ABSTRACT
This study aims to review the application of carbon fiber reinforced plastics in automotive industry. CFRP is a composite material that has been used extensively in various applications such as aerospace industry, sports equipment, oil and gas industry as well as automotive industry. CFRP has better and outstanding specific stiffness, specific strength and fatigue properties compared to conventional metals. Thus, it is suitable to be used in application required load carrying structure. In automotive industry, the advantages of CFRP are reduction in weight, part integration and reduction, crashworthiness, durability and toughness as well as aesthetic appealing. However, there are some issues hinder the application of CFRP such as cost of material, production technology and also recyclability. This review paves a way for better understanding to apply CFRP in automotive in the future.
1. INTRODUCTION
In this modern time, carbon fibre reinforced plastic (CFRP) have been regarded as potential materials to substitute conventional metals in automotive application. It is due to the fact it has better and outstanding specific stiffness, specific strength and fatigue properties compared to conventional metals. Therefore, it is suitable to be used for application required load carrying structure such as fuselage and wings in aircraft structure as well as frame structure in automotive industry.
In terms of overall application, it was reported by the association of composites companies and research institutes Carbon Compsites e.V. (CCeV) that aerospace and defence were the largest consumers of carbon fibre followed by sports/leisure sector and wind turbines in the year 2013 as shown in Figure 1. Meanwhile, automotive application continued to grow more than double over the past year to approximately 5000 tonnes of carbon fibre demand. It could be due to ramp-up phase in the production of BMW‟s i3 model [1].
Figure 1: Global carbon fibre demand by application in thousands tonnes in the year of 2013 [1]
Furthermore, the automotive industry in 2012 recorded 5% (2150 tonnes) of global carbon fibre usage and it still continue to grow in following years. It is because the car makers are now seeking for carbon fibre to produce a car with lightweight, better fuel consumption and less carbon emission. According to The Global CRP Market 2013, the growth of carbon fibre usage for automotive industry is expected to be approximately 34% annually and it would reach to almost 23000 tonnes by 2020 as depicted in Figure 2. However, in order to achieve this target, it depends on the cost of CFRP component and the development of manufacturing process [2].
Figure 2: The forecast growth of carbon fibre used in the automotive market from 2010 – 2020 (in tonnes) in the year of 2013 [1].
Generally, composite materials applied in automotive structures are limited to secondary exterior structures such as body panels, wheel housing or bumpers. Body panels are commonly made from sheet moulding compound (SMC) which consists of thermoset matrix and discontinuous glass fibres and as opposed to bumpers and front panels that are made from discontinuous glass fibre reinforced thermoplastics. These types of composite materials provide good formability, high energy absorption and resistance to corrosion, thus suitable for the aforementioned application [3].
On the other hand, carbon fibres in particular, have also been used extensively in car application. It could be seen in the production of structural element, body panels, monocoques, passenger compartments and semitrailers. Apart from that, carbon fibres could also be found in the application of trucks, motorsports as well as passenger trains. Figure 3 illustrates the current application of carbon fibres according to parts or sub-segment of automotive parts [1].
Figure 3: Carbon composites revenues in US$ million in the automotive sector according to sub- segment in the year of 2013 [1].
Nonetheless, having said that, the application of CFRP especially in automotive is still hinder by some of the factors like material cost, manufacturing capabilities, supply and recycling issues. These factors result in CFRP not to be immediately adopted in conventional road vehicle manufacturing. Thus, the following discussion will elaborate some of the advantages as well as disadvantages of using CFRP in automotive application so that it would provide a better view on the future of this material.
