PSI - Issue 41

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

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Subsequently, a decrease in resistance up to 60% of the fatigue life is observed before increasing slowly and then rapidly in the propagation phases of the crack before the final failure. The P 2 specimen, on the other hand, shows a decrease in resistance in the initial stages of the test up to about 20% of the fatigue life, followed by an almost constant trend up to about 40% of the useful life. The resistance next shows a gradual increase up to 60% of the fatigue life and then rapidly increases until the failure.

(a) (b) Fig. 7. Experimental electrical resistance change (a) and temperature variation (b) vs. fatigue life (%).

A slight reduction in resistance was also observed for P 3 specimen in the early stages of the test up to approximately 10% of the fatigue life. Subsequently, the measured experimental resistance rapidly increases from approximately 25% of the fatigue life up to approximately 33%. From this point on, the resistance decreases to about 60% of the fatigue life and then increases, first gradually and then rapidly in the final stages of crack propagation before the rupture of the sample. The P 4 specimen, on the other hand, shows a constant trend up to about 20% of the fatigue life, presenting a rapid increase starting from 20 up to 40% of the fatigue life. Subsequently it exhibits a stable behavior up to about 60% of the fatigue life. From 60 to 80%, the resistance shows a decrease and then gradually increases first and subsequently rapidly in the damage propagation phases that precede the final rupture. This latter behavior of increasing resistance in the final stages before breaking was also observed in the previous study conducted by the authors on metal specimens (Nobile et al. (2021)) of different geometry and appears to be repetitive and independent of the applied load. The variation of the average temperature, also monitored in real-time by three T thermocouples applied to the tested samples AISI 316L, showed, for all specimens, a rapid initial increase up to about 20% of the fatigue life (Fig. 7b). Subsequently, from 20 to about 80% of the fatigue life, the temperature increases linearly, with a lower slope than in the initial stages of the test. Starting from approximately 80-90% of the fatigue life, it exhibits a rapid increase in the propagation phase of the crack before the final failure. As done by the authors in the previous research activities on C45 specimens (Nobile et al. (2021)), the experimental resistance trends shown in Fig. 7a must be corrected by considering the effects of the temperature in accordance with Eq. (2) to determine the change in resistance due to only damage ( ΔR damage ). ∆ = ∆ − ∆ ℎ (2) Figure 8a shows the trends of the normalized variation of the electrical resistance due to fatigue damage (ΔR/R 0 ) damage as a function of fatigue life percent. From the graph it is observed that the electrical resistance decreases up to about 25% for the P 1 and P 3 specimens and up to 40% for the P 2 specimen. Subsequently it increases rapidly, remaining approximately stable up to about 60% of the fatigue life for the specimens P 1 and P 2 ; finally, a further rapid increase is observed up to the final failure. The P 3 specimen, on the other hand, shows a stable resistance trend from 33% up to about 50% of the fatigue life. Subsequently, after a reduction in resistance to approximately 65% of the useful life, a rapid increase is observed before the final crack, up to failure.