## 2 Composite Laminates Toughened by Polymeric Nanofibers
[PERSON] and [PERSON] [36] used various percentages of CNF (0.2%, 0.5%, and 1%) in biaxial carbon braided composites and conducted Izod impact test. CNF were in the form of nanoparticles and mixed with epoxy before producing the laminate by hand layup technique. Because of the brittle nature of the carbon fiber, the impact strength slowly increased with the increase of percentage of CNF (0.2% and 0.5%) and decreased for higher percentage of CNF (1%). [PERSON] et al. [37] conducted almost the same study and considered the influence of volume fraction (1.2 vol % and 2.5 vol %) of CNF on absorbed energy, damaged area, CAI elastic modulus, and CAI strength. The results showed that the damaged area decreased significantly (about 50%) and CAI strength increased about 1.5 times by the addition of CNF. The same group [38] comprehensively studied this topic with different stacking sequences of laminates and various fractions of CNF (Table 1). The interesting point about this research is that CNF was interleaved between prepreg layers (not  
Figure 7: Phase separation in PCL/epoxy blend [49]. mixed with epoxy before manufacturing the samples). The most important results highlighted the fact that CNF could decrease the delaminated area up to 90% percent and their effect were significantly better at higher impact energies. [PERSON] et al. [39] investigated electro-mechanical characterization of CNF-modified laminates and showed that a variation in the electrical resistance of the laminate took place in correspondence with the impact induced damage. The impact tests were conducted several times and each time electrical resistance increased as function of the increase of the damaged area. The important point is that by raising the fraction of CNT from 0.5% to 1%, the authors obtained a decrease of the electrical resistance. Oxidized carbon nanofibers (O-CNF) were also applied by [PERSON] et al. [40] for toughening CFRP prepreg. One important point which was not considered by others is the toughening mechanism by CNF. Bridging between epoxy matrix and O-CNFs, and thus, a better adhesion between them was observed due to crosslink interaction as found by FESEM investigation of composites (Figure 8). The results showed that the damage area decreased with the incorporation of O-CNFs at all the impact energy levels (10, 20, and 30 J) and a maximum reduction of 68% in the damage area was obtained at 20 J. [PERSON] and his research group [41; 42; 43] proved that the Tetra Ethyl Orthosilicate (TEOS) chemically engineered glass nanofibers are not suitable choices for toughening laminates. Drop-weight impact tests showed that the modified laminates had about 9% larger damaged area in comparison with the unmodified ones [41]. In the second step, they used numerical modeling (by means of LSDYNA) and compared the outcomes with the experiments [42]. There was good agreement between them in lower impact energies while the simulated impact loads were smaller than the experimental impact loads which resulted in a smaller bending stiffness and a weaker laminate in higher impact energies. Finally, CAI test results were reported in reference [43]. As it is expected, compressive residual strength was decreased significantly in nanomodified laminate, for instance, a 50% reduction occurred in the specimens impacted by the higher energies. There are some other types of nanofibers, such as epoxy 609 [44], Polyvinylidene fluoride (PVDF) [45], Styrene-acrylonitrile (SAN) [46], Polyvinyl alcohol (PVA) [47], Polyacrylonitrile (PAN) [48], that were reported only in one paper each. [PERSON] et al. [44] utilized co-axial epoxy 609 and SiC nanofibers for increasing impact strength of composite laminates. Lateral impact tests were conducted, and the outcomes showed that the mechanical performances of the composite laminates do not change remarkably when the interfacial nanofibrons membranes have a proper thickness and a suitable content of SiC.