## 2 Materials and Methods
### Composite Design  
The date palm/cotton/Kevlar fibre-reinforced polyester hybrid composites were manufactured in laboratory conditions. The two-component polyester resin VISCOVOSS AZUR SUPER+ type was used as the matrix and was purchased from the Romanian market (Best Tools company Brasov, Brasov, Romania). This resin has a density of 1.1 g/cm\({}^{3}\) and a viscosity of 900-1100 mPa\(\cdot\)s at 20 \({}^{\circ}\)C (Brookfield, Toronto, ON, Canada) as specified in the technical sheet. It is an unsaturated, pre-accelerated, ambient-temperature-curing polyester resin with low styrene emissions. Methyl ethyl ketone peroxide (MEKP) was used as the hardener and was added to the base component resin with a participation rate of 3% of the resin weight. Natural fibres (DP and C) and synthetic fibres (K) were used to reinforce the polyester matrix and to manufacture the hybrid composites. The design of the three types of composites is presented in Table 1.  
Each layer of the composites was firstly weighed with an accuracy of 0.01 g, and the sheets with similar weight were used for the faces in the composite structures. The average weight of the layers was as follows: the DP layers had 33.26 g for the faces and 36.81 g for the core; the cotton layers had 6.6 g; and the Kevlar layers had 24.35 g. The proportion of the fibres in the polyester resin matrix was 20 wt% for DP, 19.17 wt% for DP/C and 22.95 wt% for DP/K (Table 1). The proportion of the cotton and Kevlar fibres from the total volume of the panels is cotton fibres 9.76 vol% for DP/C and Kevlar fibres 8.18 vol% for DP/K. The polyester resin amount was calculated by multiplying the weight of each layer as follows: five times for the date palm fibres, seven times for the cotton fibres and three times for the Kevlar fibres. These rates were established according to the recommendations of the manufacturer for the synthetic fibres and after preliminary determinations for the date palm fibres and natural fibres similar to cotton [28]. The resin was applied via brush on each individual sheet of fibres. The working temperature was about 24 \({}^{\circ}\)C, and the working time for applying the resin was up to 30 min to avoid the stickiness effect as recommended in the technical sheet. On the top and bottom of the sandwich hybrid composite, white silicon baking paper was used to avoid sticking to the support where the sandwich was formed (Figure 2d). Each of the laminate composites was placed between two blockboard panels, and a weight of 20 kg was used for cold pressing the composites at a pressure of 0.019 bar for three days. Fully curing this resin needs at least 48 h, according to its technical sheet. The amounts of resin were, as follows: 490 g (DP structure), 595 g (DP/C structure), 660 g (DP/K structure). The method applied to manufacture the experimental hybrid composites is presented in Figure 3. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Code**} & \multirow{2}{*}{**Structure**} & \multirow{2}{*}{**Design 1**} & **Date Palm** & **Cotton/Kevlar** \\  & & & **Fibres Weight** & **Fibres Weight** \\  & & & **Fraction (\%)** & **Fraction (\%)** \\ \hline \multirow{2}{*}{\begin{tabular}{c} Date palm \\ (DP) \\ \end{tabular} } & 3 layers & Alternating & & \\  & (DP-DP-DP) & parallel–perpendicular & 20 & - \\  & & orientation of layers & & \\ \hline \multirow{2}{*}{\
\begin{tabular}{c} Date \\ palm/cotton \\ (DP/C) \\ \end{tabular} } & 5 layers (DP-C-DP) & Alternating & & & \\  & DP-C-DP & parallel–perpendicular & 17 & 2.17 \\ \hline \multirow{2}{*}{
\begin{tabular}{c} Date \\ palm/Kevlar \\ (DP/K) \\ \end{tabular} } & 5 layers (DP-K-DP) & Alternating & & & \\  & DP-K-DP & parallel–perpendicular & 15.6 & 7.35 \\ \hline \hline \end{tabular} 1  
The layers for the faces were oriented longitudinally in each structure.[ENDFOOTNOTE]  
\end{table} 
Table 1: The design of experimental hybrid composites. Three replicates of each hybrid composite type (DP, DP/C, DP/K) were manufactured.