PSI - Issue 68

Robert Lowe et al. / Procedia Structural Integrity 68 (2025) 173–183 R. Lowe et al. / Structural Integrity Procedia 00 (2025) 000–000

174

2

lightweight properties of natural fibres, making them an eco-friendly alternative to synthetic reinforcements. Compared to carbon and glass fibres, these materials are more environmentally friendly with reduced emissions and carbon sequestration due to their natural origins (AL-Oqla and Sapuan, 2014). However, NFRCs also come with some drawbacks compared to traditional fibres, with poor fibre matrix interaction due to their hydrophilic nature and high variability in material quality (Dittenber and GangaRao, 2012). Nonetheless, with increased concern surrounding the impacts of climate change and sustainable manufacturing, natural fibres are seen as a key part of the future of composite materials. Traditionally, engineered materials such as glass and carbon fibres have been used as reinforcing components within composites. However, though offering high mechanical performance, these materials require vast amounts of energy and create large amounts of emissions during their manufacture (Pil et al., 2016). Furthermore, due to the thermoset nature of the epoxy matrices generally used in fibre-reinforced composites, they are incredibly difficult to recycle and though holding good end-of-life value they are generally sent to landfill or incinerated (Piñero Hernanz et al., 2008). Due to the energy and emissions-intensive manufacturing process associated with traditional fibres such as carbon and glass, natural fibre-reinforced composites are being investigated as possible substitutes (Pil et al., 2016). These materials are favoured for their sustainability and recyclable nature when combined with acid cleavable bio-resins (Ferrari et al., 2021; Saitta et al., 2022). Though displaying strong potential, the continued application of natural fibre-reinforced composites is limited due to the poor interlaminar fracture toughness of the materials (Prasad et al., 2019). Composite materials have good mechanical properties with low densities (Quan et al., 2020b). The strength of composites comes from the interaction between the fibres and the epoxy, with the ability of the epoxy to transfer stress to the fibre critical to the overall performance of the material (Quan et al., 2020b). Composite materials have good in plane mechanical properties, but poor out-of-plane or through-the-thickness properties (Prasad et al., 2020). The highly anisotropic nature of composites comes from their laminate nature and poor interlaminar strength (Clyne and Hull, 2019). The main failure mechanism of these composites is the separation of the plies within their structure known as delamination. This failure mode severely limits the length of service life of composite materials (Beylergil et al., 2017). Many methods to improve interlaminar strength exist, while each comes with its drawbacks, and thus strengthening methods must be carefully chosen with end use, manufacturing capability and cost in mind. The mechanical performance of composite materials is reliant on the strength of the fibre matrix bond, with poor adhesion between the two leading to reduced mechanical properties and a shortened lifespan (Goriparthi et al., 2012; Prasad et al., 2020). Through thickness stitching method improves the interlaminar fracture properties of the composites. This method involves stitching the fibre to provide additional reinforcement in the z-direction to increase the interlaminar fracture resistance of the composite (Ravandi et al., 2016). Similarly, the Z-pinning method uses fine nail-like z-pins to bind the layers and reinforce the out-of-plane direction through friction and adhesion (Mouritz, 2007). Both these techniques improve the interlaminar fracture properties but reduce the in-plane properties of the composites. The surface of the fibre can be modified to improve adhesion through both chemical and physical treatments (Goriparthi et al., 2012). The physical methods include plasma or UV treatments, which use high-energy beams to improve the fibre's surface energy, resulting in increased fibre matrix adhesion and improved fracture resistance. These methods require surface preparation involving high energy requirements and a limited time gap between surface treatment and composite manufacturing to get the best use of these methods. (Goriparthi et al., 2012) studied the effect of chemical treatments on jute and found that while surface treatments lead to improved tensile and flexural properties they also reduce the impact resistance of the composite(Goriparthi et al., 2012). They found that though treatments reduced the mechanical properties of the fibres, the improved adhesion between the fibre and matrix resulted in treated composites having improved tensile properties (Goriparthi et al., 2012). (Prasad et al., 2020) investigated the effect of grafting TiO 2 nanoparticles onto flax fibres on the Mode I and Mode II interlaminar fracture toughness of the composite. Fibres with TiO 2 addition at 0.4 and 0.6 wt.% achieved 37% and 24% improvements under Mode I and Mode II loading respectively. The improvement in Mode I values was attributed to an improved fibre-matrix interface, while under Mode II conditions the nanoparticle addition was found to encourage the formation of hackles improving fracture toughness. One of the main disadvantages of introducing second-phase modifiers into the matrix is a lack of scalability due to difficulties with particle dispersion during mass production (Prasad et al., 2020). Additionally, unless particles are at the nano or small micro scale, the viscosity of the resin can be greatly increased with second-phase modifiers which may result in fibre modification being a more attractive alternative (Beylergil et al., 2017; Kinloch et al., 2016; Prasad et al., 2020).