The document discusses the use of composite materials, specifically carbon fiber reinforced polymers, in automobile design and manufacturing. It provides examples of the Alfa Romeo 4C and BMW i3 which make extensive use of carbon fiber composites. The 4C uses a carbon fiber unit body chassis to keep the curb weight below 900kg while maintaining strength and stiffness. BMW developed a new factory to mass produce carbon fiber body panels for the i3 through resin transfer molding, aiming to lower costs. The use of composites allows for lighter vehicles but recycling remains a challenge due to degradation of materials during the process.

1. 01/12/2014 1
The Future of Composite
Materials in Car Body
Design
-
1 December 2014
Composite Basics
The core premise behind a composite material is
the reinforcement of one material with another.
Most often, this is implemented as a matrix of a
more ductile and easily formed material with
much stronger fibres throughout. This basic com-posite
structure can be supplemented with a
sandwich layer of a third material placed between
two thin sheets of composite in order to reduce
costs, and sometimes weight, whilst significantly
increasing strength since “the flexural stiffness of
any panel is proportional to the cube of its thick-ness”
1. The combination allows for the minimisa-tion
of negative properties, and the maximisation
of desirable qualities, almost universally this
means a significant improvement on strength-weight
ratios. A common example would be a Fi-bre
Reinforced Polymer (FRP), as seen in the
Stress/Strain diagram the resultant composite will
have a mixture of the properties of the fibre and
the resin. The wide range of available matrices
and reinforcements, each with their own
strengths and weaknesses, means that a compo-site
can be tailored for almost every situation, on-ly
being restricted by costs and waste manage-ment
concerns.
Alfa Romeo 4C
The most extensive use of composites in the au-tomotive
industry is exemplified by carbon fibre
reinforced polymers, referred to as simply car-bon
fibre, in chassis and body design. Carbon fi-bre
has an excellent strength to weight ratio, and
as such is used primarily in high end vehicles to
save weight. One of the lightest production
sports cars is the Alfa Romeo 4C, which uses a
unit body chassis made entirely of carbon fibre
(Fig. 1) with aluminium front and rear sections.
This allows the 4C to have a curb weight below
900kg, without sacrificing the bending and tor-sional
stiffness required by the demands of a
performance vehicle.
Fig. 1: Stress/Strain Diagram for a Fibre Reinforced Polymer [1]
Fig. 2: Carbon Fibre Frame of the Alfa Romeo 4C [http://
en.wikipedia.org/wiki/File:Geneva_MotorShow_2013_-_Alfa-
Romeo_carbon_frame.jpg]
The composite chassis further saves weight
through the same methods as a standard steel
frame, the bulkheads give strength against bend-ing
across the width of the vehicle in addition to
increased torsional stiffness. Similarly, the
lengthwise sides of the frame has carbon fibre
formed into channel sections combined to im-prove
the stiffness of the chassis in multiple di-rections
to respond to the dynamic forces that
the vehicle generates at high speeds, particulrly
in maintaining stability during cornering. This
effect is furthered by the significant mass reduc-tion
of the vehicle, which likewise drastically re-duces
the forces acting on the structure, and
therefore increasing the stability and cornering
ability. Furthermore, Alfa Romeo have employed
the use of stiffening panels to complete the bod-ywork,
these act to complete the torsion box and
thus increase torsional stiffness. These panels
are made from a different composite material
that is much lighter, but provides similar
strength to steel.

