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1.3.6 Application: Car body

1.3.6.1    Short application description

The car body parts considered in this chapter are those called the Body-in-White and the closures.
The Body-in-White is the basic structure of a car. In case of crash this structure has to absorb as maximum energy as possible and protect the passenger space at the same time. It is also the heaviest part of a conventional car with a share between 25 and 30% of the complete car weight. Being the vehicle's largest structure, it might seem ideal for weight reduction, but normally reducing body weight involves a trade-off with body stiffness, a key characteristic which influences vehicle dynamics, durability and crash worthiness.
To complement the Body-in-White, the closures are assembled. The main automotive closures are the doors, the hood and the deck lid (hatchback or tailgate). The closures account normally for around 6% of the car's mass.
As closures are not part of the primary crash-energy management system, many automakers have started with them in the weight reduction, mainly switching from steel to aluminium and composites.
Materials used in the car body
Steel has been dominating the market for car body parts in the past decades.
However, more and more other materials are gaining market shares, as can be seen in figure 11. The main reason for material substitutions has been weight reduction.  

Figure 11: Materials used in car bodies since 1975
Source: SuperLIGHT-CAR project


 
Also important associations like EUCAR are betting on multimaterial concepts, as graphically explained in
Figure 12.










Figure 12: Trend to multimaterial vehicle structures for weight reduction
Source: SuperLIGHT-CAR project  (originally EUCAR figure)

The average aluminium content in car body is 26 kg, basically in bonnets and doors, front structure and bumper beams.
Aluminium has been used in car bodies since 1993, as a light substitute for steel. One of the typical cases are the hoods, which are typically made out of steel, but aluminium is rapidly getting more popular with auto companies. It is foreseen that the use of aluminium in the automotive industry will increase up to more than 200 kg per vehicle, according to the European Association of Aluminium. As example, Ford is working more and more with aluminium, such as in Jaguar vehicles, the Ford GT and the Shelby GR-1 concept car.
The gross yearly sales of Al sheet to OEMs for closures in 2003 was $600.000.000.
Another material in the lightweight substitution race is Magnesium. And density-wise, it has a big advantage: a steel cube with the dimension 10x10x10 cm weighs 8,6 kilograms, whereas an aluminium cube with the same measurements only weighs 2,7 kilos and if it was made of magnesium, it would weight 1,8 kg. Besides, magnesium has high strength and stiffness to weight ratios, and provides good vibration damping. Automotive manufacturers have believed in magnesium potential and Ford and VW even invested in primary production in Australia and Israel. Regarding its price, in recent years the abundance of magnesium in China has had the effect of lowering the raw material costs, at times even lower than aluminium cost.
The most widely used wrought Mg-alloy is the AZ31 (Mg-3Al-1Zn), which is available for extrusions, plates and sheets. Magnesium sheets are especially suitable for large parts like the hood or the roof. However, the application of magnesium sheets in the car body has not yet been spread. VW stopped production of the Lupo magnesium tailgate: the main reason was the cost of the corrosion protection. It's not that magnesium is not at all used in the automotive industry, but it is being mainly used in parts that are not exposed to corrosive environments, which is clearly not the case for the body-in-white or the closures. Another limitation for magnesium sheets is they have to be stamped at high temperature (200-300ºC), preheating the die as well.
To solve the corrosion problem there are diverse possible solutions: coatings at different stages of production and thin films or thin layers, as in the promising proposal of the researchers in the University of Berlin to use aluminium foil to cover the magnesium alloy.
The steel industry is not letting the other materials overpass it for car body applications and continuously introduces new materials with improved properties. Steel is being very competitive for automotive structural parts with its Advanced High Strength Steel (AHHS). AHSS such as Dual-Phase, Transformation Induced Plasticity (TRIP) and martensitic steels provide interesting characteristics because they have very high strength, and yet can be easily formed to make complex automotive parts.
Last but not least, composite materials have been in used in structural parts in the vehicles since the 1950s, when the Corvette was launched with the body made of fiberglass. However, almost 60 years later, this material has not been generally adopted in mass production. The main reasons are that the cycle times are in general still too long, raw materials (resin, fibres, etc.) are relatively expensive and it is difficult to achieve high quality surfaces. Composite materials are often used by aftermarket manufacturers, for example to construct hoods out of fiberglass, carbon fiber, or dry carbon.

