# Impact Properties of Novel Natural Fibre Metal Laminated Composite Materials
## 3 Results and Discussions  
### Impact Properties  
Table 1 shows the average and standard deviations of the impact responses. The data include two replicates with five samples each. The impact properties consist of total energy, maximum load, deflection and time to maximum load. Table 2 shows the analysis of variance (ANOVA) of the responses. \(p\)-values, less or equal to 0.05, reveal significant factors or interactions that affect responses. The \(p\)-values in bold correspond to significant second-order effects, which will be evaluated via interaction plots. Higher R\({}^{2}\) and Ryan-Joiner index values indicate that the models fit well with the data, validating the ANOVA. F-values can be used to compare the effects of each factor. Higher F-values indicate the factor that most contributes to the response variation. In this case, as shown in Table 2, aluminium treatment is the main factor affecting impact properties, especially the maximum impact load. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**SiRAL Condition**} & **Total Energy** & **Max Load** & **Total Deflection** & **Time to Max Load** \\  & **[J]** & **[Kn]** & **[mm]** & **[ms]** \\ \hline \(0^{\circ}/90^{\circ}\) & 31.77 \(\pm\) 0.72 & 5.44 \(\pm\) 0.25 & 9.16 \(\pm\) 1.21 & 4.53 \(\pm\) 0.50 \\ \hline \(0^{\circ}/90^{\circ}\) Al-Treated & 32.76 \(\pm\) 0.89 & 4.13 \(\pm\) 0.33 & 12.18 \(\pm\) 1.38 & 6.36 \(\pm\) 0.48 \\ \hline \(+45^{\circ}/-45^{\circ}\) & 32.09 \(\pm\) 0.86 & 5.99 \(\pm\) 0.46 & 7.34 \(\pm\) 1.14 & 3.65 \(\pm\) 0.67 \\ \hline \(+45^{\circ}/-45^{\circ}\) Al-Treated & 34.05 \(\pm\) 0.62 & 3.81 \(\pm\) 0.20 & 12.63 \(\pm\) 1.29 & 4.47 \(\pm\) 1.52 \\ \hline Random & 31.61 \(\pm\) 0.69 & 6.33 \(\pm\) 0.27 & 7.73 \(\pm\) 0.49 & 3.98 \(\pm\) 0.46 \\ \hline Random Al-Treated & 33.56 \(\pm\) 0.76 & 4.17 \(\pm\) 0.22 & 10.91 \(\pm\) 0.82 & 5.67 \(\pm\) 0.61 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Impact properties of the SIRAL specimens. Figure 2: Impact test: (**a**) initial setup; (**b**) indentor; (**c**) overview of the drop tower machine; (**d**) fixtures for the samples. Figure 3 shows some typical load histories of the conditions tested. The treated Al panels reach the lowest maximum load and the highest absorbed total energy, deflection and time to maximum load (Table 1). In this sense, the treated aluminium skins provide a higher capacity for absorbing impact energy. This behaviour can be explained mainly by considering (i) the fragile interfacial bond between the skins and the core, and (ii) the high deflection of the core composite material. The impact load is first transferred to the skin material as normal stress, with subsequent shear deformations at the interface and core, similar to the bending behaviour. When a rigid and fragile bond at the interface is present, a sudden delamination occurs; thus, the core material is subjected to premature loading, causing it to deflect more than in the presence of a more ductile interface. The oscillation of the treated Al samples' reaction force indicates the fraying of the SSI fibres, which is commonly observed in natural reinforcements under normal stress [8; 25]. Additionally, especially for the case of the treated Al samples, the crack propagating along the interface consumes more energy at the beginning of the test, with a subsequent drop after delamination. This behaviour can be ascertained by observing the peak load (P1), with a more extended signal period typically associated with delaminated specimens [23]. In contrast, the untreated Al skin samples show a less rigid interfacial bonding with higher impact loads and lower energy absorption. The impact behaviour of the sandwich panels is generally opposite to the one observed during the static bending tests. Skin-core delamination [26; 27] and/or fibre pull-out [28] are considered to be between the main energy absorption mechanisms acting under dynamic loads. The treated SiRALs provide not only a premature delamination, but also larger deflections, including in the composite core.