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

Specifically, the trends of the damage parameter first gradually increase with a lower slope up to about 60 % of the fatigue life and subsequently, starting from 60 % of the useful life, they show a further increase with a greater slope, first gradually and then more rapidly in the damage propagation phase until the sample breaks. These results are in good agreement with the stiffness degradation parameter D k . From the experimental results, it was observed that the increase of the damage dissipative parameter (D diss ) from 5-10 % of the fatigue life is coherent with the observed increase of the electrical resistance variation due to only damage (ΔR damage ) (Fig. 6a). Starting from 60 % of the fatigue life, the trends of the parameter (D diss ) and of the electrical resistance variation showed a rapid increase resulting both in good agreement. The thermal profiles relating to specimen A6 (Fig. 8b) show a rapid increase of the D diss parameter due to the evolution of the damage already starting from the early stages of the fatigue life of the specimen up to the final failure. These results agree with the stiffness degradation of the sample. However, from the graph of Fig. 8b, it can be observed that the D diss parameter increases more rapidly with a greater slope than the D k parameter. The general trends of the thermal profiles relating to the D diss parameter, were also in this case in agreement with the observed increase in the electrical resistance variation observed starting from about 20% of the fatigue life of specimen A6. For specimen A5, the thermal profiles of the dissipative damage parameter show a remarkable rapid increase from 0 % to about 60 % of the fatigue life, resulting in agreement with the increase of the parameter D k . However, the latter increases with a smaller slope than the D diss parameter. Starting from 60 % of the fatigue life, the general trends of the thermal profiles relating to the Ddiss parameter show a decrease due to the considerable damage (see Fig. 9b). From the experimental data obtained, it has been observed that the trend of the dissipative parameter, determined for the selected areas (ROIs), is coherent with the increase in resistance observed between 30-60 % of the fatigue life for sample A5. However, from 60 % of the fatigue life, the trends of the D diss parameter and the change in electrical resistance differ. In the opinion of the authors, this latter result can be explained by the fact that the damage due to fatigue which also affects the surface of the specimen near the hole (Fig. 9b), determines a reduction of the surface temperature in the ROIs thus generating a reduction of the dissipative damage parameter (D diss ), while in this phase, the resistance further increases with a higher slope as highlighted earlier in Figure 6a.

(a) (b) Fig. 9. (a) Comparative analysis between dissipative damage parameter D diss and D k against fatigue life [%] for A5 specimen; (b) Thermal map at 80 % of fatigue life for A5 specimen with selected ROIs. 4. Conclusions In the present work, Non-Destructive methods based on the measurement of electrical resistance and surface temperature changes were applied to a batch of composite specimens in an open hole configuration, obtained by LCR process, to monitor fatigue damage in real-time. From the results obtained, a slight decrease in resistance was observed in the initial phases of the test which could be due to self-generated heat in the initial phases due to load cycles.