Elsevier

Composite Structures

Volume 258, 15 February 2021, 113208
Composite Structures

Improving the mechanical properties of natural fibre reinforced laminates composites through Biomimicry

https://doi.org/10.1016/j.compstruct.2020.113208Get rights and content

Highlights

  • Helicoidal configuration reduces and delays natural fibre damage in NFRP laminate.

  • Stacking NFRP laminate helicoidally significantly improves its loading performance.

  • The large matrix damage in helicoidal NFRP laminate dissipates more impact energy.

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 environmental sustainability. Instead of focusing on the mechanical properties of natural fibers, this study demonstrates 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 different 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 configurations. Helicoidal NFRP laminates are also shown to absorb more energy under impact as compared to the cross-ply and quasi-isotropic NFRP laminates.

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], [4], [5] because NFRP components reduce cost by 20% and weight by 30% [3]. While most of these components are currently for automobile interiors, 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, aerodynamic 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], [9], [10], [11], [12], [13]. Bessa [10] studied various combinations of surface treatments. These included cleaning with water, treatment with alkali, coupling of functional groups like benzoyl, amino and epoxy groups and corona treatment. 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 laminates 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. Undoubtedly, 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], [15], [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 considering the size of the striking club, which is typically 10 mm long [19]. The exoskeleton of mantis shrimp consists of chitin fibers and a mineral matrix [20] (analogous to NFRP laminate). The microstructure of the enlarged heel comprises a helicoidal arrangement of laminates of these chitin fibres [21].

Research inspired by the helicoidal microstructure in crustaceans has found that stacking unidirectional carbon fiber plies helicoidally leads to an improvement in out-of-plane loading and impact performance [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33]. Liu et al. [26] conducted out-of-plane loading of helicoidal laminates with inter-ply angles ranging from 2.5° to 20° and reported that spiraling matrix splitting was the dominant damage mechanism in helicoidal laminates with small inter-ply angles. Fiber damage seen in common cross-ply and quasi-isotropic laminates was suppressed by the spiraling matrix split. Mencattelli and Pinho [23], [33] conducted low-speed impact tests on thin-ply carbon fiber helicoidal laminates and also observed spiraling matrix splits in helicoidal laminates. They reported that helicoidal laminates with smaller inter-ply angles showed less fiber damage.

Since fiber damage appears to be reduced in helicoidal carbon fiber reinforced laminates under out-of-plane loading and impact, the helicoidal configuration may also improve fiber damage resistance in NFRP laminates where the relatively weak natural fibers are more prone to damage. To the authors’ knowledge, there is no report on whether helicoidal configuration can be an effective alternative to strengthen the mechanical performance of NFRP laminates. Experiments are conducted in this study to determine the out-of-plane loading and impact performance of helicoidal flax fiber-epoxy laminates together with cross-ply and quasi-isotropic laminates as reference.

Section snippets

Specimen preparation

Flax-epoxy composites were fabricated using FLAXPREG-T-UD-110 supplied by Eco-Technilin. It is a pre-impregnated material based on an epoxy resin system and pre-treated unidirectional flax fibres with an area density of 220 g/m2 consisting of 50% flax fibre and 50% Huntsman XB3515 epoxy resin by mass. The material properties of FLAXPREG-T-UD-110 are provided in Table 1.

Individual plies of unidirectional FLAXPREG-T-UD-110 prepregs were cut into 100 × 100 mm squares and laid manually to the

Out-of-plane loading results

Fig. 3 shows the out-of-plane load vs displacement curves of the natural fiber specimens. It is seen the average peak loads of helicoidal laminates increase sharply as inter-ply angles increases from 3° to 9° (3H, 6H and 9H), the peak load then decreases gently as inter-ply angle increases further from 18° to 45°. It is seen 9H laminates significantly outperform the cross-ply and quasi-isotropic laminates in terms of peak load. In addition, the 9H specimens can also sustain a larger deflection

Conclusion

By adopting the Bouligand structure found in the exoskeleton of crustaceans, this study shows that the helicoidal configuration can significantly improve the out-of-plane strength of NFRP laminates compared to the common cross-ply and quasi-isotropic configurations. The spiralling matrix split, a distinct feature reported for helicoidal carbon fibre reinforced laminates, was also observed in the helicoidal NFRP laminates.

Helicoidal NFRP laminates were found to be more resistant to fiber damage

CRediT authorship contribution statement

Enquan Chew: Resources. J.L. Liu: Resources. T.E.Tay: Resources. L.Q.N. Tran: Resources. V.B.C. Tan: Resources.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

The support through SIMTech-NUS Natural Fibre Composites Jointlab is gratefully acknowledged.

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