## 3 Results
### Constant Amplitude Tests Performed under Reactor Pressure Vessel Boiling Water Conditions  
To investigate the influence of application-relevant BWR conditions (c.f. Chapter 2), various fatigue tests were carried out in a respective environment. In this context, the course of the stress amplitude and the open-circuit potential were used to characterize the fatigue behavior. Figure 6 provides the results from two CATs performed under BWR conditions with total strain amplitudes of \(0.3\times 10^{-2}\) and \(0.55\times 10^{-2}\), respectively. Since with the CATs under BWR conditions, only a measurement of the specimen elongation at the specimen shoulders outside the autoclave is possible, a correlation between the elongation measured directly on the cylindrical gauge length of the specimen and the elongation determined at the specimen shoulders must be made on the basis of the fatigue tests in air as it was already described before. For this purpose, a strain measurement has been developed that makes it possible to perform tests after measuring the total strain amplitude at the gauge length of the specimen while observing symmetrical elongation amplitudes related to the measurement of two inductive displacement transducers at the specimen shoulders. After each loading cycle, the resulting strain amplitude is automatically evaluated online by the control system, and, if necessary, the elongation amplitude is increased or reduced in steps to subsequently obtain the desired total strain amplitude. By doing so, correlation curves were determined, which reflect the specimen elongations to be applied to the specimen shoulders in order to achieve constant strain amplitudes of the test length as a function of the number of cycles. These curves must be redetermined for all tests with different parameters via a best-fit correlation.  
Figure 5: Constant amplitude tests in distilled water for (**a**) the initial condition and (**b**) the aged condition of AISI 347 steel.  
Within the CAT with \(\varepsilon_{a,t}\) = 0.3 \(\times\) 10\({}^{-2}\) (Figure 6a) it can be seen that the course of the stress amplitude remains almost constant from the beginning up to about 100 cycles and slightly decreases in the following, which is due to a cyclic softening of the material. Subsequently, saturation occurs, followed by a strong softening at specimen failure, indicated by a drop of the stress amplitude. With regard to the \(E_{OCP}\) signal, this remains constant at a value of approx. 0 for the first 1000 cycles, after which there is a continuous increase until specimen failure. As it was already explained before, the signal changes in the open-circuit potential being dedicated to surface defects caused by the fatigue process.  
Since the initiation of damage in the high cycle fatigue (HCF) regime almost certainly starts from the surface, the information obtained from this measurement technique is essential for the design of the components used in the field of nuclear energy. The exposure of new surfaces first leads to an increase in the corrosion current, with the open-circuit potential dropping locally. The subsequent re-passivation processes then lead to a respective increase in the open-circuit potential, which can be seen in Figure 6a.  
In Figure 6b, the results from a CAT with a total strain amplitude of \(\varepsilon_{a,t}\) = 0.55 \(\times\) 10\({}^{-2}\) are shown. Due to the higher total strain amplitude when compared to the results in Figure 6a, cyclic deformation behavior changes to a continuous cyclic hardening behavior over the entire lifetime indicated by a continuous increase in the stress amplitude. The \(E_{OCP}\) signal also shows a behavior comparable to the CAT with the lower total strain amplitude, changing by about 55% from its initial value to specimen failure.