# Impact Properties of Novel Natural Fibre Metal Laminated Composite Materials
## 1 Introduction  
Fibre metal laminate (FML) is a hybrid composite used in various applications, ranging from aerospace to construction fields. FMLs are based on the combination of polymeric layers from different fibre-reinforced laminates and thin metal sheets [1]. Classical aluminium alloys possess low damage tolerance and fatigue properties, but those can be enhanced by designing hybrid structures consisting of fibre-reinforced composites and metal layers [2]. During the last two decades, many FML concepts have been developed based on different polymeric reinforcements. Amongst them, it is worth mentioning commercial FMLs, such as ARALL (aramid), CLARE (glass) and CARALL (carbon) [3]. Several research groups have also expanded the FML baseline design in recent years. The authors of [4] introduced the use of Co-Fe electric steel skins with magnetically susceptible properties for electrical machine applications. [PERSON] and [PERSON] [5] have developed a new thermoplastic matrix FML based on a self-reinforced polypropylene manufactured in a simple compression moulding process. This particular type of FML has been investigated to assess its impact behaviour and general mechanical properties. The demand for sustainable and eco-friendly materials has driven the development of FMLs based on natural fibres [6; 7; 8]. The use of natural fibres in FMLs, such as mixed jute/carbon [9] and bamboo [10], has shown promising behaviour for different structural applications [3]. In a previous work [11], the authors evaluated the mechanical properties of bidirectional SiRAL (0/90) for potential applications in the transport industry. Recently, two FMLs made from natural fibre composites were investigated under tensile and bending loading [12; 13]. A kenar bastfibre-reinforced polypropylene composite bonded to aluminium skins was developed by [PERSON] et al. [12], and 3D jute fibre composites with aluminium skins were evaluated by [PERSON] et al. [13]. The chemically treated kenaf/PP FMLs showed a significant increase in terms of tensile modulus and flexural strength (-74%) compared to the laminate composites with the chemically untreated kenaf fibres [12]. Jute-based FMLs with an interlocked 3D woven fabric architecture showed consistent larger mechanical properties than analogous jute fibre-reinforced composites [13]. Low-kinetic energy impacts are a likely occurrence during the service life of a structural composite. The resulting damage generated by these impacts is generally in the form of delamination. If the impact energy is large enough, fibre breakage and matrix failure can also be observed. Most low-velocity impact studies have been performed using hemispherical impactors [14; 15]. Impact resistance is one of the most critical issues related to composite structures [2]. The resistance to impact perforation of a thermoplastic (polypropylene)-based fibre metal laminate was investigated, showing superiority to its monolithic aluminium and thermoset-based counterparts [5]. [PERSON] et al. [16] analysed the impact response and post-impact fatigue behaviour of GLARE compared to 2024-T3 monolithic aluminium. The dynamic failure of fibre metal laminates was also investigated by [17]. Three types of woven fibre fabrics (basalt, S2-glass and Kevlar-29) were used to reinforce the aluminium sheets. Different deformation/failure modes of the FMLs were obtained by varying the impact energy. The results showed that the total deformation increased with the impact velocity. An evident reduction in total deformation could be observed after penetration. Damages to the FML panels, such as tearing of the aluminium by, fibre fracture, matrix cracks and debonding/delamination, were also detected around the impact region. The results indicated that basalt-reinforced aluminium laminates perform very well in terms of impact resistance. Autoclaved carbon and glass FMLs (steel skins) have been tested under low-velocity impact [18]. These FMLs have been evaluated by considering the influence of different fibre stacking sequences.