1.3.6.2    Nanotechnology impact

Nano-enabled products are not used in the automotive structural parts yet. The products currently in the market are mainly with aesthetic purposes (waxes, paints, etc.) in the high-class or luxury vehicles.  Still, nanostructured metals and polymer nanocomposites have an important potential to contribute to further weight reduction in the car bodies. Lighter bodies without compromises to the stiffness and crash resistance means less material and indirectly less fuel consumption.
Nanostructured metals are stronger than their traditional counterparts. For this reason, they can have an important role in reducing the weight of the vehicles. Moreover, they have improved properties in corrosion resistance and can even be tailored with different characteristics depending on the requirements of the zone. When the technology is able to produce large parts (bigger than a screw) with a competitive price, nanostructured aluminium and magnesium will be suitable to be widely used in structural parts, replacing steel. Also for nanostructured steel, as the properties would have improved, the material could be reduced, and thus the weight.
The same functionality is applicable for polymer nanocomposites. Polymer nanocomposites could introduce tailored, light materials to the car body applications. Carbon nanotubes and inorganic nanoparticles can be added to conventional materials for special properties.  Applied in the automotive industry, the matrix polymeric composite makes possible that the composite parts can be painted together with the rest of the auto body and treated in the same process as the metallic material.
As an example of applied research in this area, Ford researchers are working at Northwestern University with nanotechnology to develop stronger and lighter structural materials, such as metals and plastic composites. These metals and plastics use nanoparticles as fillers that reduce weight and increase strength.
Apart from lighter structural parts, there are more applications for nanoenabled products in the automotive industry. As introduced in the top of this chapter, the driver that is currently opening doors for nanotechnology is aesthetics. This will be the main selling point for antiscratch, self-healing and dirt-repellent nanocoatings, paints and varnishes.

1.3.6.3    Functional requirements and boundary conditions

As the car body is the main structure of the vehicle, its functional requirements regarding crash resistance, fatigue and static performance are very stringent and if the vehicle does not comply with the European standards, the car cannot be commercialised. All these standards and regulations respond to passenger protection measures, aimed at reducing the number of fatalities in the European roads.
In case of crash, the Body in White has to absorb as much energy as possible, but with a controlled deformation, so as to cause as few damages as possible in the driver and passengers space. The most known tests are Euro NCAP front crash, side impact, rear impact, natural frequencies and static stiffness, in which accelerations, intrusions, intrusion velocities, torsions and bending are measured in specific parts of the car body.
For hoods, pedestrian safety is as well an important boundary condition for design. In Japan and Europe, regulations have come into effect in recent years that place a limit on the severity of pedestrian head injury when struck by a motor vehicle. This is leading to more advanced hood designs.
As well, the external car body parts have to have high quality surfaces (Class A), corrosion resistant and be UV-light resistant.
Like the rest of the vehicle, car body parts also have to be assessed according to their compliance with stringent regulations for recycling. This is specially an issue when several materials are mixed in the car body, since the recovery is more complicated.
 
1.3.6.4    Product examples

• AEROSIL(Degussa): Nanoparticles used in the body shell for pigment stabilisation, rheology control and corrosion resistance. It is present in car varnishes as well, for improvement of the scratch resistance.

• Nanopaints used in Mercedes Benz models (Daimler AG)

• Mercedes-Benz also makes a major contribution to exemplary long-term quality and value retention with a scratch-resistant clearcoat based on nanotechnology. This innovative paint system, which celebrated its world debut at Mercedes-Benz at the end of 2003, is a standard feature of the new C-Class and is used for both metallic and non-metallic finishes.

• General Motors introduced their nanocomposite thermoplastic olefin (TPO) step-assist for the 2002 Chevrolet Astro and GMC Safari minivans.

• Renault Clio and Megane are also using a nanotube filled polyamide / polyphenylene ether (PA/PPE) blend for their sport fenders.

1.3.6.5    Some selected key companies

• Daimler
• Volkswagen
• Renault
• General Motors
• Degussa
• Faurecia
• ArcelorMittal
• Hydro Aluminium
• Fraunhofer Gesellschaft Institutes
• Fiat
• Magna International
• Electrovac
• Ford Motor Co.
• Aveka Group
• GE Plastics
• Synkera
• Emil Bröll GmbH
• Toyota
• Nanocor Inc.
• Blackhawk Automotive Plastics Incorporated
• Bayer AG
• NanoX