Carbon Fibre Composites Compared with Traditional Metallics
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Introduction
According to Elaheh Ghassemieh (2011), the automotive industry has experienced lots of changes that occurs by day through the application of composite materials in the manufacturing of motor vehicle parts and body. Several reasons have been advanced in support of this shift from the traditional use of metals. In comparison to the metallic counterparts, many composite materials exhibit relatively greater strength characteristics as compared to the metallic materials. They are also comparatively lighter than the metals and thereby reducing the fuel cost per passenger in the vehicles. It is also believed that composites exhibit higher resistance to fatigue from repetitive use and thus reducing the maintenance cost of the vehicles and increasing the usage time.
In reference to Long, A. C. (2005), the composite material can be defined as a material consisting of strong carry-load materials (reinforcements) embedded in a relatively weaker material (matrix). The purpose of the reinforcement is to provide the strength stiffness, rigidity and mechanical properties needed to support the structural load. The matrix on the other hand acts to provide a fixed orientation of the reinforcement and in many cases is more brittle.
Question 1
Advantages of carbon fibre reinforced polymers in over metallics
Carbon fibre reinforced polymers (CFRP is one of the classes of the composite materials that are used in vehicle construction industries; it provides far-reaching advantages and is outdoing the traditional metallics. Some of the advantages are discussed below.
In reference to Elaheh Ghassemieh (2011), they are light in weight compared to the metals, and this is of importance because of the increased emphasis that is being laid by environmentalists for the reduction of the greenhouse gases through improved fuel efficiency. Many car manufacturers, component producers and even assemblers are investing heavily in research of lightweight materials with CFRP being a focal point for most research works. This is with the aim of moving towards the use of lightweight materials for better fuel efficiency and reduced operating cost. Most composite materials however are expensive, and this is hindering the transit from the utilization of the heavy metals to the light weight composites. Many farms in the vehicle industry are thriving by day to develop lightweight materials that are affordable to enable a shift from heavy materials to light materials, this is according to Elaheh Ghassemieh (2011). A reduction in weight is seen as the most efficient means of reducing fuel consumption and also the production of greenhouse gases in the transport sector. Numerous works have shown that a 10% weight reduction leads to an improved fuel economy by about 7%. Further for every kilogram of weight reduced it is estimated that there is a decrease in carbon dioxide emission by 20 kg.
In line with Long, A. C. (2005), CFRP provides for higher safety and crashworthiness factors compared to the metallic counterparts, and this is is critical in the design and construction of vehicles. By definition, crashworthiness can be said to be the potential to absorb energy through controlled failure modes and mechanisms that provide for gradual decay in the load profile during absorption. The design requirement by law in automobile industry provides for an impact at speeds up to 15.5 m/s with solid immovable objects, the passengers are not suppose to experience a force producing a net deceleration that is greater than 20 g. In the light of this many manufacturing and assembling industries, are using CFRP that provide for reduced weight of vehicles. CFRPs exhibit opposite composite failure in compression to the metals, and this is because most composites experience a brittle response to load rather than the ductile response that is exhibited by metals. It can also be noted that metals collapse under a crush as opposed to CFRPs that fail through a sequence of fracture mechanisms. The sequence of damage and the mechanism of failure in CFRPs is dependent on the geometry of the structure, lamina orientation, type of trigger, crush speed all of which can be designed against to provide a high energy absorbing mechanisms. The CFRPs deformation and progressive failure behavior that is characterized in terms of stiffness, yield, strain hardening, elongation and strain at break are key factors in determining the energy absorption capacity of the vehicle.
