PSI - Issue 42

Kai Donnerbauer et al. / Procedia Structural Integrity 42 (2022) 738–744 Kai Donnerbauer / Structural Integrity Procedia 00 (2019) 000 – 000

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applied to the specimen shafts and allows to track changes of open circuit potential due to microstructural changes during fatigue loading. A carbon electrode was used as counter electrode and a silver chloride electrode as reference electrode.

Fig. 2. Instrumented experimental setups for (a) tests in air; (b) tests in distilled water.

3. Results from fatigue tests and StrainLife Fig. 3 (a) shows the results from a strain-increase test in distilled water. Step length was 1800 s and step height Δε a,c = 0.05·10 -2 respectively. At strain amplitudes ε a,c of 0.3·10 -2 and lower the step size was reduced to 0.025·10 -2 with the intent of generating more data in mostly elastic loading parts. The correlated total strain amplitude ε a,c is plotted in black, stress amplitude σ a in green, Hall voltage HV in blue and open circuit potential E OCP in red. Since the E OCP follows the triangular load function of the controlled total strain amplitude of a cycle quite well it is possible to determine an E OCP amplitude E OCP,a plotted in yellow. Data processing was done in MatLab using lower and upper envelope calculations from the original signal. The resulting value for a strain amplitude of 0.6·10 -2 is shown graphically in Fig. 3 (b). E OCP signals from steps with strain amplitudes 0.4·10 -2 shown in red and 0.2·10 -2 in blue respectively are also plotted. During SIT the material undergoes pronounced cyclic strain hardening upwards from strain amplitudes of 0.3·10 -2 . This is well recognizable from the stress amplitude increasing inside a step but also in the Hall voltage signal. Hall voltage provides a more pronounced signal with increasing ferromagnetic phase volume. Therefore, stress amplitude and Hall voltage are correlating well, since martensitic transformation leads to cyclic hardening. The absolute signal from open circuit potential is well suited to indicate crack growth in a stainless steel. As the crack grows, new free surfaces without a passive layer are getting in touch with the electrolyte leading to a sharp drop of the potential at the end of the test. The onset of the drop is even earlier than the drop in stress amplitude due to crack growth resulting in a smaller effective cross-section. The E OCP,a correlates well with σ a because it is following the triangular loading function. It is especially interesting for steps around strain amplitudes of 0.4·10 -2 , where a lot of cyclic hardening occurs. Here, the amplitude decreases slowly during a step, indicating a relation between the electrochemical measurement and changes in the material’s microstructure. Only surface related effects can be measured, because electrolyte contact is necessary. Therefore, slip bands, intrusions and extrusions might be the reason for this behavior.