Materials & Design, Vol. 18, Nos. 4r6, pp. 203]209, 1997 Q 1998 Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved 0261-3069r98 $19.00 q 0.00 PII: S0261–3069(97)00049–6 Technical Report New cars } new materials Arno Jambor U , Matthias Beyer Daimler-Benz AG, Adv Development Cars EP/ VF;HPC:G250,D 71059 Sindelfingen, Germany Received 10 July 1997; accepted 30 July 1997 Due to more demanding requirements of car occupants in relation to comfort and safety enhancing measures, the weight of cars has been increasing, and as a result additional difficulties have been encountered in making lighter cars. In the development of every new car there is a search for new ways to combine the demands of the customers with reducing the weight of new cars. Further progress in optimizing steel body design can only take place gradually. Reinforced steel or tailored blanks are already in common use today. Even further reductions can be achieved by design in aluminium, magnesium or plastics. At Daimler]Benz, for example, the hard-top of the SL-sports-car is made of aluminium and the petrol tank partition panel of the SLK-roadster is made of die-cast magnesium. Lightweight design and, consequently, fuel saving will only be successfully realized, if proper materials are selected for appropriate parts. Q 1998 Published by Elsevier Science Ltd. All rights reserved. Keywords: cars; materials; design; parts Introduction The entire automobile industry is under considerable pressure to reduce the fuel consumption and therefore the emissions of their products. Over the last few decades, traffic density has continued to grow, because both the vehicle population and the mileage driven per vehicle have risen considerably. At the same time, even more demanding requirements in relation to comfort and safety-enhancing measures have tended to increase the weight of vehicles. Total energy consumption of a motor ¨ ehicle The aim of all car makers is to reduce fuel consumption. It is known that the energy consumption over the full life cycle of a vehicle is essentially determined by the fuel consumed during active use Ž Figure 1.. The chart shows the percentage of energy consumed during the life cycle of a car. This figure has to be reduced. These reductions can be achieved by various measures Ž Figure 2 .: v v v v v Disciplined driving style Improvement of drive efficiency and tyre rolling resistance Improvement and optimisation of the ancillary components Reduction of drag Reduction of vehicle weight Within this presentation, the remainder of this paper U Correspondence to Dr A. Jambor, Tel.: q49 703 1909820; fax: q49 703 1900998 Figure 1 Energy consumption during life cycle will concentrate briefly on the effect of vehicle weight on fuel consumption followed by examples, which show how we can reduce the vehicle weight. Influence of ¨ ehicle weight on fuel consumption Road traffic generally moves at permanently changing speeds. The influence of mass acceleration can be clearly seen from the example of the fuel consumption figures calculated according to the ELA ŽEuropean Legislative Average.. Tyre rolling resistance also depends on mass Ž Figure 3 .. The influence of vehicle mass on fuel consumption depends more on the kind of the engine than on the Materials & Design Volume 18 Numbers 4 / 6 1997 203 New cars } new materials: A. Jambor and M. Beyer Figure 4 Impact of weight reduction Figure 2 Impact on fuel consumption Figure 3 Impact of vehicle weight on NEDC Žwith C 180. category of the car. If we assume, that the axle-drive ratio is adapted for same flexibility Ž60]120 km hy1 ., the Daimler]Benz product range has the following values according to the ELA Ž Figure 4 . Lightweight vehicle construction Vehicle weight factors The overall weight of our cars is distributed about 50% to the engine drive train and running gear and 50% to the body. The chart shows the weight distribution of a Mercedes-Benz C-class Ž Figure 5 .. The increased use of lightweight materials in the engine and drive train brings a potential weight saving of 1]2% in relation to the gross vehicle weight. For the running gear, the potential weight saving from the use of these materials is about 6%. Greater potential weight savings can be obtained in the vehicle structure, with the body in white providing the largest contribution through the use of new technologies or new materials. 204 Figure 5 Vehicle weight distribution Mercedes-Benz C-Class ŽC 180. Some examples will be considered in detail in this paper. Lightweight design concepts in the body The possibilities for reducing the weight of the vehicle body start with an optimised all-steel body in white}with a potential weight saving of about 7% Žin relation to the body-in-white. }and span all the way to the all-aluminium car. Here, weight reductions of 30]50% are possible. More extreme lightweight designs can only be obtained by using fibrous composite materials Ž Figure 6 .. Between the extremes of all-steel and all-aluminium, there are solutions that combine steel with lightweight materials. It should be noted that costs do not increase in relation to the use of lightweight materials. Reducing these costs is a primary aim. Lightweight design requirements A lightweight design must also meet the following Materials & Design Volume 18 Numbers 4 / 6 1997 New cars } new materials: A. Jambor and M. Beyer Figure 8 Methods of producing a lightweight vehicle Figure 6 Cost curve of various lightweight design concepts Figure 7 Body shell requirements criteria Ž Figure 7 .: v v v v v v Low-cost production in high quantities Requirements of strength, stiffness and crash resistance Durability Recycling Repair concept Acoustic properties