Parking assist systems, airbags, antilocking systems, seat heating – cars are becoming increasingly comfortable and safe, but also heavier. For example, the popular sport utility vehicles, SUV for short, weigh two tonnes or more. However, even most small and medium-sized vehicles weigh more than 1.2 tonnes.
But with stricter threshold values for the carbon dioxide emissions of vehicles to apply in Europe from 2020, it is now time to slim down. Once the new limits come into effect, the average new car is to emit a maximum of only 95 grammes of CO2 per kilometre – 130 grammes being the current figure.
In order to fulfil these requirements, cars must become lighter. If a car weighs 100 kilogrammes less, its petrol requirements will decrease by 0.4 litres per 100 kilometres; carbon dioxide emissions will decrease by up to ten grammes. The car body offers an opportunity to save weight. Here, car manufacturers continue to use mostly steel.
A study by management consultancy Berylls Strategy Advisors on the future of automotive lightweight shows that this will change. In future, greater use will be made of lightweight construction materials such as high-strength steel, aluminium, magnesium and composite materials.
Lightweight construction is of interest not just for car manufacturers. Manufacturers of aeroplanes, trains, wind turbines, machines and plants also want to reduce the weight of their products. The McKinsey study "Lightweight, heavy impact" forecasts that the global market for lightweight construction materials will grow by 8% per year to more than EUR 300 billion in 2030.
"In times of diminishing resources and a rising environmental awareness, lightweight construction is one of the key technologies," stresses Professor Andreas Büter, a spokesman for the Fraunhofer Lightweight Design Alliance. But while engineers have many years of experience in the use of steel, manufacturing with lightweight metals, metal foams and composite materials is only just beginning. There remains a need for research and development here.
"It is necessary to find an economic compromise between weight reduction on the one hand and sufficient stiffness, stability and engineering strength on the other hand," says Büter. "The challenge is to use the right material in the right place."
Innovative lightweight construction solutions can do more
Carbon-fibre-reinforced plastics (CFRP) offer major potential for lightweight construction. CFRP components, often referred to as carbon components, are frequently only about half as heavy as steel components but are just as crash-proof. Formula 1 has made use of the ultra-light material for years. And even in commercial aircraft, CFRPs are gradually replacing metal.
The situation in car manufacturing is different. The lightweight material is rarely used in series-produced cars. This is because CFRP components are still considerably more expensive than the same components made of steel. In addition, manufacturing is complex. Nevertheless, the first automotive manufacturers are beginning to use carbon fibres.
"Innovative lightweight construction solutions can do more than just reduce weight," says Professor Frank Henning of the Fraunhofer Institute for Chemical Technology (ICT), based in Pfinztal, near Karlsruhe, Germany. The expert manages the Chair for Lightweight Construction Technology at the Institute for Vehicle System Technology at the Karlsruhe Institute of Technology (KIT) as well as the Polymer Engineering department at the ICT.
"Thanks to new manufacturing processes, even complex components can be produced completely in one piece, combining various functions," raves Henning. ICT researchers combined two production techniques in order to produce a crash-relevant car seat crossmember along with cable entries and integrated seat fixtures from fibre-reinforced composites, ready for series production.
The component can be manufactured in less than four minutes. First, a workpiece is woven using fibres. "Structures produced using weaving technology absorb a great deal of energy and ensure enormous damage tolerance," explains Michael Karcher, Project Manager at the ICT. Another advantage is that the highly automated robot-supported process delivers reproducible components and hardly any waste. The woven workpiece is then filled with resin and hardened in a press under heat and pressure.
"This high-pressure RTM (resin transfer moulding) technology is suitable for producing large and complex component geometries in series. The finished components have a good surface quality and a low cavity and pore content and possess outstanding material and component properties," emphasises Karcher.
Production in series
Despite all their advantages, complex components made of fibre-reinforced plastics are still rarely used for series-produced products. This is because manufacturing costs are often too high. This is set to change. Fraunhofer researchers are working on new manufacturing processes that are also suitable for large numbers of units.
If lightweight construction components are to establish themselves on the market in future, not only must it be possible to manufacture them economically and in series, but they must also work safely and reliably. Therefore, Fraunhofer scientists are working on calculations with which the damage tolerance of the materials can be determined.
They are also using special processes to analyse the resistance of the components in the face of the strongly alternating mechanical and thermal loads faced in daily use.
So far, the experts have used ultrasound processes to test the quality of components made of fibre-reinforced plastics (FRP). Researchers from the Fraunhofer Institute for Nondestructive Testing (IZFP) are developing the system further. With the sampling phased array (SPA) technology, even complex fibre composite components can be checked quickly and reliably for possible errors.
A further challenge for lightweight construction is that components and materials are to be recyclable after use. "Innovative lightweight construction must be considered across the entire life cycle – from design to production, testing and use, up to recycling," stresses Prof Büter.
How fibre-reinforced plastics (FRP), for example, can be made more environmentally friendly is shown by researchers at the Application Center for Wood Fiber Research (HOFZET) of the Fraunhofer Institute for Wood Research, at the Wilhelm Klauditz Institute (WKI) in Braunschweig, Germany. They combine carbon fibres with various bio-based textile fibres made of hemp, flax, cotton or wood.
The result is that the components are cost-effective, are very firm, have good acoustic properties and are considerably more ecological than pure carbon components.
Light metals and metal foams
Fibre-reinforced composites are lightweight construction materials with potential. However, the potential of metals has not been exhausted yet either. High-strength steels, aluminium and magnesium help to reduce the weight of cars and other products considerably. In future, automotive manufacturers in particular will make greater use of high-strength steel.
However, light metals, too, can help to reduce the weight of their products. New opportunities are being opened up by metal foams that can be used to manufacture lightweight and stable components. They have a similar structure to bones. A pioneer in the development of the foamed metals is the Fraunhofer Institute for Manufacturing Technology and Advanced Materials (IFAM) in Bremen, Germany.
Today, many groups – including the IWU – are working on the airy materials. In most cases, the metal foams are offered as a sandwich – with a foam core between two massive cover sheets. Such structures are not only lighter than massive sheets but also possess higher resistance to bending.
Fraunhofer researchers are laying important foundations to ensure that cars, aeroplanes, machines and plants consume less energy in future. They help to make lightweight construction fit for series production.