Improving the mechanical properties of natural fibre reinforced laminates composites through Biomimicry Enquan Chew a , J.L. Liu a,⇑ , T.E. Tay a , L.Q.N. Tran b , V.B.C. Tan a,⇑ a Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, 117565, Singapore b Singapore Institute of Manufacturing Technology, Agency for Science, Technology and Research (A*STAR), Singapore ARTICLE INFO Keywords: Natural fiber laminates Biomimicry Damage mechanism Out‐of‐plane loading Out‐of‐plane impact ABSTRACT Although they have lower stiffness and load bearing capabilities than composites reinforced with synthetic fibers, there is increasing attention on natural fiber reinforced plastic (NFRP) laminates because of their envi- ronmental sustainability. Instead of focusing on the mechanical properties of natural fibers, this study demon- strates that the mechanical performance of NFRP laminates can be improved by adopting a helicoidal laminate stacking configuration that is found in the exoskeletons of crustaceans. Helicoidal NFRP laminates with differ- ent inter‐ply angles were fabricated from flax‐epoxy prepreg and tested under out‐of‐plane and impact loads. The results show that a helicoidal configuration with 9° inter‐ply angle improved the peak load of the NFRP laminate by 72% and 52% respectively when compared against the common cross‐ply and quasi‐isotropic con- figurations. Helicoidal NFRP laminates are also shown to absorb more energy under impact as compared to the cross‐ply and quasi‐isotropic NFRP laminates. 1. Introduction Natural fiber reinforced plastic (NFRP) laminates have drawn increasing interests as an environmentally sustainable material. Not only are they renewable and biodegradable, they also require less energy to produce compared to synthetic fibers like glass and carbon [1,2]. The automotive industry has already started to adopt NFRPs [3–5] because NFRP components reduce cost by 20% and weight by 30% [3]. While most of these components are currently for automobile inte- riors, there has been a push to incorporate NFRPs on exterior structural parts as well. For example, flax composite doors and spoilers have been installed on the Porsche 718 Cayman GT4 Clubsport [6]. Exterior automobile parts are designed to withstand impacts from debris, aero- dynamic loads, drag and crashes. As natural fibers are mechanically weaker compared to synthetic fibers, NFRPs are mechanically more inferior to their synthetic counterparts [7]. Improving the out‐of‐ plane loading and impact performance of NFRPs is important for the wider application of NFRPs in the automotive as well as other industries. Current research into strengthening NFRPs is mainly through the way the fibers are processed such as fiber treatment [8–13]. Bessa [10] studied various combinations of surface treatments. These included cleaning with water, treatment with alkali, coupling of func- tional groups like benzoyl, amino and epoxy groups and corona treat- ment. These combinations of treatments have shown mixed results in improving the mechanical performance of NFRPs. Fiore [11] identified that sodium bicarbonate‐treated flax‐epoxy laminates showed a 20.9% improvement in flexural strength compared to untreated ones. Huner [12] also found that NaOH‐treated flax‐epoxy composites had 42% higher flexural strength compared to untreated ones while Wu [13] reported that vinyltrimethoxy silane‐treated flax‐β‐polypropylene lam- inates flax fibers had 144% higher flexural strength compared to untreated ones. However, the treatment led to a 19% and 28% decrease in tensile strength and impact energy absorption. Undoubt- edly, these treatments will continue to play a significant role in improving the performance of NFRPs. The mechanical performance of NFRP laminates can also be improved through the structural design of the laminate [14–16]. Such an approach has long been adopted in nature. The peacock mantis shrimp is a crustacean with a unique attack mechanism. The enlarged heel of its second thoracic appendage is used to strike at targets in a rapid swinging motion with a maximum velocity of 23 m/s and an average peak force of 226 N [17,18]. Such a force is extraordinary con- sidering the size of the striking club, which is typically 10 mm long [19]. The exoskeleton of mantis shrimp consists of chitin fibers and https://doi.org/10.1016/j.compstruct.2020.113208 Received 8 August 2020; Revised 26 September 2020; Accepted 22 October 2020 Available online xxxx 0263-8223/© 2020 Elsevier Ltd. All rights reserved. ⇑ Corresponding authors. E-mail addresses: e0011565@u.nus.edu (J.L. Liu), mpetanbc@nus.edu.sg (V.B.C. Tan). Composite Structures xxx (2020) 113208 Contents lists available at ScienceDirect Composite Structures journal homepage: www.elsevier.com/locate/compstruct Please cite this article in press as: Chew E et al. Improving the mechanical properties of natural fibre reinforced laminates composites through Biomimicry. Compos Struct (2020), https://doi.org/10.1016/j.compstruct.2020.113208