2. 01/12/2014 2
The 4C’s chassis uses a combination of process- es to manufacture:
 Pre-preg: The reinforcement material is pre-impregnated with the resin, but it has not been set. The resulting ductile and slightly adhesive sheet is cut and formed, then applied to the mould.
 Vacuum bagging: The moulded and pre- preg is then sealed with a plastic layer and vacuumed out to increase the pressure on the composite, allowing for higher fibre content and fewer voids.
 Autoclaving: The vacuum bagged pre-preg is placed into an autoclave (essentially a pressurised oven) to cure the resin.
 Wet or Hand Lay-up: Finally, the now set composite frame is coated with a thin layer of resin by hand to fill any surface defects and create a smooth finish which cures un- der ambient conditions.
These processes require quite a lot of skilled la- bour and there is limited automation possibility. The production is therefore quite limited, in this case to 3500 units per year. Additionally, the cut- ting and discarding of the pre-preg excess re- sults in a large amount of waste with limited re- cyclability. The recycling process of carbon fibre involves burning away the epoxy, which also shortens the length of the individual fibres of carbon. Subsequently, the epoxy used in the composite is completely lost, and the carbon drops in quality with each subsequent recycling [2], similar to the effects of recycling paper.
BMW i3
The i3 is the first high volume production car to be made mostly out of carbon fibre. With the ever increasing push to expand electric vehicles there have been a number of investments to counteract the issue caused by the very large weight of the batteries necessary for competitive vehicle travel range. Most commonly this has been solved by the class of Hybrid vehicles, but without a massive leap forward in battery tech- nology, the most efficient way of maintaining performance and range of an all-electric car is by cutting weight elsewhere. BMW have elected to do this through the extensive use of carbon fi- bre. However, this is not intended to be a very high end car, as is associated with the use of composites, rather, it costs around 30,000GBP, almost 20,000 pounds cheaper than the Tesla Model S. This has been accomplished through the investment in new factory dedicated to man- ufacturing carbon fibre bodywork, with the ma- jority of the stages of production being done in- house, only importing the raw materials neces-
sary. The new plant has even been located close to a hydro-electric plant to minimise the environmen- tal impact of the high energy processes required to manufacture carbon fibre.
The advantages seen from the Alfa Romeo’s use of carbon fibre in the 4C translate across, but the i3 is designed as a city car, and has a different focus for design decisions. The i3 is a taller and narrower car, and as such the need for torsional stiffness is primarily to maintain ride comfort. However it should also be noted that due to the larger mass, and higher centre of gravity, the importance of maintaining traction in corners at lower speeds is also a concern. It is also clearly visible that the i3 values the cabin space of the vehicle, compared to the compactness of the 4C.
BMW are using Resin Transfer Moulding (RTM) to mass produce the carbon fibre parts. RTM stacks layers of woven fibres cut into their precise shapes then stacked, much like a 3D printer. This stack of fabric is then shipped to the final manufacture plant, where it is placed into moulded presses and the resin is transferred into the mould at high pressure. The process is very similar to casting, and as such it requires the component to be trim- mer down, BMW use water jets for this process.
Conclusion
The development of applications and feasibility of composite materials has only been increasing over time, from the first polymer matrix compo- sites (fibreglass) in the early 1900s to the exten- sive use of carbon fibre in aerospace and high end automotive industries. The constant invest- ment into improving the quality of the materials as well as the cost of manufacturing means that the industry is now crossing the threshold where true mass production of majority composite vehi- cles is possible. However it is of vital importance, not only environmentally, but also from supply- chain management, that there is an increasing focus on recyclability and renewability of materi- als.
Fig. 3: BMW i3 Frame being build [3]

3. 01/12/2014 3
References
1. Gurit. Guide to Composites. Online: Issuu, 2012
2. Polek Gregory. Composites Go Green Through Recyling, http://www.ainonline.com/aviation-news/aerospace/2012-11- 05/composites-go-green-through-recycling (5 November 2012, accessed 1 December 2014).
3. Sloan Jeff. The making of the BMW i3, http://www.compositesworld.com/blog/post/the-making-of-the-bmw-i3 (23 April 2014, accessed 30 November 2014)
4. Think Engineering. Composite Materials in Automotive Engineering, http://www.thinkengineering.net/104/composite- materials-in-automotive-engineering/automotive-engineering/ (2009, accessed 29 November 2014).
5. Bal Claire. Alfa Romeo 4C is a 'masterpiece,' says Fiat product boss, http://www.autonews.com/article/20130905/ COPY01/309059953/alfa-romeo-4c-is-a-masterpiece-says-fiat-product-boss (5 September 2013, accessed 30 Novem- ber 2014).
6. CMS Industries. CMS Industries & the Alfa Romeo 4C, http://www.cmsna.com/blog/2013/08/cms-industries-the-alfa- romeo-4c/ (19 August 2013, accessed 29 November 2014).
7. Howard Bill. BMW i3: Cheap, mass-produced carbon fiber cars finally come of age, http://www.extremetech.com/ extreme/162582-bmw-i3-will-bmws-new-ev-finally-be-the-breakthrough-for-carbon-fiber-cars (30 July 2013, accessed 1 December 2014)