Lighter, stronger, cheaper. As automotive designers and engineers scour the industry for materials that cut vehicle weight while meeting strength and cost requirements, competition becomes more fierce among producers of aluminum, high-strength steel and plastic composites. Mike Dickison, manager of body engineering at the Motor Industry Research Association of the United Kingdom, discussed lightweight vehicle structures during a conference this month in Cernobbio, Italy. Here is an edited version of his talk, co-authored by Stephen Buckley and Keith Clemo.
In recent years, the necessity for reducing fuel consumption has intensified in automobile design and manufacturing. The fuel consumption of cars is directly linked to their mass. Weight reduction is, therefore, essential to reduce fuel consumption.
Other factors are increasing the weight of vehicles. Consumers demand increased levels of in-car equipment and passive safety components such as airbags. To meet new, more stringent impact scenarios, body-in-white weight increases as a result of more structural reinforcement.
A disadvantage of weight reduction is that as vehicle mass is reduced, accident severity may be increased. Studies by the U.S. National Highway Traffic Safety Administration show that an average 99-pound reduction in mass could cause more than 10,000 more injuries a year.
Vehicle mass can be reduced by using lightweight components such as aluminum axles, wheels and subframes, as in the 1996 BMW 5 series. The body-in-white of a vehicle represents a significant portion of its mass - for example 26 percent for the Mercedes S class and 25 percent for the Mercury Sable. Many attempts to design lightweight vehicles have therefore concentrated on reducing the mass of the body-in-white.
STRIDES IN ALL FIELDS
In order to achieve this, several novel designs have incorporated aluminum or composites, while high-strength steels have been developed for the same purpose. At the same time, advanced joining techniques, such as weld-bonding and clinching, have permitted metal thickness to be reduced, allowing better optimization of the structure.
Three basic approaches have been adopted to utilize aluminum for the vehicle body:
1. The aluminum-bodied Honda NSX was developed using methods of unitary construction similar to steel-bodied vehicles, with a combination of seam and spot welding. The NSX also uses aluminum extrusions in the sill sections of the vehicle.
2. Audi was the first manufacturer to bring a medium-volume aluminum space-frame car into the market, with the A8. The technique, which it developed with Alcoa in a £47 million ($79.5 million) venture, joins the extrusions by welding them to aluminum diecastings at nodes. It also uses internal panels, such as the floorpan and inner wheel arches, to brace the structure.
3. Alcan has developed a method of joining aluminum panels with a combination of spot welds and structural adhesive supplied by Ciba Geigy. The technique was developed on a pilot batch of Volvo 960 front structures and was implemented by Ford on its Aluminum Intensive Vehicle Project.
STEEL MAKES PROGRESS
Steel dominates high-volume car production, and the fact that more than 50 percent of modern automotive steels have been introduced since the mid-1980s is sometimes overlooked by the automotive media.
New and modified high-strength steels can provide efficient contributions to the weight reduction of automobiles. For the future, dual-phase and bake-hardened steels are the most promising, since they offer the best combination of strength and ductility.
In the UltraLight Steel Auto Body program, a consortium of 33 sheet-steel producers aims to produce a lightweight steel car structure with a reduction of 20 percent in the weight of the body-in-white, while meeting a wide range of performance targets, including exemplary crash protection.
Breakthroughs in the program were achieved when the twin approaches of hydroformed pieces and optimized monocoque design were combined.
Advanced composites, as a family of materials, appear to offer the designer of lightweight structures almost unlimited potential, and all the indications are that we are at the beginning of the process of exploiting this.
LOOKING TO THE FUTURE
Where once we simply selected a suitable material for the job in hand, we now have the ability to configure variables such as particle size or fiber orientation to give precisely the properties we require. Soon, we may be able to create detailed structures at a microscopic level in order to achieve the kind of performance previously seen only in biological materials such as wood and bone.
The most familiar present-day composite materials, namely the glass, carbon or aramid-fiber thermoplastics, have been used with great success for structural purposes in the aeronautical, marine, sporting goods and racing car fields, as well as in more exotic applications. However, structural applications in the mass-production automobile field are still rare.
As pressure mounts for cars to become more energy-efficient, it seems almost inevitable that the use of composites in structural parts will increase.
The competition among the three advanced materials - aluminum, plastic composites and high-strength steels - is clearly growing by the day. The vehicle industry faces technical challenges for each material, and it remains to be seen whether there will be a return to a position of dominance by one technology, or whether a diversity of approaches will continue.