employing RTM for mass production. Still, the three core problems remain—raw material costs, process time and the handling of stress point loads.
I have been working on the high-volume, low-cost problem since 1998 and in 2007 formed Gordon Murray Design with investment from the U.S.A.-based Mohr Davidow Ventures to develop and industrialize the world's first high-volume structural composites system for the automotive industry. We've developed an actual car, the T.25, to demonstrate the concept.
We call this i-Stream (Steel tubular reinforced exoframe-advanced manufacturing) and set out to solve the three basic high-volume problems:
1) Material costs. By using glass instead of carbon for the reinforcement, relying on a low-cost compound for the matrix instead of epoxy resin and incorporating recycled paper instead of either Nomex or aluminum for the honeycomb core, we have drastically reduced raw material costs. For example, a T.25 monocoque delivered to the assembly plant is about $160.
2) Process time for i-panels is 100 seconds—the process employs low temperature and low pressure and is therefore very low energy.
3) The point load issue is solved by using a simple, low-cost steel frame to handle point loads; the stabilizing function is performed by the continuously bonded i-panels with very low unit stress in the bonded joint.
The composite panels are very "adjustable" to suit load input requirements for torsion, bending, NVH and crash. The skin thickness and glass content are variable. One more F1 racing concept is used whereby we introduce unidirectional fibers whose format and direction have been determined by the same program we used for designing modern carbon racing monocoques. The i-Stream system allows these reinforcements to be co-bonded in the same 100-sec. cycle time. This is truly F1 technology for the everyday motorist.
What direction then for the future of structural composites in our automotive world?
I think we will see more RTM carbon structures and panels for low-volume, high-price sports cars; perhaps this technology will appear in mid-price, medium-volume products sometime in the future.
As for i-Stream, our team is already working on i-Stream II, which will include advance composites for medium-volume niche products and i-Stream III, which will use totally sustainable, natural materials for composite panel construction, coming full circle to the concept of the tree.
What is a Composite?
A composite material usually has two components—a matrix and reinforcement. The matrix is the liquid part that is filled with the reinforcement, usually a fibrous material. The composite of liquid and fibers is normally "set" or formed by temperature and/or pressure. The liquid or resin component comes in two categories: thermosets or thermoplastics. Thermosets are more common and once set (or cured) the final shape is retained for the life of the part whereas thermoplastics can be reheated and reformed.
The most common thermoset structural composite is glass-reinforced plastic (GRP), which is made from a polyester resin reinforced with glass fibers, arranged as either random long fibers or a woven mat. The job of the matrix is to provide a molded finish to the shape and to hold the fiber in place. The matrix can be adjusted to be either stiff or tough, while the fibers carry the loads and give the part its stiffness. The most efficient composite structure is a laminate of two thin composite skins either side of honeycomb material. A single skin composite is referred to as a monolith.
Most racing cars use carbon for the fiber and epoxy resin for the matrix with a Nomex or aluminum honeycomb core as this combination produces the stiffest and lightest part.
Other supercars in the early '90s used a variety of materials for their primary structures. The Jaguar XJ220 stuck with aluminum; the Bugatti EB110 used a mixture of carbon panels, aluminum and steel; the Porsche 959 was steel; and the Ferrari F40 had a twin-tube steel frame with some composite panels. Ferrari was first to follow the McLaren F1 with a full carbon chassis in their 1995 F50.
Some companies adopted carbon for the primary structure—always a pre-preg, hand lay-up process used in racing car manufacturing with vacuum and autoclave curing. Pagani in 1998 and Koenigsegg in 2002 adopted carbon for the safety cell but used metallic subframes to manage suspension and powertrain loads. Porsche followed in 2003 with the Carrera GT, which came closer to the purity of the F1 chassis by using carbon for the engine bay structure, but they remained with steel for the front crash structure. In 2002 the Ferrari Enzo was the first road car to adopt an all-carbon approach since the McLaren F1 in 1992. The Enzo uses carbon for all its structures and bodywork.