The requirements regarding acoustic properties, stiffness and crash safety are now sufficiently satisfied, as has been proved by well-known models of aluminium construction. These questions will therefore not be considered further in this paper. Lightweight design methods Vehicle weight can be reduced by an optimised design and by lightweight materials Ž Figure 8 .. Figure 9 Straight members in new A-class Lightweight design shapes Lightweight design shape means a design that is optimised for the expected load and for the material. This can be illustrated by two examples of the recently presented A-class Ž Figure 9 .. The concept of this car gave us the possibility to design the front side member straight and without offsets, so that in the event of a frontal crash it is optimally axial-loaded. By contrast, bent members are always subject to bending forces, and the generated strains have to be compensated by extra material Ž Figure 10 .. A second example is provided by flat panels with cambered impressing. This type of panel was used for the floorpan of the A-class. These cambered impressions increase the impedance of the vehicle floor in the low frequency range. In consequence, the thickness of the fused sheets otherwise required can be reduced. The result is a reduction of 50% in weight. Lightweight design materials Steel materials. Steel, as the traditional automobile material, has long since proved its worth. New highstrength sheet metal is now more and more being used for parts exposed to high stresses. Materials & Design Volume 18 Numbers 4 / 6 1997 205 New cars } new materials: A. Jambor and M. Beyer can be reduced by 0.1]0.2 mm with the result of an additional weight reduction of 5]7 kg at the E-class. Aluminium as lightweight construction material. The lightweight construction potential of aluminium is generally well-known, and so there is no need to explain it in detail here. The disadvantages in terms of processing, recycling and costs are also outside the scope of this paper. After a brief history, only the current state of development at Daimler]Benz will be described here. Figure 10 Cambered panels High-strength panels. These micro-alloyed panels enable us to reduce the panel thickness and achieve reductions in body weight andror improve the vehicle properties Žstrength. at reasonable costs. Modern strength ratings of up to 540 MPa compared to 180 MPa for conventional body panels permit thinner panels in strength-relevant and crash-relevant areas Ž Figure 11.. At present, the proportion of micro-alloyed highstrength steel panels in the E-class is 20%. The increased use of these panels is at present restricted by their limited formability. However, new steel materials combining the qualities of higher strength and improved formability are currently being tested. These new materials are essentially dual and triple-phase steels. In addition to their improved formability, increased strength is a prime aim. These steels will increase the proportion of high-strength steels in future models, and therefore the proportion of lightweight design. One example is a tunnel panel that could not previously be pressed in high-strength steel Ž Figure 12 .. Continuing research into the above materials is highly promising. It shows, that the thickness of the panels The use of aluminium at Daimler]Benz. Our sports cars were already making extensive use of aluminium Žand magnesium. before the war and in the post-war years. One representative example is the 300 SLR, a racing car dating back to January 1956 Ž Figure 13 .. Another example from the past, this time a standard production model, is the 230 SL dating from 1963. This vehicle had an inner door panel of die-cast aluminium. The weight of the part was 6.6 kg, representing a weight saving of about 5 kg compared to a steel construction Ž Figure 14 .. In today’s Daimler]Benz model range, the hard-top of the SL sports car is made of aluminium. The weight of the roof structure has been reduced by 52%. The customer has the advantage of a light and therefore easily removable hard top. This all-aluminium design is cost-intensive. The costs are about 30 DM per kg of weight reduction compared to steel Ž Figure 15 .. Interior parts are made of aluminium, too. For example, the structure of the new A-class seats is assembled of aluminium parts: the seat back is a tubular space frame, the seat rails are made of extrusion parts and the seat cushion pan consists of a panel. Con¨ ertible in aluminium design (study). In order to research lightweight aluminium design at reasonable costs using the latest technical knowledge, a lightweight vehicle design study was carried out by Daimler]Benz on the basis of the current Roadster SL. The development goal was to prove that all specific requirements for the body in white}as already mentioned}could Figure 11 Material properties of various steels 206 Materials & Design Volume 18 Numbers 4 / 6 1997 New cars } new materials: A. Jambor and M. Beyer Figure 12 Tunnel E-class Figure 15 SL-hardtop in aluminium Figure 13 Mercedes-Benz 300 SLR 1956 Figure 16 Aluminium body in white SL Žstudy. zones of the body. The following tests were performed on the cars: v v v v v Figure 14 Mercedes-Benz 230 SL 1963 be met by an all-aluminium concept also for a convertible Ž Figure 16 .. The body in white consisted of panels, extrusion parts and die-cast components. Die-cast parts have the advantage of high integration: there are fewer individual parts and therefore less costly joining work and an ideal design e.g. with ribbing in the stiffness-relevant Static and dynamic stiffness measurements Frontal crash Frontal offset crash at 65 km hy1 with 40% overlap