2. ADVANTAGES OF CFRP
2.1 Reduction in Weight
The potential of CFRP as material for automotive parts has drawn attention from the industry due to its properties lightweight and superior specific strength and stiffness. Thus, it offers a great reduction in vehicle weight which in turn increasing the fuel efficiency and fuel economy as well reducing CO2 emission. It was reported that by replacing steel with CFRP would reduce the vehicle weight by 60%, increasing fuel efficiency by 30% and also cut the CO2 emission by 20% [4]. In other word, it is estimated that a reduction of 100 kg weight of vehicle would reduce the fuel consumption approximately by 0.3-0.4 litres per 100 km. Moreover, reducing the weight of vehicle body structure can lead to secondary weight reduction of other main systems such as chassis, gears and brakes [3]. In addition, the top speed of any super cars able to exceed 322 kilometre per hour (kph) and accelerates from 0 to 100 kph under four seconds, by applying CFRP in the structure [4].
Currently, a large amount of CFRP parts is used in high end sports vehicles which are produced approximately 500 units per year. A common example of CFRP component is CFRP monocoque as depicted in Figure 4 and engine carrier of Porsche Carrera GT. In addition, the passenger cell incorporates sandwich structure for either side of plastic or aluminium foam material to enhance its stiffness and lightweight. On the other hand, the engine carrier at the rear of the car is bolted onto passenger cell to form main load carrying structure [3].
Figure 4: The CFRP monoque structure used in the Porsche Carrera GT [3].
Figure 5: BMW Z22’s CFRP side frame [6]
There are some structural parts of automobiles are considered to employ CFRP. However, it is still under concept car and yet to be mass-produced. For instance, the BMW Z22 concept car uses CFRP for occupant compartment in addition to its body structure that has approximately 20 components including CFRP as side frame as illustrated in Figure 5. By introducing CFRP, the reduction of weight is nearly 50% as compared to conventional steel based material [3]. Furthermore, BMW M3 CSL car model utilises CFRP for a production of roof structure. The mass of CFRP roof structure is 6 kg (50%) lighter than the conventional roof and thus, the centre of gravity of the car is lower due to roof location [6].
Apart from that, the project financed by European Union involving three car companies namely Volkswagen, Renault and Volvo had developed CFRP floor panel for car as shown in Figure 6. The parts developed in this project contributed to reduce the weight of the car by 50% as well as lessen the number of parts by 30%. The technology was also able to produce 50 units per day by utilising technological advances in heavy tow carbon, preform technology and resin transfer moulding (RTM) [7].
Figure 6: The CFRP floor panel demonstrator part developed within the TECABS project [6]
Carbon fibre in the form of continuous fibre has the ability to tailor the specific properties according to the corresponding direction. It is called as anisotropy condition. Thus, the fibres can be oriented along the path of the highest stress. The thickness, on the other hand, can be locally tailored to the high or low stress areas and it is challenging to perform in a stamped sheet metal. Hence, these types of carbon fibres are 75% lighter than steel parts, 40% lighter than aluminium and 50 – 60% lighter than glass fibre sheet moulding compound [5].
2.2 Parts Integration and Reduction
Despite reduction of weight, the number of individual parts can also be reduced significantly if high-volume composite car concept is introduced. It can be achieved by integrating parts with the components of the car, for instance fasteners and supports integrated with other car components [4]. Although the shape of structural parts or body assemblies is complex, it still can be produced on one machine tool; hence translating into a substantial reduction in the number of individual parts required to produce the vehicle body.
A report by Technologies for Carbon-Fibre Reinforced Modular Automotive Body Structures (TECABS) in producing a composite BIW (Body in White) concept explained the number of parts that could be reduced to 30% by using complex shaped multi-functioning and smart joining technologies of carbon fibre. Thus, it is expected that by reducing the number of parts, less tooling and joining would be used in producing that concept car. BIW is the stage in automotive design or automobile manufacturing in which a car body's sheet metal components have been welded together [6]. In one of the research carried out by ACA (Automotive Composite Alliance), it shows that using low-density structural composite in one-piece moulding trunk compartments, for example, can reduce the mass up to 50%. The composite materials are also capable, in one assembly, to replace eight metal stampings and thus, saving up to 70% of tooling investment [8].
