## 3 Results
### Comparison, Initial Condition and Agged  
SITs and CATs performed in distilled water were used to compare the initial condition with the aged condition of AISI 347 defined previously in Chapter 2. Figure 4a (same as Figure 3b) shows results from an SIT for a specimen under fatigue loading for AISI 347 in the initial condition showing the development over the time for the correlated total strain amplitude \(\varepsilon_{a,t,cov}\) (yellow), the stress amplitude \(\sigma_{a}\) (green), the open-circuit potential \(E_{OCP}\) (red) and the magnetic field strength \(\Delta M_{tang}\) (blue), respectively, all up to specimen failure at \(\varepsilon_{a,t,cov}\) = 0.95 \(\times\) 10\({}^{-2}\). Here, \(\sigma_{a}\) increases stepwise up to \(\varepsilon_{a,t,cov}\) = 0.35 \(\times\) 10\({}^{-2}\). At \(\varepsilon_{a,t,cov}\) = 0.40 \(\times\) 10\({}^{-2}\) onwards, a large increase in \(\sigma_{a}\) can be seen until immediately before specimen failure. Afterward, there is a large decrease in stiffness at \(\varepsilon_{a,t,cov}\) = 0.95 \(\times\) 10\({}^{-2}\), which is accompanied by a decrease in \(\sigma_{a}\). From \(\varepsilon_{a,t,cov}\) = 0.40 \(\times\) 10\({}^{-2}\) onwards, a stepwise decrease of the \(E_{OCP}\) signal can be seen inverse to \(\sigma_{a}\), which decreases more significantly from \(\varepsilon_{a,t,cov}\) = 0.85 \(\times\) 10\({}^{-2}\) onwards. \(\Delta M_{tang}\) also decreases stepwise with increased \(\varepsilon_{a,t,cov}\), although the decrease intensifies from \(\varepsilon_{a,t,cov}\) = 0.35 \(\times\) 10\({}^{-2}\). In the range 0.40 \(\times\) 10\({}^{-2}\) < \(\varepsilon_{a,t,cov}\) < 0.70 \(\times\) 10\({}^{-2}\), a strong increase occurs within the load steps, which  
Figure 3: Results obtained from strain increase tests in (**a**) in air and (**b**) distilled water for specimens of the initial condition of AISI 347 steel. can probably be explained by the martensitic phase transformation. The specimen failure at \(\varepsilon_{a,t,cor}\) = 0.95 \(\times\) 10\({}^{-2}\) is reliable, indicated by a sudden decrease of the \(\Delta M_{tang}\) signal. Figure 4b shows the SIT for the aged specimen. Comparatively, it can be observed that the aged specimen fails four load steps below the one in the initial condition (Figure 4a) at \(\varepsilon_{a,t,cor}\) = 0.95 \(\times\) 10\({}^{-2}\). The slopes of \(\sigma_{a}\), \(\Delta M_{tang}\) and \(E_{OCP}\) are comparable to the initial condition up to a step of \(\varepsilon_{a,t,cor}\) = 0.25 \(\times\) 10\({}^{-2}\). Thereafter, the slopes are significantly different from the \(E_{OCP}\). The \(E_{OCP}\) signal of the aged specimen decreases considerably in the range 0.25 \(\times\) 10\({}^{-2}\) < \(\varepsilon_{a,t,cor}\) \(\leq\) 0.40 \(\times\) 10\({}^{-2}\) within the steps. In the following three steps, the \(E_{OCP}\) signal remains approx. constant in the step and again decreases continuously from \(\varepsilon_{a,t,cor}\) = 0.60 \(\times\) 10\({}^{-2}\) until specimen failure. The increase of \(\sigma_{a}\) in the first three load steps of the two SITs is due to a pure elastic behavior, which is reflected by the linear stress-strain relationship. From \(\varepsilon_{a,t,cor}\) = 0.40 \(\times\) 10\({}^{-2}\) for the specimen in the initial condition or from \(\varepsilon_{a,t,cor}\) = 0.45 \(\times\) 10\({}^{-2}\) for the aged specimen, the relative increase in \(\sigma_{a}\) can be explained by cyclic hardening effects typical for that material under cyclic loading. The decrease in \(\sigma_{a}\) in the last load step is due to progressive macro crack propagation in both the initial and aged conditions. Prior the fatigue tests, the specimens were electrochemically stabilized in a load-free period at the beginning of the test, such that there were no active surfaces in the medium-material interface zone, as can be read from the constant \(E_{OCP}\) slope.