# Impact Properties of Novel Natural Fibre Metal Laminated Composite Materials
## 4 Conclusions  
This work described the drop-weight impact properties of sisal fibre-reinforced aluminium laminates (SiRALs), and the main conclusions are:  
i. The ANOVA revealed that all individual factors and their interactions affect impact properties; however, aluminium treatment is the most contributing factor.  
Figure 6: Interaction plot representing the maximum load (**a**), the total energy (**b**) and maximum deflection (**c**).  
2. Sisal fibre orientation affects the fracture behaviour of the panels. Cores made with 0\({}^{\circ}\)/90\({}^{\circ}\) fibres lead to longitudinal fractures (90\({}^{\circ}\)). Randomly oriented fibre cores show an X-shaped fracture that extends to the end of the sample. Cores made of fibres oriented at \(\pm\)45\({}^{\circ}\) exhibit crack propagation along the diagonal direction of the laminate, with significant impact energy dissipation.
3. Treated aluminium panels sustain the lowest values of maximum loads, with higher levels of total energy, deflection and time to maximum load. This effect is mainly attributed to premature delamination between the core and the skins, including the wrinkling effect of the upper skin metal sheet.
4. In contrast, FMLs with untreated aluminium sheets provide the highest maximum impact loads with a less rigid surface bonding and reduced delamination effect.
5. Skin-core delamination and/or fibre pull-out are considered to be energy absorption mechanisms acting under dynamic loads.
6. There is no perforation of the bottom skins; however, macro-cracks are commonly present in the untreated aluminium samples.
7. Panels composed of cores made from random fibres or \(\pm\)45\({}^{\circ}\) sisal fabric along the untreated aluminium skins provide the lowest deflections.
8. Random sisal fibres induce the dissipation of impact energy over a larger area in different directions, leading to greater impact load.
9. SixRALs made with a random fibre core and untreated aluminium sheets give the highest maximum impact load (6.3 kN). However, FMLs with \(\pm\)45\({}^{\circ}\) fibre-oriented cores with treated Al skins provide the highest deflection (12 mm) and total impact energy (34 J).
10. Sisal fibre-reinforced aluminium laminates (SiRALs) are very promising in several areas of engineering. They could be used, for example, in civil construction sectors such as room partitions, signage and wall coverings. SiRALs combine sustainability, low cost and structural performance.  
Conceptualization, T.H.P. and J.C.C.R.; methodology, T.H.P.; formal analysis, L.M.G.V.; investigation, L.M.G.V., J.C.d.S. and Y.D.; resources, T.H.P., J.C.C.R. and F.S.; data curation, L.M.G.V.; writing--original draft preparation, L.M.G.V. and J.C.d.S.; writing--review and editing, T.H.P. and F.S.; supervision, T.H.P., J.C.C.R. and F.S.; project administration, T.H.P. and J.C.C.R.; funding acquisition, J.C.C.R. and T.H.P. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Brazilian Research Agencies, FAPEMIG (PPM), CAPES (PhD scholarship) and CNPq (PQ 309885/2019-1 and 394208/2019-1).  
No new data were created or analyzed in this study. Data sharing is not applicable to this article.  
The authors would like to thank the Brazilian Research Agencies, CNPq, FAPEMIG and CAPES, for the financial support granted, the University of Bristol and the Bristol Composites Institute for the support provided.  
The authors declare no conflict of interest.