2.3 Crashworthiness
The ability to absorb the impact energy and be survivable for the passengers is called the crashworthiness of the structure in vehicle. It is defined as the potential of absorption energy through controlled failure modes and mechanisms that provides a gradual decay in the load profile during absorption [5]. Hence, the application of CFRP in automotive could improve the crashworthiness due its strength and stiffness [2]. The improvement in crashworthiness can be done by aligning 7 nm fibres based on the anticipated load path through a certain part or component. The technique that CFRP fibres are laminated is also plays important role in crashworthiness performance of CFRP [7]. German car maker, BMW also testified that CFRP produced by Zoltek (the commercial name is PANEX) exhibits exemplary crash behaviour when incorporated into carbon fibre composite body components [11].
In addition, there was also a research that had been carried out on the application of CFRP for joint technology in automotive as depicted in Figure 8. Structural adhesive by using CFRP was applied to seat anchor and seat belt anchor attachments. These two attachments are of critical for passenger safety especially during crash or impact event. Based on the results, CFRP showed an outstanding performance compared to conventional steel [12]. It was also reported that the safety feature of the CFRP BIW had demonstrated good energy absorption performance when the energy absorption of CFRP body was 1.5 times better than steel body. In order to accommodate the energy absorption, CFRP crush tube was developed and attached to the front side component of the body. Head-on collision proved that the CFRP crush tube absorbed more energy compared to the current design made from steel as illustrated in Figure 9 [12].
Figure 8: The example of anchor attachment mechanical test [12]
Figure 9: Energy absorption of CFRP during collision test [12]
Additionally, composite provides the utmost specific-energy absorption of all other structural materials. CFRP crush cones and other similar energy-absorbing structures can absorb 120 kJ/kg or 250 kJ/kg of energy approximately if they are made of a thermoset resin or thermoplastic resin respectively. This performance is far better than steel because the steel records 20 kJ/kg only [13]. Meanwhile, in door application, CFRP provides better option in the area that required great strength and/or stiffness. CFRP door design has an integrated side of impact beam; a component that assemble in the outer panel of standard door systems. This crash-relevant component comes with a woven and filled with integral foam that provides a flexible carbon fibre chamber profile [14].
2.4 Durability and Toughness
CFRP has proven its capability in extreme condition application such as military helicopter and jet fighters. It has infinite fatigue strength if the strain is maintained at reasonable values, for example 0.3%. The epoxy resins in CFRP enable the production of engine compartment and primary vehicle structure due to glass transition temperature of 150°C. Carbon fibre components have also successfully passed rough road durability, crash simulated hail testing and hot/cold slamming test [5].
On the other hand, the layered structure of CFRP, particularly in door design, is designed to withstand specific stresses while saving the part weight by 4 kg and 11 kg if compared to aluminium and steel respectively. CFRP provides the design potential of high precision in the shaping wall thickness, fibre orientation and layer structure without compromise the strength of the door. It is due to the fact that the fibre orientation can be accustomed without affecting the ultra-thin wall strength in other areas. Hence, it is feasible to dispense the reinforcement sheets used in standard systems against the stress caused by door lowering, wind forces and torsion in the door inner panel [14].
2.5 Aesthetic Appealing
The other aspect that makes CFRP comparable and better than bending sheet metal is the ability to realise essential styling which is long, sweeping curves that improve aerodynamics and enable the vehicle to stay intact on the road. Carbon fibre also relates with things that can move very fast, for instance, Formula 1 racing car, jet fighters and rockets. Putting carbon fibre onto a sports car delivers the same value to the driving enthusiast. It delivers a true performance advantage, and is still relatively rare, since it involves price premium that need to be considered. In some circumstances, the situation of carbon fibre is similar to the leather seat twenty years ago, a luxury that not everyone can afford. In application where carbon fibre is exposed and coated, the look is considered “high-tech” [5].