In reference to Long, A. C. (2005), it is also important to note that CFRPs are preferred in the vehicle manufacturing industries because of their corrosion resistant; corrosion of metals is a significant cost and forms a constant maintenance problem for vehicles. Therefore, corrosion resistance of composites results in major savings in the supportability costs. Metal contacts experience galvanic corrosion when in direct contact with each other, this can only be mitigated at by the use of bonding glass fabric that provides for electric insulation layer with all the metal interfaces. Fatigue is also another challenge that can be overcome by the utilization of the CFRPs through working within the reasonable strain levels during the design phase, this is in accordance with Long, A. C. (2005). The use of CFRPs also leads to a reduction in the assembly costs by about 50%, this is through the reduction of the number of fasteners and also due to the fact that detailed parts can be combined into a single cured assembly at the initial cure or by secondary adhesive bonding. A graphical representation of the structural efficiencies comparing steel, Aluminium and the CFRPs is shown below;

Fig 1: showing the relationships between structural efficiencies of steel, aluminium and CFRPs, Elaheh Ghassemieh. (2011)
Question 2
In the last decade, significant strides have been made in the vehicle manufacturing industry, this has been led to increased component performance and reduced components weight. Composites are fast becoming acceptable in the automotive industry for design of vehicle parts. CFRPs are being used in the manufacture of most parts of the vehicle that includes the body and also used in the development of some engine parts. In this essay we are solely discussing the CRFPs, this is because other widely used composite reinforcement fibres that are glass and amid have limited use in the bodybuilding of vehicles. The reason behind the limited use of the of glass fibres is because of its weight and lack of stiffness, on the other hand, aramid is unsuitable for such use mainly because of the moisture absorption property. The use of the CRFPs, therefore, dominates the motor vehicle manufacturing industry.
Today, pre-impregnated materials are applied to nearly all composite components in the automotive sector. As mentioned above, almost all volume of materials employ the use of CRFPs, the carbon unidirectional pre peg has been growing at a stronger pace. However, change in the trend is looming with the invention of the reliable processing routes with resin transfer moulding (RTM) or otherwise called resin film infusion (RFI) with a couple of other processing routes that are connected to the essential methods. As can be shown in the figure below.
Fig 2. Showing New process technologies that are driving the carbon fibre textile processes Hearle & Cusick, (2005)
According to Elaheh Ghassemieh (2011), there is a consensus among the manufacturers that lower service costs overrides the higher components cost. It is believed that consumers may bear the higher acquisition cost with the expectation of lower running costs (by the use of lighter automobiles). From the general process of manufacturing carbon fibre yarn, it is understood that the production cost decreases with an increase in tow count for the same type of yarn. For example the cost difference between a 7 mm diameter, 240 Gpa modulus, 400 Mpa strength yarn can be approximated as below;
3000 filaments, 3K100%>6000filaments, 6K75%>12,000 filaments, 12K(50%)
Carbon fibre yarn is not a low-cost raw material, and it is, therefore, important that in this essay we describe carbon fibre fabric materials on the basis of 3K yarns towards more cost-effective 12K and 24K yarns. Normally, the high-cost pre-preg materials are supplied by a selected range of suppliers and is used in the running of the composite textile development. However, textile materials have lower properties as compared to pre-preg and therefore they cannot be used for widespread use, this is in line with Elaheh Ghassemieh (2011), . Future works are targeting the development of resins and other components that complement the carbon fibre textile properties hence leading to improved mechanical properties that include impact, compression and interfacial strengths, these properties lie below those attainable with advanced carbon fibre pre-pregs.
Developments in woven fabric applications
Using standard pre-preg processing
In reference to Long, A. C. (2005), Pre-impregnated carbon fibre fabric forms the main composite material being utilized in the motor vehicle industry, there is a lot of developments in the development processes of these materials to cut down on the cost and improve the material properties. In the industry, minimum two-ply carbon fabric layer over honeycomb core (used to seal the structure) has always been in use. The sheets are made by 3K fabric pre-preg in ply thickness of about 0.2 mm (for plain weave: 200g/m2), 0.25 mm (for 5-harness satin 285 g/m2) or 0.4 mm (for 8-harness satin: 370 g/m2).