against a deformable barrier Rear impact crash Comprehensive road testing The result of this study is that the high demands of Daimler]Benz regarding passive safety, stiffness, strength and vibration comfort can be fully satisfied in all functional criteria}even in convertibles]by an aluminium body designed to meet the requirements of the material. The overall weight reduction compared to the steel body in white of the SL including doors, bonnet and boot lid, is 40%. Despite this remarkable weight reduction, the costs prevented the industrialization yet Ž Figure 17 .. Magnesium. Magnesium is an even lighter construction material than aluminium. The use of magnesium in sheeted panels is not feasible at present. The sheet has to be deformed at high temperatures Žover 3008C.. Materials & Design Volume 18 Numbers 4 / 6 1997 207 New cars } new materials: A. Jambor and M. Beyer Figure 17 Aluminium body in white SL Žstudy.; crash test Figure 19 Magnesium die-casting Figure 18 Magnesium die-casting for structure parts Therefore, heated tools are necessary. This makes the process expensive. Consequently, magnesium can only reasonably be used in die-cast production. A well-known example of the use of magnesium for structural components is the seat frame of the present Roadster Žstart of production 1989.: this seat frame is entirely made of die-cast magnesium, and the complete seat structure weighs only about 8 kg, even though the attachment of the restraint systems to the backrest imposes very strict strength requirements Ž Figure 18 .. A second standard production part made of magnesium is the petrol tank partition panel in our new SLK sports car. The original steel panel weighed 6.7 kg and was replaced, for weight reasons, by an aluminium panel with the weight of 4.0 kg. Since a magnesium die-cast version allowed a further weight reduction to 3.2 kg, the decision was made, despite the higher costs, to introduce this lightest version in series production Ž Figure 19 .. Other possible applications for magnesium are in the doors. Due to the possibility of casting thin-walled parts Ž1]1.5 mm wall thickness ., the door inner panel could be built of die-cast parts combined with extrusion parts or die-cast as a single piece. A weight reduction of 40% is imaginable Ž Figure 20 .. One problem with the use of magnesium is the corrosion that occurs in contact with steel or other 208 Figure 20 Door in magnesium Žstudy C-class. materials. In this case, the materials must be kept separate, e.g. by plastic intermediate layers or special coatings. Plastics. The outstanding advantages of plastics are their low specific weight. Known or possible uses include both the outer body skin and the load-bearing structure, mainly with the use of fiber-reinforced plastics. An example of an outer skin part at Daimler]Benz is the plastic wing of the A-class. The material is an unreinforced high-quality thermoplastic polyamide blend ŽPPOrPA. Ž Figure 21.. The weight reduction compared to a steel wing is 45%. Another essential advantage is the increased customer benefit due to the reduced risk of minor damage. The costs of the plastic wing are on a similar level to the steel variant. Other applications for plastic outer skin components include doors, hatches and lids. Necessary extra strength can be achieved by a two-shell construction. The hatch of the A-class was built using this concept. The outer shell Žunreinforced thermoplastic. is divided into a lower covering section and a rear roof spoiler. In Materials & Design Volume 18 Numbers 4 / 6 1997 New cars } new materials: A. Jambor and M. Beyer Figure 23 LK-GTR in carbon fibre Figure 21 Wing A-class Figure 22 Hatch A-class the inner part made of GMT, the formability of the material is fully exploited by the integration of the lock fixing, hinges, rear washrwipe fixing and number plate lamp housing Ž Figure 22 .. The door is fully assembled as a module by the supplier and is painted off-line. This concept offsets a part of the higher material costs compared to steel. However, the manufacturing costs are about 15% higher than for the steel variant. The weight reduction of the plastic door is 3 kg Žabout 25%.. Structural components. For load-bearing body in white structures, only fibre-reinforced composite plastics with appropriately oriented reinforcing fibres offer suitable lightweight construction potential, even compared to aluminium. The technical feasibility and advantages of carbon fibre reinforced plastics are already well-known from applications in aviation and space travel as well as in motor-racing. As long as high stiffness remains a major concern, carbon fibres should be preferred, whereas strength Figure 24 Crash behaviour reinforced plastic requirements can also be very well met by glass or}this is perhaps new}natural fibres. With this technology we can achieve a weight reduction of 50%. Fibre reinforced plastics, depending on the structure and orientation of the fibres, not only have high stiffness and strength but also a much higher energy absorption potential than metals, so that in principle they can even be used as lightweight materials in crash-relevant structural areas Ž Figures 23 and 24 .. Conclusion In the course of this paper, I have shown with the aid of a few examples the possibilities that we now have for using materials other than steel in order to reduce vehicle weight. These methods must be improved for future vehicles in order to meet our high demands and specifications, whether self-imposed or laid down by law. We must combine our efforts to attain these goals to preserve the car itself in its full fascination. Materials & Design Volume 18 Numbers 4 / 6 1997 209