# The High-Velocity Impact Behaviour of Kevlar Composite Laminates Filled with Cork Powder
## 2 Experimental Procedure  
Composite laminate plates involving Kevlar bi-directional woven fabrics (taffeta with 281 g/cm\({}^{2}\)) and an Ampreg 22 epoxy resin with an Ampreg 22 hardener standard (both supplied by Gurit) were manufactured with a useful size of \(300\times 300\times 3.3\) mm\({}^{3}\) by hand lay-up technique. For this purpose, nine ply laminates, all in the same direction, were used. After that, the system was placed inside a vacuum bag and a load of 2.5 kN was applied for 48 h to ensure a uniform laminate thickness. During the first 10 h the bag remained attached to a vacuum pump to eliminate possible air bubbles in the composite laminates. Finally, the post-cure was carried out in an oven at 45 \({}^{\circ}\)C for 48 h to guarantee a convenient curing stage. Similar composite laminates with epoxy matrix filled by cork powder were also produced by the same methodology. The cork powder used consists of a residue obtained during the final sanding phase of cork-stoppers and it was collected at Amorim Cork S.A. (Valada, Portugal) industrial facilities. The cork powder presents a particle size, in terms of percentile, of d (0.1) = 18.6 \(\upmu\)m, d (0.5) = 78.9 \(\upmu\)m and d (0.9) = 208.3 \(\upmu\)m; a bulk density of 0.1095 g.cm\({}^{-3}\); and a humidity of \(\approx\)5.1%. The low moisture content observed is explained by the fact that the raw material from which the cork powder was obtained underwent several heating and drying operations [23]. Regardless of the low moisture content observed, to improve the adhesion with the resin, the cork powder was still dried in an oven (Heraus, model UT 6060) at 120 \({}^{\circ}\)C for 2 h and stored until used in a desiccator. Finally, the cork powder (3 wt.% of the epoxy resin-hardener mixture) and epoxy resin were mixed at 900 rpm for 2 h, with resource to an electrical mixer, in an ultrasonic bath for an uniform distribution of the cork particles into the resin. In order to avoid the formation of air bubbles, the mixture was degassed in a vacuum oven, followed by addition of hardener agent. However, more details about the cork fraction used and respective manufacturing process can be found in [32; 33; 34]. According to [PERSON] et al. [24], cork's structure is formed by hollow polyhedral prismatic cells, which have a honeycomb shape when observed from the radial direction and rectangular when observed from the transversal direction [9]. The area of the prism base is 4 to \(6\times 10^{-6}\) cm\({}^{2}\), with a mean prism edge of 13-15 \(\upmu\)m and a prim height of 30-40 \(\upmu\)m. The mean cell volume is around \(2\times 10^{-8}\) cm\({}^{3}\) and the number of cells per unit is 4 to \(7\times 10^{7}\) cm\({}^{-3}\). The cell walls are thin, with a thickness of 1 to 1.5 \(\upmu\)m, in which is concentrated the solid mass volume fraction of the cork [36]. The cork powder keeps the same cellular structure intact; however, when the size of cork particles deceases, the number of closed cells decreases and the external surface areas of the particles increase, and consequently, the number of open through-cut cells increases [36]. Based in studies reported by [PERSON] [36], a cork particle of volume 0.015 mm\({}^{3}\) (around 0.25 \(\times\) 0.25 \(\times\) 0.25 mm\({}^{3}\)) contains about 500 cells (7-9 cell per one row) in which only a fraction of 6 to 8 cells in one row are closed. This evidence and respective particle shape and morphology are well reported by scanning electron micrographs, in a review paper compiled by [PERSON] [36], for cork particles similar to those used in this study. The high-velocity impacts were carried out on plates of Kevlar bi-directional woven laminates with surface of 100 \(\times\) 100 mm\({}^{2}\). The specimen's size was big enough that the damaged area did not reach the border of the laminate. The projectile used for impact tests was of steel with a spherical geometry of 7.5 mm diameter and a density of 7800 kg/m\({}^{3}\).