Composite materials reinforced with carbon and glass fibres have grown in popularity over the last few decades due to their advantageous properties and decreased cost compared to their metal counterparts. However, the ever-worsening condition of our planet because of unprecedented climate change is becoming an increasingly pressing issue, and as a result, new environmental legislations have redirected research into more sustainable, cost effective, and environmentally friendly lightweight composite materials [1
There are different types of natural fibres that have been used in composite reinforcements. The commonly used fibres include hemp, flax, jute, kenaf, and sisal. Natural fibre-reinforced polymer composites are emerging as alternative materials in many non-structural engineering applications because of their higher specific properties (strength and modulus) and good ecological attributes compared to their conventional counterparts, such as carbon and glass fibre-reinforced composites. However, their susceptibility to moisture absorption, variation in their properties, poor mechanical properties due to weak fibre-matrix interface, and low fire resistance have been a hindrance for their use in high-performance structural applications [4
Many attempts have been made over the years to improve the fibre matrix interface and water repellence behaviour of natural fibre composites by using various processes including chemical and physical fibre treatment methods, such as [8
]. The findings from these works suggested that appropriate chemical and physical treatments such as alkalisation by using sodium hydroxide (NaOH), acetylation, cyanoethylation, silane coupling agent, and heating can reduce water absorption behaviour and improve mechanical and thermal properties of natural fibre composites through the enhancement of interfacial properties [11
In recent years, there have been a significant number of research works undertaken dealing with the use of hybrid techniques in order to improve the properties of natural fibre-reinforced composites [14
]. Hybridisation between natural and synthetic fibres is a technique in which the benefits of each material can be combined to achieve a composite that can demonstrate both higher mechanical performance and improved environmental features [16
]. The work carried out by Fiore et al. [17
] using basalt fibre as external hybridising material in flax/epoxy composites reported that because of a hybrid effect there was a significant improvement in flexural and impact properties. Recent works on hybrid effect by Almansour et al. [18
] and Li and Sain [19
] reported that natural fibre hybridised with glass and basalt fibre composites provided improved fracture toughness as well as stiffness and tensile strength. There were several reported works investigating the effects of glass and basalt fibres hybridisation on the performance of natural fibre-reinforced composites [20
]. These reports on hybrid composites revealed that both basalt and glass as hybrid materials played a synergic role in improving various mechanical properties (tensile, flexural, fatigue, and impact) and thermal properties (glass transition temperature, improved degradation behaviours) [26
Carbon fibres have become an important reinforcing material for many lightweight composite applications such as aerospace, automotive, and sports equipment due to their low density, high tensile strength and modulus, and lower susceptibility to corrosion (Table 1
]. However, carbon fibres are created from unsustainable fossil-based materials through energy intensive processes. As a result, products manufactured from carbon fibre-reinforced composites have a large carbon footprint [30
]. Through life cycle assessment techniques, it was shown that 1 kg of carbon fibre composite consumes up to 300 MJ of energy for production [31
]. In addition to this, limited recyclability and non-biodegradability of carbon fibre have become a growing concern when disposing of waste end of life products. Studies carried out by Das et al. [32
] highlighted that wood and biochar biocomposites exhibited highest mechanical properties (tensile and flexural) and improved fire resistant behaviour when compared to other waste biomasses. Additionally, hybridisation was used to improve mechanical properties as well as limiting oxygen index of waste-based biochar/wood hybrid composites [33
Due to their superior properties, carbon fibres are often hybridised with natural fibre composites in order to create composites with balanced properties. The work carried out by Dhakal et al. [34
] on hybridisation of carbon fibre into flax fibre epoxy composites reported that more ductile behaviour could be realised through hybridisation. Moreover, their work revealed that water repellence behaviour and thermal properties of flax fibre composite was significantly improved through hybridisation with carbon fibre. There is clear evidence from the literature that the hybrid approach can offer a synergistic effect and provide the best properties of each of the constituent components in the resultant composites. However, for the hybrid systems to be effective, the compatibility of hybridising constituents, how they fail and an understanding of their structural performances relating to hybridisation are important. In this regard, there are not many reported works highlighting the failure modes and damage mechanisms of hybrid composites. In particular, there are insufficient reported works investigating carbon fibre hybridisation with natural flax fibres, as well as analysing their synergetic effects on light-weight critical applications.
Flax (Linum usitatissumum
) is a widely used natural fibre. The average chemical composition of flax fibre is cellulose (71%), hemicellulose (19.6%), while other constituents are pectin (2.2%), lignin (2.2%), and wax (1.5%) [10
]. It is an attractive reinforcement material due to its several attractive properties such as specific tensile strength and modulus compared to conventional glass fibres. However, for natural fibre-reinforced composites to be used in light weighting semi-structural and structural applications such as automotive, marine, and aerospace, their mechanical properties such as tensile strength and modulus need to be improved so that the design specifications assigned by the Original Equipment Manufacturers (OEMs) are met and their damage mechanisms are understood.
A recent comprehensive review on the lightweight application of composites carried out by Pervaiz et al. [1
] argued that greenhouse gases (GHGs) generated by automotive vehicles counts for more than a quarter of all GHGs generated. Despite several drawbacks of carbon fibres in terms of their poor environmental performances, they strongly suggested that the lightweight and excellent mechanical properties of carbon fibre can help in reducing vehicle weight significantly, and hence help in reducing the overall CO2
emission. Taking this scenario into consideration and as a motivating factor, this study focuses on the experimental investigation into the effects of carbon fibre hybridisation on the tensile properties of flax fibre-reinforced epoxy composites. In order to understand the critical factors for improving mechanical performance of carbon/flax hybrid composites, the present work further assesses the damage mechanisms on the fractured surfaces of plain flax and plain carbon composites in comparison with the damage mechanisms of carbon/flax hybrid systems by using environmental scanning electron microscopy (E-SEM).