{-2}\) and (**b**) a total strain amplitude of 0.55 \(\times\) 10\({}^{-2}\) for the initial condition of AISI 347 steel. Equation (1) is a relationship by [PERSON] [27], which is commonly used to describe the stress amplitude (load variable) and to be a function of plastic strain amplitude (material response). \[\sigma_{a}=K^{\prime}\times(\varepsilon_{a,p})^{n^{\prime}} \tag{1}\]  
By using the load quantity \(L\) and the material response \(M\), Equation (1) can now be described as follows. \[M={K^{\prime}}_{M}\times(L)^{n_{M^{\prime}}} \tag{2}\]  
Here, the coefficient \({K^{\prime}}_{M}\) and the exponent \(n_{M^{\prime}}\) are additionally marked with the index \(M\) to express that this formula is used for a quantity other than the mechanical plastic strain amplitude. Those relationships provide the basis to determine S-N curves may be derived from an SIT, where the load quantity could be considered to be a total strain amplitude \(\varepsilon_{a,t}\) and the material response electrical resistance \(\Delta R\) written then as  
\[\Delta R={K^{\prime}}_{\Delta R}\times(\varepsilon_{a,t})^{n_{\Delta R^{ \prime}}} \tag{3}\]  
The next step is to calculate the average values of the change in electrical resistance of each load level of the SIT and then plot the total strain amplitude as a function of these values (see Figure 7a). In addition, two CATs are performed, which are also added to the same plot. In the respective example, the change in electrical resistance at the same total strain amplitude in the LIT and CAT may not be identical, which can be attributed to a difference in load-time history between LIT and CAT, resulting in different material conditions with respect to damage accumulated up to the respective point, as well as cyclic strain hardening (Figure 7b). To correct this a ratio, \(Q\), between the total strain amplitude of the CAT and the SIT is calculated according to Equation (4). \[Q(\varepsilon_{a,t,1/2})=\frac{\Delta R_{1/2}(\text{CAT})}{\Delta R_{1/2}( \text{SIT})} \tag{4}\]  
Using the calculated corrective values, \(Q\), for both total strain amplitude levels applied in the CATs, a fitting curve for further extrapolation of the data is calculated, where an exponential fit has been chosen. By using this relationship, the stress-strain behavior of the SIT is transferred to the stress-strain behavior for strain-controlled CATs (see Figure 8). Figure 7: (**a**) Morrow curve for the SIT and the values of the change in electrical resistance \(\Delta R\) at \(N_{f}/2\) of the two CATs in distilled water for the initial condition and (**b**) the CATs in distilled water for the initial condition of AISI 347 steel. If electrical resistance is considered to be the relevant material response \(M\), then this may need to be differentiated between a predominantly elastic and predominantly plastic strain region when plotting the relationship between the material response \(M\) and the load variable \(L\).