Eco-Efficiency Performance for Multi-Objective Optimal Design of Carbon/Glass/Flax Fibre-Reinforced Hybrid Composites
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1. Introduction
Fibre-reinforced hybrid composites are created by reinforcing a matrix with two or more types of fibres. Previous research [1] has demonstrated that, for layered composite materials, the flexural strength can be enhanced through the hybridisation of carbon and glass fibres. Additionally, the inclusion of higher strain-to-failure glass fibre plies has been found to improve the strain-to-failure [2]. The observed hybrid effect holds the potential to be a valuable strategy for achieving a well-balanced composite material that optimises both the cost and weight.
Natural fibre-reinforced composites have garnered significant research interest due to their numerous advantages. These composites, characterised by their lightweight nature, cost-effectiveness, abundant raw materials, and excellent recyclability, offer a compelling solution for various applications. The exploration of natural fibre composites presents a promising avenue for addressing the recycling of agricultural residues, thus contributing to sustainable waste management practices. Research investigations have demonstrated the potential of natural fibre composites to replace conventional glass fibre composites in a variety of applications [3].
Two crucial design objectives are the minimisation of the weight and cost. These objectives often conflict with each other, necessitating a trade-off. The optimisation challenge aimed at minimising both the cost and weight of composites is referred to as a multi-objective optimisation problem.
In our previous research, we employed NSGA-II (Non-Dominated Sorting Genetic Algorithm II) to minimise the cost and weight of both unidirectional [4,5] and multidirectional [6] carbon/glass fibre-reinforced hybrid composites. These studies involved determining the flexural properties of composites through an analytical approach based on the principles of cross-laminated timber (CLT). However, the application of NSGA-II coupled with finite element analysis (FEA) rendered the optimisation infeasible due to excessive time consumption. To address this, a previous study introduced a design rule-based optimisation approach for carbon/glass fibre-reinforced hybrid composites [5]. This approach involved developing a set of design rules based on theoretical and numerical analyses. By employing these design rules, various stacking configurations were generated. The connection between the flexural strength and fibre volume fractions was established using FEA and regression analysis. To meet the specified minimum flexural strength, an optimisation process was carried out for the hybrid composite under flexural loading, with the primary goals being the reduction in both the cost and weight.
While carbon and glass fibre-reinforced hybrid composites have excellent mechanical properties, their environmental impact is a concern. On the other hand, flax fibre composites are more sustainable but face challenges in terms of supply and performance. Hybrid composites that combine these materials aim to balance performance and sustainability. A study has found that a carbon/flax hybrid system is 15% cheaper, 7% lighter, and displays 58% greater vibration damping qualities over a full carbon fibre composite [7]. Flax has a higher fibre content, which causes less pollution in the synthetic polymer matrix, and is significantly lighter, which may reduce the amount of driving fuel required for transporting the fibres and their applied components [8].
Life cycle analysis (LCA) is a tool used to assess the environmental impacts associated with all stages of a product’s life, from raw material extraction through materials processing, manufacture, distribution, use, repair and maintenance, and disposal or recycling [9]. This approach, often referred to as a “cradle-to-grave” analysis, helps to provide a comprehensive view of the environmental aspects of the product and its potential impacts [10].
In the context of composites, LCA is particularly relevant. Composites, especially green composites made of natural materials, are claimed to have lower negative environmental effects due to their sustainability and easier recyclability. However, to substantiate these claims, a thorough LCA is needed [11].
Flax fibre is suitable for a particularly low-density reinforcement as it has a high resin uptake that makes the laminates considerably thicker, for a given weight of reinforcement, than would be for commonly used carbon or glass fibre. The environmental aspect of the use of flax is that it is a natural plant fibre that uses production methods with low environmental impacts, and it requires no irrigation [11]. The LCA of Dissanayake [12] found that flaxes are better sustainable alternatives to glass fibres for the reinforcement of polymer matrix composites. However, this may not always be the case as the LCA conducted by Deng and Tien [13] found that flax polypropylene floor mats (mat-PP) have 0.8–2 times higher environmental impact values than the glass mat-PP in most environmental impact categories over the production and end-of-life (EoL) phases due to the use of less-efficient technologies in flax cultivation and fibre processing in China. Similarly, Jacobsson [14] found that the production and use of fertilisers contribute to 70–90% of the total life cycle environmental impacts of flax fibre production in Sweden. It appears that the environmental impacts of flax fibre vary across regions.
It is shown from the literature that no research has been conducted for the optimisation and LCA for the carbon/glass/flax fibre-reinforced hybrid composite. The novelty of this study is to explore the sustainability benefits of the use of carbon/glass/flax fibre composites, which are structurally sound and meet the standard or technically feasible specifications, but their environmental and economic implications warrant further investigation to come up with a decisive strategy. In addition, the methodology or the framework that is considered in this paper is applied for the first time to this composite material-based research to determine the eco-efficient options. Lastly, to the best of our knowledge, this is probably the first study on the carbon/glass/flax hybrid composite, which aims at combining the advantages of carbon/glass and carbon/flax hybrid composites.
