## 2 Composite Laminates Toughened by Polymeric Nanofibers
On the other hand, interleaving the laminates improved the critical force by 8% (using the 0.5 g/m\({}^{2}\) mat), 42% (using the 1.5 g/m\({}^{2}\) mat), and 45% (using the 2.5 g/m\({}^{2}\) mat). In addition, damage growth rates decreased by 12, 32, and 48%, respectively. The University of Bologna research group published three papers regarding the impact response of carbon/epoxy and GLARE fiber/metal laminates interleaved by NY66 [27; 28; 29]. In the first study [27], two different nanomodified configurations were investigated (Figure 3) and their responses were compared with virgin specimens. Before and after low velocity impact tests, the stiffness, the harmonic frequencies and damping of all samples were examined to consider the effect of nanofibers on these properties. Scanning electron (SEM) and optical microscopes were also used to evaluate the toughening mechanism and damages occurred during the impact. The results of the tests on non-damaged samples proved that the stiffness and the first harmonic frequency of nanomodified samples were 10% lower, but the damping ratio was 160% higher than the non-modified ones. On the other hand, the post-impact analysis of non-modified samples showed a decrease in the stiffness and harmonic frequencies, proportional to the impact energy level. Modified samples presented unexpected effects: Both the stiffness and the first harmonic frequency increased up to 14% and 12%, respectively, after 6 J impact. The outcomes also showed that Nano1 configuration (Figure 3) had better damping factor than the virgin and Nano configuration before impact test, but all these three samples had the same damping effect after impact energies of 6 and 12 J. The SEM pictures also illustrates the toughening mechanism occurred in the NY66-modified laminates. As the curing temperature of laminates is normally less than NY66 melting point, so the nanofibers were available with their initial configuration (Figure 4). Therefore, the nanofibers could make bridge between composite layers and stop the crack from propagation. In the second study, the research group focused, for the first time, on toughening fiber/metal laminates (GLARE) using NY66. No nanofibers were put between glass/epoxy layers and only two nanofibrous mats were applied between aluminum (AL) layers and composite laminate. The results showed that nanofibers could increase the adhesion strength between AL and laminate, which led to a decrease of the damaged area between 42% and 62% depending on the impact energy level (Figure 5). [PERSON] et al. [30] used a new method for producing nanomodified laminates. In this method, the nanofibers were firstly electrospun on dry glass fibers, then cast resin film was transferred to them. The method is called RFI and more details about it can be found in [31]. They conducted impact and CAI tests, but only the results of the second test were reported. According to the outcomes, with an enhancement in the areal density of nanofibers, an increase in the residual compressive strength was obtained; for instance, about 20% increase was achieved by applying 0.4 g/m\({}^{2}\) of nanofibers. [PERSON] et al. [32; 33] used NY6 and 6.9 for considering their areal density on impact response (14, 28, 41, 54, 67, 79 J) of glass/epoxy laminates. The results proved that areal density did not have significant effect on impact parameters and their efficiency on damaged area was almost similar. In the lower impact energies, the modified and non-modified laminates had the same delaminated area, but nanofibers could decrease it up to 25% in higher impact energy levels. According to SEM pictures, the toughening mechanisms of NY  
surface\" technique and introducing cohesive parameters of the reference and modified samples, the delaminated area was determined in low-velocity impact tests. By comparing the numerical and experimental results they showed that the difference was only 0.6% ([PERSON] et al., 2018), and that the nanofibers could decrease the delaminated area by 34%.