PSI - Issue 41

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R. Nobile et al. / Procedia Structural Integrity 41 (2022) 421–429 Riccardo Nobile et al. / Structural Integrity Procedia 00 (2019) 000 – 000

(a) (b) Fig. 8. (a) Normalized electrical resistance due to only damage vs. fatigue life [%]; (b) normalized stiffness degradation data of each specimen subject to fatigue. A similar trend was also observed for the P 4 specimen. After a reduction in resistance up to 20% of the fatigue life, the latter increases rapidly, maintaining a stable trend up to about 65%. Subsequently, after a decrease up to about 70% of the fatigue life, the resistance due to only damage increases first gradually and then rapidly in the propagation phases of the crack before failure (Fig. 8b). This trend as also observed in the previous research activity, could be indicative of the end of the nucleation phase of the fatigue crack and the beginning of the propagation of the latter which further increases the resistance until the final failure. For all tested specimens AISI 316L, a general trend of a first interesting rapid increase in resistance from approximately 20-40% of fatigue life was observed (Fig. 8a). This increase in electrical resistance is associated with fatigue damage that causes a change in the resistivity of the material located in the area where the notch is present. Subsequently, a general trend of rapid increase in resistance from approximately 65-75% of fatigue life was observed (Fig. 8a) which becomes more pronounced starting at approximately 80% of fatigue life in crack propagation phase. This latter behavior is coherent with the reduction in stiffness of the tested specimens AISI 316L, obtained by processing fatigue data. From the stiffness curves shown in Fig. 8b, a reduction with a lower slope from 60-70% of the fatigue life is generally observed, while in the fatigue damage propagation phase starting from about 85-90%, the stiffness of the specimens suddenly decreases with a greater slope up to the final failure. As also observed by the authors in previous research activity on C45 specimens (Nobile et al. (2021)), the rapid increase in resistance to about 20-40% of the fatigue life in this case is interesting because it is associated with irreversible damage due to fatigue localized at the tip of the notch. Also, in this study it is possible to note that the percentage change in experimental resistance and that due to damage are inversely proportional to the level of stress applied and approximately linear (Fig. 9). Table 3 summarizes the data relating to the percentage changes in resistance.

Fig. 9. Correlation at 20- 40% of fatigue life of ΔR damage /R 0 (a) and ΔR exp /R 0 (b) vs. stress amplitude.