# The High-Velocity Impact Behaviour of Kevlar Composite Laminates Filled with Cork Powder
## 3 Results and Discussion
They show typical images obtained from visual inspection for the back side of the plates. Figure 4 shows the specimens of Kevlar and Kevlar with cork powder impacted at similar velocities, close to the perforation velocity, while the specimens of Figure 5 were impacted at higher velocities. From the visual inspections of the back sides of the plates (Figures 4 and 5), it is visible that the failure mechanisms are: fibre failure and delamination. Although matrix cracking is not possible to identify by visual inspection, this is one of the first damage mechanisms in composite laminates, and therefore, it should appear since it induces other failures modes, as delamination [45]. The images of the damaged area that were obtained by C-scan in the samples subjected to different impact velocities are shown in Figures 6 and 7. The impact velocities coincide with the impact velocities of the samples showed in Figures 4 and 5. In all specimens inspected by C-scan, it was possible to observe that the damaged areas had circular shapes, a behaviour that also appears in woven glass laminates [46]. Figure 4: Damage area of the back side. (**a**) Specim of Kevlar impacted at 231 m/s and (**b**) specimen of Kevlar + cork powder impacted at 230 m/s. Figure 5: Image of damage area of the back side. (**a**) Specimen of Kevlar impact to 438 m/s and (**b**) specimen of Kevlar + cork powder impacted to 439 m/s. Figure 6: Images of damage areas by C-scan. (**a**) Specimen of Kevlar impact to 231 m/s and (**b**) specimen of Kevlar + cork powder impacted to 230 m/s. Figure 7: Images of damage areas by C-scan. (**a**) Specimen of Kevlar impact to 438 m/s and (**b**) specimen of Kevlar + cork powder impacted to 439 m/s. The specimens impacted at higher velocities, around 300 m/s, showed fibre breakage close to the impact point, as reported in Figures 5 and 7. Simultaneously, the C-scan images clearly show that the damage caused by the bullets not only causes local damage (macro damage), but also affects the vicinity of the damaged areas clearly seen in different colours (micro damage), which agrees with the studies developed by [PERSON] et al. [47]. Moreover, the extension of damage is more located around the impact point at higher velocities, as can be seen in Figure 7. In this context, the extension of damage area determined by the C-scan inspection is larger than the damage established by the visual inspection on both sides of the laminate (front and back face).Therefore, in addition to macro-damage, it is evident that the impact also induces internal micro-damage that is not visible to the naked eye, but should be considered when quantifying the damage size [47]. Figure 8 shows the extension of damage area for all specimens measured by C-scan. In all structures, the larger damaged area was found for velocities around the minimum perforation velocity. For velocities below the minimum perforation velocity, the damaged area increases with velocity, while for velocities greater than the perforation velocity, the damaged area decreases with increasing velocity. This behaviour was observed by [PERSON] et al. [48] for glass woven laminates, and it is produced by the bending of the plate due to the transverse wave travel, which leads to delamination. Since the contact time is maximal at the perforation velocity, the distance travelled by the transverse wave is also maximal, and thus the delaminated area is too [46; 49]. On the other hand, when the damaged areas are compared between laminates with neat resin and resin filled by cork powder, in all conditions there is a clear tendency for the last ones to have smaller damaged areas. For example, comparing the damaged areas shown in Figures 6 and 7, it is possible to observe that for impact velocities around 230 m/s, they are around 11.9% lower for laminates with cork powder. However, this value was around 45.7% lower for impact velocities of 438 m/s. Nevertheless, for the highest impact velocities, the damaged areas tend to be similar.