PSI - Issue 47

R. Nobile et al. / Procedia Structural Integrity 47 (2023) 176–184

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R. Nobile et al. / Structural Integrity Procedia 00 (2019) 000 – 000

exception. The latter, subjected to a greater load than the other two samples tested, generally exhibits greater temperature variations as expected; but in the final stage, before failure, no further increase is observed. In this phase the temperature continues to increase almost linearly until the final breaking of the sample.

(a) (b) Fig. 5. Trend of experimental resistance (a) and temperature variation (b) against fatigue life [%].

For the tested specimens, the experimentally determined resistance is the sum of the change in resistance due to temperature (ΔR th ) and that due to damage (ΔR damage ) as reported by the authors in the previous work (Nobile and Saponaro (2021)). Therefore, the electrical resistance data due to thermal contribution were subtracted from the experimentally obtained value (ΔR exp ) to obtain the resistance value of the damage alone: ∆R damage =∆R exp −∆R th (4) Figure 6a shows the obtained results of resistance variation due to only damage, against fatigue life percent for each tested specimen. From the general trend of the curves relating to the tested specimens, a slight reduction in resistance can be observed in the initial stages of the test, probably due to an early initial damage to the material, as also observed by the reduction in stiffness in the early stages of fatigue life (Fig. 6b). Figure 6c shows an example of final fracture for specimen A3.

(a) (c) Fig. 6. Electrical resistance due to only damage (a) and normalized stiffness degradation (K/K 0 ) (b) vs. fatigue life [%]; (c) Example of final fracture for specimen A3. Subsequently, for A3 specimen, starting from 5% of the fatigue life up to approximately 20 % and from 20-30 % of the fatigue life up to approximately 40 %, for specimen A6 and A5 respectively, the contribution of resistance due to damage shows a first rapid increase, much more evident for specimens A3 and A6 than for specimen A5. Starting at 20-40 % of the fatigue life for specimens A3 and A6, respectively, the electrical resistance shows an increase with (b)