Thus, this study aims at filling this technical gap. It should be noted that a region-specific study on the LCA of the use of flax fibre in hybrid composites is important for Australia as no such study has been carried out in Australia. Also, it is equally important to assess the economic viability of the use of flax in hybrid composites to find out the mix with the lower environmental and economic impacts. An eco-efficiency framework has been utilised as it helps integrate the environmental cost results resulting from the LCA analysis to determine the eco-efficiency performance of the hybrid composites. This study is the first of its kind in Australia, as it applies the eco-efficiency framework under Australia’s conditions to assess the eco-efficiency performance of hybrid composites reinforced by flax as opposed to carbon and glass fibres.
3. Results and Discussion
For comparison, the selected candidates for the carbon/glass/flax fibre-reinforced hybrid composite from the optimisation are shown in Table 9 and Table 10. The selected candidates were given codes for facilitating the analysis and interpretation of results.
Figure 4 shows that the increase in the use of flax in the hybrid composite materials of 1000 MPa can help achieve eco-efficiency by reducing the use of expensive and energy-intensive carbon/glass fibre. The specimens without flax are not eco-efficient as they are below the diagonal lines. Most of the specimens using flax are eco-efficient as they are above the diagonal line. Only two specimens that use flax (i.e., A and B) were not found to be eco-efficient due to having a lesser amount of flax. In the case of hybrid composites of 1300 MPa, all composites using flax are eco-efficient as the use of a higher amount of energy- and carbon-intensive carbon/glass fibre can be avoided at a higher flexural strength.
There are mainly two reasons why the hybrid composites using flax are eco-efficient. Firstly, the hybrid composites using flax are cheaper than those without flax. The average cost of hybrid composites with a flexural strength of 1000 MPa using flax is 7% cheaper than those without flax. For the hybrid composites with a flexural strength of 1300 MPa, the hybrid composites without flax have a 13% higher cost than those without flax, resulting from the higher cost of carbon and glass fibres (Table 4 and Table 5). Secondly, the hybrid composites without flax have higher environmental impacts than those without flax (Table 11). The hybrid composites using no flaxes have 12% to 13% more environmental impacts than the ones with flaxes. The impacts that are mainly responsible for increasing the overall impact are global warming impacts (55%), photochemical smog (13%), and acidification (11%) (Figure 5). Figure 5 shows the breakdown of impacts based on the average values of the impacts for hybrid composites with a flexural strength of 1000 MPa. Other studies also found GWPs as the dominant impacts for both flax and glass/carbon fibre-reinforced hybrid composites [13,27].
The GWP or carbon footprint has been found to be the hotspot and the replacement of carbon/glass fibre with flax-reinforcing materials can reduce GWPs by 12.5%. This carbon footprint-saving potential of flax-reinforced hybrid composites has a significant bearing on achieving the net zero emissions target.
4. Conclusions
The breakthrough of the research in this study is that it could probably be the first study on the carbon/glass/flax hybrid composite, which aims at combining the advantages of carbon/glass and carbon/flax hybrid composites. Hybrid composite materials using flax as a reinforcement material have been found to have a better eco-efficiency performance than those using conventional carbon/glass-reinforcement materials for different flexural strengths under the Australian situation. This study proved that bio-based reinforcing materials could not only be a suitable substitute for conventional ones like carbon and glass fibres but they also have sustainability benefits, as confirmed by the eco-efficiency analysis that was applied for the first time in this material science research to the best of our knowledge. Both the economic and environmental benefits of the use of flax in hybrid composites increase with the increase in the flexural strength. Flax fibre-reinforced hybrid composites have been found to have a lower carbon footprint compared to ones using carbon/glass for reinforcing. This will assist manufacturers to achieve their net zero emissions targets.
Future research should consider durability and fatigue tests in order to determine the service life or longevity of these hybrid composite materials, as it is a determinant of resource efficiency in a resource-constrained world. In addition, social impacts can be carried out to assess the overall sustainability implications of the use of bio-based materials in hybrid composites as a replacement for non-renewable and carbon-intensive materials.
In addition to the above, a future study could consider the recyclability aspect of this hybrid composite material in an LCA study. It is a limitation of this study as it considered the “cradle to gate” approach or did not consider impacts during the use and end-of-life stages of the LCA for these hybrid materials. Usually, most (up to 90%) FRP waste will end up in a landfill as this is deemed to be the economically viable option. There are mechanical, thermal, and chemical recycling processes although the mechanical crushing of FRPs currently seems to be the only viable option for industrial applications [28]. However, the advantage of the use of flax fibre in this situation is that it is biodegradable, and thus improves the overall recyclability of the composite as it decomposes naturally over time, reducing the amount of non-degradable material (such as carbon and glass fibres) that would otherwise have remained in the environment [29].
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