## 2 Composite Laminates Toughened by Polymeric Nanofibers  
One of the most promising methods for producing nanofibers is electrospinning which uses electrical field to produce polymer fibers with diameters ranging from nanometers to micrometers. The polymers for electrospinning applications can be used in solved or melted forms, however, the solved form is more common. Generally, an electrospinning machine consists of three main parts: 1--a high voltage power supply, 2--a feeding system like injection pumps, and 3--a collector plate or a cylinder. Figure 2 shows an electrospinning machine made by SPINBOW company (Italy). Various factors affect the quality and final configuration of produced nanofibers including: solvent type, applied voltage, feed rate, distance between the needle tip and the collector, polymer concentration in the solvent, environmental temperature, humidity and etc. The identification of the best factors is very important for conducting a fast and optimized process. For instance, there are different solvent systems for producing Nylon 66 such as pure Formic Acid (FA) [12], mixture of FA/Chloroform [13; 14; 15] and mixture of FA/Trifluoroethanol (TFE) [16; 17; 18; 19]. The use of the first two solvents results in a very slow electrospinning process (about 0.2-0.3 mL/h), while the third one allows a very fast process with about 0.8-1.2 mL/h [16; 20]. Up to now, 34 papers have been published regarding the effect of nanofibers on impact response of composite laminates. According to Table 1, different types of Nylon (NY) nanofibers have attracted researchers' attention more than other nanofibers (with 15 published papers). [PERSON] and his research group [22; 23; 24; 25; 26] applied Nylon 66 nanofibers between carbon/epoxy laminates to study their behavior under low-velocity impact loading. In the first paper [22], the diameter of the nanofibers was 65-120 nm and areal density of its mat was 0.7 g/m\({}^{2}\). The composite laminates consisted of 24 layers with stacking sequence of \([-45/90/45/0]_{\text{3S}}\). All composite layers were interleaved by nanofibrous mats. In addition, a layer of nanofabric was placed on the top and bottom surfaces of the laminate. The results showed that nanofibers could decrease the delaminated area significantly at lower impact energy levels, but at higher impact energies they found the opposite phenomenon. They also introduced the concept of \"critical force\" which corresponds to the damage initiation in the laminate during impact. The application of nanofibers improved the critical force from an average of 4.5 to  
Figure 2: Electrospinning machine [21]. 4.7 kN, representing a 4.4% improvement. CAI tests were also conducted on impacted laminates which showed a compression strength improved by 10% in the nanomodified laminates. A similar study was conducted in [24] in which the areal density of nanofibers was 1.6-2 g/m\({}^{2}\), whereas the impact energies considered were in the range 0.46-1.8 J. The results showed that the presence of nanofibers decreased the delaminated area considerably. The authors also proved that nanofibers increased the critical force by about 60%, it reduced the rate of impact damage growth with impact height to one-half, and reduced the impact damage from 0.115 to 0.105 mm\({}^{2}\)/N. In [24; 25], [PERSON] et al. used the data published in reference [24] and compared the results with commercial T800H/3900-2 composites interleaved by Polyamide particles. The results showed that the improvements obtained by applying the nanofibers were comparable to that of the commercial T800H/3900-2 composites, but with no thickness increase penalty, no loss of in-plane properties and no multiple glass transition temperatures. [PERSON] and [PERSON] [26] considered the influence of the areal density of nanofibrous mat (0.5, 1.5, and 2.5 g/m\({}^{2}\)) on impact response of carbon/epoxy laminates. By applying the thickest mat, total thickness of the laminates increased by 2.5%.