PSI - Issue 47

R. Nobile et al. / Procedia Structural Integrity 47 (2023) 176–184 R. Nobile et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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a lower slope up to about 60 % of the fatigue life. The variation of the electrical resistance for the A5 specimen instead shows a gradual increase from about 45 % to about 60 % of the useful life, in accordance with the stiffness degradation. From 60 % of the fatigue life (Fig. 6a), a rapid increase in resistance due to the fast progressive increase in damage is observed for all specimens tested. Specimen A3 exhibits a faster increase in electrical resistance than the other two tested specimens. Specimen A5, stressed with a greater load, shows a minor increase in resistance compared to the other two specimens A3 and A6. The latter results agree with the decrease in stiffness. With reference to the curves relating to specimens A3 and A6 (Fig. 6b), at 60 % of the fatigue life it is observed that the stiffness of specimen A3 decreases more rapidly and earlier respect to specimen A6; first slowly up to about 80% of life and then quickly until the final rupture of the specimen. Specimen A5, on the other hand, showed a different behavior. The stiffness presents a linear reduction of up to approximately 90 % of the fatigue life and subsequently it slowly decreases until the sudden break of the sample. This different behavior could be due to a different evolution of the damage in the final stages of the test. 3.2. Thermographic monitoring results A preliminary qualitative analysis was performed on each specimen tested, highlighting a repetitive pattern in terms of damage modes. In Fig. 7, the temperature maps referred to a specific instant of fatigue life (10 %, 23 %, 51 %, 71 % and 90 %) are shown as an example, normalized with respect to the initial load level for specimen A3. From the thermal maps, small temperature changes are observed at 23 % of the fatigue life in the hole region. At 23-51 % of the useful life, there is a constant increase of the temperature values, with an asymmetrical distribution located around a large area in the hole zone as illustrated in Fig. 7. Subsequently, at 71% of the fatigue life, the evolution of the damage is observed with a large damage arranged diagonally near the hole. In the final stages, the heat generation related to the dissipative sources rapidly increases in the delamination propagation stages until sudden failure.

Fig. 7. Normalized thermal maps at selected fatigue instant for A3 specimen.

The analyzed damage parameter, related to dissipative sources (D diss ), was plotted as the fatigue life varied for each sample tested, considering different regions of interest (ROI) near the hole (B1, B2, B3, B4 and B5). and compared with the stiffness degradation parameter (D k ). For specimen A3, both D k and D diss trends show a similar evolution of the damage (Fig. 8a).

(a) (b) Fig. 8. Comparative analysis between dissipative damage parameter D diss and D k vs. fatigue life [%] for A3 (a) and A6 (b) specimens.