CFRP has the capability to be formed in any shape including large and complex shape. Its amenability enables to provide more stylish look on the exterior [7]. On the other hand, Hexcel has developed carbon fibres that are flat, lightweight and provide more uniform coverage than conventional non-crimp fabrics. This is a major advantage when producing thin composite structures requiring a cosmetic appearance. The material is applied for BMW M-series roof and it provides additional aesthetic appealing for the customer towards the car [15].
In addition, UK-based sports car manufacturer, TVR uses SP Systems‟ SPRINT CBS for its car body panels. This sand-able surfacing film makes the body panel easy for painting whereby it takes approximately 10 minutes/m2 with a 600 grit abrasive. Hence, the print-through of carbon fibre is not an issue anymore. Moreover, SP‟s testing has shown that the quality of the painted surface is maintained even with humid conditions and repeated cycling to temperatures of over 100°C [10].
3. DISADVANTAGES OF CFRP
3.1 Cost of Material
Nonetheless, one of the factors that contribute as technological barrier of CFRP to be used in automotive application is the cost of material. Even though the material cost have decreased to a certain satisfactory level, the conversion cost to turn the materials into the finished part is still high, resulting in part prices of $55 to $250 per kilogram [5]. CFRP is known for its performance in racing car, owing to its low weight, high stiffness and strength as well as the crashworthiness. But, in production car sector, CFRP has been neglected due to its cost, until recently. In 1992, when GM developed its carbon concept car, Ultralite 1992, the monocoque body weight was 191 kg and stiffer than steel. Nevertheless, the cost would be 18 times more expensive than the steel [10].
Approximately, 51% of the cost of carbon fibres is proportional to the cost of its precursors, namely PAN (polyacrylonitrile), rayon and petroleum pitch [13]. For PAN-based carbon fibre, the combination of the price of precursor and low volume production as well as specialised equipment has led to its high cost [16]. In fact, the cost of precursor is one of the major contributions to the higher cost of carbon fibres [17].
The limited availability of low cost precursors is a big issue with the industry since it influences the cost of producing carbon fibres. Thus, the carbon fibre‟s manufacturers, for example, Akzo Nobel and Zoltek have made a long-term supply arrangement by buying Courtaulds and Magyar Visoca, respectively, as their carbon fibre precursor suppliers. Hence, they would be able to produce low cost and high tow of commodity-grade carbon fibre. Zoltek also targets to produce carbon fibre at a cost of $5/lb by building production capacity ahead of demand in order to break "low demand, therefore high price, and therefore low demand" circle. Some of the existing textile acrylic fibre production capacity is able to be converted to produce precursor fibres of the quality needed for automotive applications. But, it depends on its type of production technology [17].
In addition, higher volume production of carbon fibre is needed to lower the unit capital and labour costs [16]. The research has also been focused on developing alternative precursors (such as renewable lignin) and alternative processing technologies (e.g., advanced oxidation reactors and microwave-assisted plasma technologies), which is believed could reduce the cost of carbon fibres production [13].
3.2 Issues in Production Technology
In carbon fibre processing, the fibres can be in the form of tows, roving, mats, weaves, knits and braids. Tows and roving could be cut to produce chopped fibre, whereas the other forms are in the form of continuous fibres. As for the automobiles application that optimised for low cost, the fibres are commonly in the form of either chopped fibres and randomly distributed or continuous forms such as random-strand mats. In various moulding, the fibres and resin could be either combined prior to insert in the mould or combined directly in the mould and are shaped in mould cavity [16]. These processing types can also contribute to the cost of carbon fibre and thus, becomes the barrier of carbon fibre growth [5].
The common technique of producing carbon fibre composites with pre-preg are by cutting the piles into flat shape and laminate layer by layer inside one sided of mould. After completing the layup, the laminate is covered by a separator film and a sealed vacuum bag placed over the mould. While vacuum removes air from the laminate, the external pressure is applied in a pressure vessel known as autoclave. The surrounding air inside the autoclave is elevated to a cure temperature setting and hold at that temperature before cooling off the mould. The finished part is then removed from the mould. Cure cycles including cooling and heating steps of Formula 1 and supercar components are 5 to 10 hours typically. Thus, it would take one to two days to build a vehicle. However, this cycle time is relatively at slow rate and less attention is given to improve the situation [5].
