# Impact Properties of Novel Natural Fibre Metal Laminated Composite Materials
## 3 Results and Discussions
### Impact Properties
Premature delamination and deflections contribute to absorbing more impact energy. In contrast, the untreated skins withstand larger impact loads because of the less premature delamination occurring in those samples, with a consequent reduction in bending deflection. Figure 4 shows examples of delaminated samples made with treated (a-c) and untreated (d-f) aluminium skins after impact. There is no evidence of perforation of the bottom skins; however, macro-cracks are present in the samples with untreated Al skins. The damage induced by the impact on the SiRALs can be divided into two categories: (i) visible damage in the form of local plastic indentation and (ii) internal damage represented by fibre failure, matrix cracking and delamination. SiRALs exhibit the distinctive characteristics of impact deformation and cracking (see Table 3). Treated Al sheets provide larger deflections, including wrinkling of the upper skin metal sheet; the latter deformation mechanism is mainly attributed to premature delamination. The area marked in black in Figure 5 highlights the cracks for each core material reinforced with fibres oriented at 0\({}^{\circ}\)/90\({}^{\circ}\) (Figure 5a), \(\pm\)45\({}^{\circ}\) (Figure 5b) and randomly (Figure 5c). Sisal fibre composites oriented at 0\({}^{\circ}\)/90\({}^{\circ}\) show a crack propagation along the longitudinal direction of the sample (Figure 5a). Cores made with \(\pm\)45\({}^{\circ}\) fibre orientation show a diagonal crack following the 45\({}^{\circ}\) fabric orientation (Figure 5b). X-shaped cracks are evident in those cores reinforced with random fibres (Figure 5c). Therefore, random sisal fibres appear to dissipate the impact energy over a larger area in different directions and sustain the highest impact load (Table 1). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Experimental Factors**} & \multicolumn{2}{c}{**Total Energy**} & \multicolumn{2}{c}{**Total Deflection**} & \multicolumn{2}{c}{**Max Load**} \\ \cline{2-7}  & \(p\)**-Values (\(\leq\)0.05)** & \multicolumn{1}{c}{**F-Value**} & \(p\)**-Values (\(\leq\)0.05)** & \multicolumn{1}{c}{**F-Value**} & \(p\)**-Values (\(\leq\)0.05)** & \multicolumn{1}{c}{**F-Value**} \\ \hline
**Fibre orientation (FO)** & 0.005 & 14.25 & 0.009 & 11.43 & 0.002 & 20.57 \\ \hline
**Al Treatment (Al T)** & 0.000 & 174.46 & 0.000 & 276.29 & 0.000 & 940.27 \\ \hline
**FO \(\times\) Al T** & **0.030** & 6.69 & **0.013** & 9.92 & **0.002** & 21.65 \\ \hline
**R\({}^{2}\) (adj)** & 95.05\% & & 96.62\% & & 98.93\% & \\ \hline
**Ryan-Joiner** & 0.057 & & \textgreater{}0.100 & & \textgreater{}0.100 & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Analysis of variance (ANOVA). Figure 3: Typical impact curves for SiRALs. Figure 4: Bottom skins after test: (**a**–**c**) treated Al and (**d**–**f**) untreated Al. Figure 5: Typical cracks for (**a**) \(0^{\circ}/90^{\circ}\), (**b**) \(\pm 45^{\circ}\) and (**c**) randomly oriented fibre cores. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Type** & **Front** & **Rear** & **Side** & **Core** \\ \hline
0/90 & & & 5 mm & \\ Pre-Al-treated & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Impact damage of SIRALs.