# Impact Response of Composite Sandwich Cylindrical Shells
## 2 Material and Experimental Procedure  
Composite cylindrical shells and composite sandwich cylindrical shells were produced by hand lay-up and the overall dimensions are shown in Figure 1. In detail, Figure 1a shows a view of the manufacturing process used in this study and Figure 1b the geometry and dimensions of the specimens.  
A system of SR 1500 epoxy resin and a SD 2503 hardener standard, both supplied by Sicomin, was used as matrix and different number of layers and fibres, all in the same direction, were used to produce cylindrical shells with the following stacking sequences: 8C; 6C; 4C; 4C + cork + 4C and 4K + cork + 4C. The \"number\" represents the number of layers used, C = carbon bi-directional woven fabrics (taffeta with 160 g/m\({}^{2}\)), K = Kevlar bi-directional woven fabrics (taffeta with 281 g/m\({}^{2}\)) and cork is a 2 mm-thick NL 20 CORECORK layer (supplied by Amorim Cork Composites) inserted into the middle of the sheets. In detail, the carbon fibres used are of the T300 type, supplied by Toray, which have a density of 1.76 g/cm\({}^{3}\) and a filament diameter of 7 \(\upmu\)m, while for Kevlar fibres the density is 1.44 g/cm\({}^{3}\) and the filament diameter is 12 \(\upmu\)m, supplied by DuPont. The density of NL200 cork is 0.25 g/cm\({}^{3}\). These systems were placed inside a vacuum bag (see Figure 1a) for 24 h and a maximum vacuum pressure of 0.5 bar was applied for 9 h in order to maintain a constant fibre volume fraction and a uniform thickness to eliminate any  
Figure 1: (a) Manufacturing process; (b) geometry and dimensions of the specimens in mm (t = thickness showed in Table 1).  
air bubbles existing in the laminate. According to the supplier's datasheet, the post-cure procedure was carried out in an oven at 60 \({}^{\circ}\)C for 16 h.  
Table 1 summarizes the different thicknesses in terms of materials used and respective configurations.  
Static tests were carried out using a Shimadzu AG-100 universal testing machine, with the same support and loading nose used along the impact tests (Figure 2), equipped with a 100 kN load cell at a displacement rate of 3 mm/min. The displacement was obtained directly from the crosshead displacement that is automatically measured by the testing machine with a precision of \(\pm\)0.01 mm. These tests were carried out under conditions similar to those used in the impact tests, but, in this case, aiming to obtain the static characterization. All tests were carried out at room temperature, and five specimens were tested for each configuration.  
Low-velocity impact tests were also performed using a drop weight testing machine IMATEK-IM10, and details of this impact machine can be found in [43]. An impactor diameter of 10 mm with a mass of 2.826 kg was used, which will impact the centre of the samples with free support on the curved edges while the straight edges are bi-supported  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Stacking Sequence** & **Schematic Lay-Up** & **Thickness (mm)** \\
**(Standard Deviation)** \\ \hline
8C & & 2.53 (0.15) \\
6C & & 1.58 (0.03) \\
4C & & 0.92 (0.02) \\
4C + Cork + 4C & & 4.23 (0.09) \\
4K + Cork + 4C & & 4.11 (0.03) \\ \hline \hline \end{tabular}
\end{table}
Table 1: Stacking sequences and thicknesses of the samples tested.  
Figure 2: Support used in the experimental tests.  
(Figure 2). An impact energy of 5 J was used, which was previously selected to promote visible damage, but without perforation of the specimens. The impact tests were performed in accordance with the considerations reported in ASTM D 7136 and, for each configuration, five specimens were tested at room temperature.