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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Fig. 3. (a) Evolution of NDT signals during SIT; (b) Graphical explanation of open circuit potential amplitude E OCP,a .

In Fig. 4 results from two CATs are presented, where the same NDT methods were applied as in the SIT described above. Hall voltage and stress amplitudes are relating well for both correlated strain amplitudes ε a,c = 0.4·10 -2 , Fig. 4 (a) and 0.8·10 -2 , Fig. 4 (b). At the higher strain amplitude the magnetic method demonstrates a stronger and earlier change in the signal due to a faster increase of the ferromagnetic phase volume. Absolute signal of E OCP is again well suited to indicate macrocrack growth at the end of test. As in the SIT the signal decreases even before the stress amplitude does. At the beginning of both tests the E OCP,a decreases sharply and then slowly almost linearly drops until macro crack growth sets on. The first part is probably related to the pronounced strain hardening behavior while the second part is more likely explained by microstructural surface effects and possibly a change of chemical composition of passive layer as a result of an increasing martensite phase content, as also reported for AISI 304 by Jinlong and Hongyun (2013).

Fig. 4. Evolution of NDT signals during CATs with strain amplitudes of (a) 0.4·10 -2 ; (b) 0.8·10 -2 .

From the results presented in Fig. 3 and Fig. 4 it is possible to evaluate a total strain S-N curve using the short time evaluation procedure StrainLife, Acosta et al. (2018). With StrainLife it is possible to utilize the data from NDT instrumentations for fatigue life evaluations and not just stress-strain data. The data development versus life time from electrochemical and magnetic signals can be an additional source of information about development of the microstructure as discussed above.