# The High-Velocity Impact Behaviour of Kevlar Composite Laminates Filled with Cork Powder
## 3 Results and Discussion  
Figure 3 shows the absorbed energy regarding impact energy for all the specimens. The variation of the absorbed energy with impact energy is very similar for both type of plates. Below perforation, the plates absorb all the kinetic energy of the projectile, mainly by damage. When the impact energy increases above perforation, the absorbed energy decreases asymptotically. It was not possible to calculate the minimum perforation velocity in a determinist manner, as an impact-velocity interval exists in which the structure may or may not be entirely perforated. In addition, the residual velocity of the projectile from the gas gun cannot be fully controlled. For this reason, a high level of scatter was found in the data obtained from the experimental tests around the minimum perforation velocity. To obtain this velocity, a least-squares method was used, by fitting Equation (1) which is similar to that proposed in the model of [PERSON] [39]. This method has been used successfully to predict the minimum perforation velocity in laminate and sandwich structures subjected to high velocity impacts [30; 40; 41; 42]. \[v_{r}=\left\{ \begin{array}{c}0\ if\ 0\leq v_{i}<v_{bl}\\ v_{r}=A\left(v_{i}^{p}-v_{bl}^{p}\right)^{\frac{1}{p}}\ if\ v_{i}>v_{bl} \end{array} \right. \tag{1}\]  
where: \(v_{r}\) is the residual velocity, \(v_{i}\) the impact velocity, \(v_{bl}\) the minimum velocity of perforation and \(p\) and \(A\) are fitting parameters. Therefore, the minimum velocity of perforation is about \(206.35\pm 4.20\) m/s for Kevlar laminates with neat resin, while for that with resin filled with cork powder is around \(209.71\pm 4.63\) m/s. Regardless of  
Figure 3: Impact energy vs. absorbed velocity. the small difference in terms of average values (1.6%), both values are similar because they are within the experimental dispersion. In fact, cork is a hydrophobic material with a surface energy of 18 mN.m\({}^{-1}\), while the typical cured epoxies have surface energy of about 45 mN.m\({}^{-1}\) and a poor adhesion occurs for substrates with a surface energy of 30 mN.m\({}^{-1}\) or lower. However, this weakness is countered by the larger external surfaces of the particles [36]. Regardless of the improvements obtained in terms of surface energy using surface treatments [43; 44], [PERSON] et al. [44] report that some of them destroy part of the honeycomb structure of the cork cells (erosion of cork cell walls). In this case, the damage caused to the particles can facilitate the penetration of the resin with the consequent decrease on the absorption of energy, because the damping effect of cork is lost. Therefore, while the bonds that occur when the adhesive diffuses into cell walls to build chemical bonds with the cell's chemical components are weaker, the particles' morphology promotes the physical bonds with the penetration of the epoxy resin into the open cells and consequent mechanical interlocking. In this context, the cork particles are surrounded by resin, and the closed cells that contain air inside can deform when they suffer an impact force and absorb that impact [44]. For this reason, small particles show less improvement than larger particles, because the number of open through-cut cells increases, and the resin does not allow the cell deformation. Consequently, they cannot absorb as much energy. Therefore, the response of the cork/resin composite is influenced by the number of closed cells in the particle [44]  
In addition to the benefit obtained in terms of impact, it is unquestionable that the cork powder reduces the amount of resin in the composite, making it more environmentally friendly. These results are in good agreement with the open literature, where [PERSON] et al. [34] observed, for example, that fatigue life is insensitive to the presence of cork powder into the resin. Simultaneously, these authors obtained benefits in terms of impact strength and glass transition temperature [32; 33]. To understand these results, the failure mechanisms are shown in detail and Figures 4 and 5.