# Effectiveness of Nanotechnology Treatments in Composite Aircraft Applications
## 3 Corrosion and Fatigue Test
### Fracture Area and SEM Analysis  
Once the fatigue tests were completed, the failure areas were analyzed. An initial analysis was performed to identify the portion of the area attributable to fatigue failure and that attributable to static failure, which occurred once the residual section of the sample was no longer able to withstand the applied mechanical stress. From Figure 9, where the results are divided by specimen category, it is possible to observe that, as could be expected, the extent of the fatigue failure section was more significant when the specimen under consideration was subjected to a lower stress: a lower mechanical load means a higher number of cycles, therefore a greater failure area due to fatigue. The graph also highlights what has already been observed previously: in each group of specimens, the best resistance to mechanical stress was always obtained with nanotechnology coating protection. From the electron microscope (SEM) analysis, as reported in Figure 10, production damage (scratches and streaks) could be seen on all specimens subjected to corrosion as well as signs of surface pitting [19]; however, they were more consistent in size and number in the case of raw aluminum and primer treatment, where a detachment from the metal surface was observed. Pitting corrosion also occurred in the case of aluminum treated with nanotechnologies, but with characteristics at first sight decidedly more limited than what occurred in the other two cases. Figure 8: S-N curve: blue: no corrosion; black: corroed specimens; green: primer protection; red: nanotechnology protection. Considering the limited number of specimens considered in this study and observing Figure 11, it can be cautiously stated that at the same magnification, the corroded specimen, MCF 008, had the same number of holes and imperfections as the uncorroded one, MCS 012, both treated with nanotechnology, and only increased in size. The raw aluminum samples showed notable differences: an increase in the number of imperfections in the corroded sample, MCF 002, compared to its uncorroded counterpart, MF 000. This aspect shows a general improvement in the ability to resist corrosion for the nanotechnology-treated specimens. A significant presence of slip bands could also be noted in all specimens with a general direction of development of the phenomenon orthogonal to the application of the load [20,21]. The analysis conducted by an SEM microscope provided better evidence of how corrosion affected the surface finish of the metal: to observe the same type of damage related to the machining and fabrication of the metal specimens, the corroded raw aluminum specimen, MCF 002, was analyzed at a lower magnification, compared to the uncorroded raw aluminum specimen, MF 000, see Figure 12.  
Figure 10: SEM analysis: (**A1,A2**) MCF002; (**B1,B2**) MCF005; (**C1,C2**) MCF008. Figure 9: Comparison of failure areas for each type of load. The authors are further investigating the mechanism underlying the improvement in fatigue resistance in metallic materials when treated with silicate-based nanoparticles. At present, we can state that the initial cracks were noted to originate from pitting damage at numerous locations, as highlighted also in [20], where a detailed narrative of the entire damage evolution process in corrosion-nucleated fatigue was provided. A similar conclusion was derived in [21], where all the prerecorded Al 2024-T4 specimens under investigation fractured from cracks associated with pitting. Results indicated that quantities such as pit surface area and surrounding pit proximity were as important as pit depth in determining when and where a crack would form. The same material as in the present work (Al2024-T3) was analyzed in [19], and similarly, a pit-to-crack transition was successfully observed using digital video techniques. Since a reduction in the number and size of corrosion pits was observed in specimens treated with nanotechnologies, this could correlate with the extension of fatigue life detected by experimental tests.