A current area of research in most vehicle industries has been for the industry to develop lower-cost woven fabrics solutions, by moving from the 3K to 6K pre-preg fabrics. Improved weaving techniques and advancement in technology have made it possible for the same fabric quality with the same coverage that can be achieved by moving from 3K to 6K yarns. A similar trend can be observed with the application of 12 K fabric pre-pregs, this is in reerence to Long, A. C. (2005). The figure below shows schematics of achieving the same fabric weight and thickness with different carbon tow counts.

Fig.3: Showing the schematic of achieving the fabric weight and thickness having different carbon tow counts Long, A. C. (2005).
Question 3
According to Long, A. C. (2005), there is a consensus among motor vehicle manufacturers that the use of CFRPs in the manufacturing process provides for an ideal combination of strength and weight. This is however challenged by the fact that it is difficult to use it in automated production as it offers an opportunity for hand built race cars that are extremely low volume and very expensive. This creates an opportunity for the consideration of aluminium. These factors put into consideration have made some companies to build their models based on aluminium rather than the CFRPs. A case in point is the Ferrari that is creating its model from aluminium. Other firms are also following in line with the advancement of arguments that more pieces of cars can be produced with the application of aluminium in a day as compared to the use of labor intensive pre-impregnated carbon fibre. The shift has also been pegged on the fact that carbon fibre construction techniques have failed to realize the material weight saving potential. This they argue that can change once intensive and low rate technologies are in place for use, and this was according to the companies report.
From the companies reports it is also evident that aluminium use is better suited for higher production volumes, it entails the use of several alloys that are tailor made to suit specific applications within the cars, this is according to Long, A. C. (2005). Heat treatment of aluminium can be done in order to improve its strength, further it could be welded, screwed, riveted and epoxy bonded to other sheets in order to make better components parts. For example, in the development of 458, Ferrari makes use of five different alloys in the formation extrusions of the car, three different alloys for the formation of its castings, and three different sheet metal alloys, this in reference to Elaheh Ghassemieh (2011). Aluminium is capable of producing up to 320-MPa yield strength that permits extrusions with wall thickness made as thin as 1.6 mm (0.063 in). The deformability of aluminium can also be improved by heat treatment to improve its crash absorption. The formation of a superplastic is possible since 5000-series aluminium can elongate several hundred percent under the subjection of the controlled strain at temperatures of between 450-5000C.
In reference to Long, A. C. (2005), in the manufacturing techniques in most companies, like Ferrari are optimising the use of aluminium and a variety of fastening methods. According to reports of companies some of the models, for example, a 458 Ferrari model is constructed using 70 m of welds and 8 m of bonding adhesive. Further, some companies use a lower-temperature form of MIG (metal inert gas) welding, and this is because it causes less heat distortion of the welded materials and also encourages the automation. It is anticipated that more of bonding will be used than welding in the near future this is due to the increase of use of alloys that are not suitable for welding. Ferrari one of the companies that are using aluminium is employing the use of epoxy joining technique this is due to the development of aluminium material, metal matrix composite. That is an aluminium mesh soaked in epoxy that yields stiffness that is comparable to steel. The development of these composite of metal matrix is estimated to reduce the weight of the new models by 15-20% from the previous aluminium cars.

References
LONG, A. C. (2005). Design and manufacture of textile composites. Cambridge, Woodhead. http://site.ebrary.com/id/10131786.
U.M.I.S.T. SYMPOSIUM, HEARLE, J. W. S., & CUSICK, G. E. (2005). High performance fibres, textiles & composites: symposium : 25th-27th June, 1985. Manchester, UMIST, Dept. of Textiles.
(2008). Carbon & high performance fibres directory. London, England, Chapman & Hall.
ELAHEH GHASSEMIEH. (2011). Materials in Automotive Application, State of the Art and Prospects. InTech. http://www.intechopen.com/articles/show/title/materials-in-automotive-application-state-of-the-art-and-prospects.