In addition, curing the pre-preg is relatively expensive and time consuming. Similarly, the autoclave process requires big capital and operating cost, whereas hand laminating process is labour intensive process that leads to increase in labour cost. Efforts have been in place to overcome these shortcomings by introducing a new resin with curing time is reduced nearly to 10 minutes. Yet, the cooling and heating of the mould is still time consuming. Apart from that, the mould is also developed in such a way that the layup and curing can be performed simultaneously. Thus, productivity is increased [5].
The improved process of autoclave is also introduced by eliminating the pressure in autoclave. However, it is done by developing new materials system such as semi-impregnated sandwich dry fabrics and interleaved with the resin films. But, the curing time is little longer to almost one hour. For the type of textile fabrics, liquid moulding technique such as Resin Transfer Moulding (RTM) and Structural Reaction Injection Moulding (SRIM) can be used as manufacturing technology. Nevertheless, the class A finishing of body panel is somewhat hard to achieve as well as minimum thickness requirement to facilitate the resin flow [5]. Tooling cost including integral injection points, sprues and heaters are also limit the advantage of RTM in carbon fibre production [10]. Woven fibres, on the other hand, have fibres that are crimped, and hence weakened in at least one-axis [10].
Surface finish has been an issue for composites using Sheet Moulding Compound (SMC) because it is difficult to remove “paint pops”. “Paint pops” is pinhole imperfections that can arise in painted surface due to slight outgassing of the company during high temperature spray operation. Thus, the alternative approach is to mould the colour inside the mould. Limited shelf life is also a problem that needs to be solved. It is echoed by the difficulty to ensure that the thickness of SMC‟s batches is not beyond the acceptable limit [10].
3.3 Recyclability
Another aspect that CFRP may face the challenges is the recyclability issue. In Europe and Japan, a significant emphasis is given to manage the End-of-Used Products (EOUPs) or vehicles that are commonly called as ELV (End-of-Life Vehicles). The ELV directive is bringing about intense changes in infrastructure associated with ELV recovery and recycling [18]. It is targeted to increase the recovery rate of materials from an ELV to 95% (by weight) by 2015.
A component-based application of composite materials leads to various types of resins used. By recycling the large number of different resins, it increases the dismantling costs and hence reducing the recovery value of car. These resins are not commonly recycled, instead landfilled as automotive shredder residue (ASR). In turn, it lowers the overall recycled content of a steel car [16]. Thermoplastics are more recyclable compared to thermosets even though the properties of both resins will degrade over time. Nonetheless, the amenability of thermoplastics on repeated thermoforming provides its advantage. Meanwhile, high fractions of dense carbon fibres can reduce the ability for recycling [10].
Grinding up the end-of-life material and sending it to landfill has little appeal due to non- degradability of carbon thermosets and the health and safety risks they posed [19]. Furthermore, the embodied energy of CFRP is among the highest compared to other materials as shown in Table 1. The embodied energy is defined as the total energy required in producing each constituent material including fibres and resins. Carbon fibre is more energy intensive due to the high temperatures required for graphitisation. As for resin, the embodied energy is intermediate between glass and carbon fibres. Hence, recycling resins environmentally may be an area to pay attention as well. [20].
In a survey carried out by The University of Michigan Transportation Research Institute to the automotive industry, one of major barrier in applying composite materials in vehicle components is recyclability. When composite applications are sufficiently abundant to generate large quantities of material for disposal, then recycling will indeed emerge large [21].
Table 1: Embodied energies of common composite constituent materials and two common
4. CONCLUSION
The application of CFRP in automotive industry was successfully reviewed. The benefit of CFRP in automotive is far more encouraging despite some of hindrance that may drag its potential. It is worthwhile to mention that the research that has been carried out will provide the stepping stone to exploit the potential of CFRP in automotive industry to the fullest.
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