Not composite yet
Carbon fibre reinforced polymer (CFRP) composites first found their way into Formula One during the mid-1970s; however their full potential was not universally recognised until 1981 when John Barnard pioneered the carbon fibre monocoque with the McLaren MP4/1. After John Watson’s crash at Monza that season silenced sceptics’ concerns over structural integrity during impact, all Formula One teams followed suit, developing their own composite monocoques. The carbon fibre revolution had begun.
The superior specific strength and stiffness of CFRP to that of metals (Table 1), as well as its formability, have made it the primary choice for highly loaded and aerodynamically optimised components of the modern Formula One racecar. More and more parts have evolved from metallic to composite construction as the teams’ engineers fight to reduce weight. More than three-quarters of a contemporary Formula One car’s volume is now made from CFRP, while it accounts for less than a third of its mass.
So as this material offers such performance benefits, why aren’t even more components constructed from carbon composite? Outside of Formula One, the greatest obstacle would be cost (design, materials and production), but in the highly competitive world of motor racing this is unlikely to be prohibitive. The technical regulations restrict the use of materials for certain components, forbidding the application of composites in some areas, but one of the major limiting factors is structural integrity at elevated temperatures above 400 C.
Carbon fibres themselves have very high temperature resistance (carbon sublimes at above 3000 C); however it is the polymer matrix that limits the ultimate operating temperature of the composite. The predominant matrices are thermosetting polymers, examples being epoxies and polyesters, with Formula One composites being dominated by the former. Epoxy systems exhibit maximum glass transition temperatures (Tg) of around 180 C above which the matrix softens, taking on a rubber-like state, greatly reducing the load-carrying capability of the composite. There are alternative polymers, including bismaleimide (BMI) and cyanate ester, which can achieve transition temperatures of up to 380 C.
Less widely used thermoplastic matrices consisting of polymers such as PEEK (polyetheretherketone), polycarbonate and PEI (polyetherimide) offer transition temperatures of up to 215 C. These again are incapable of continuous exposure to the high temperatures experienced by some Formula One components, such as the exhausts and brake discs, which can both encounter temperatures in excess of 950 C.
Composites have been used for brake discs and pads in the form of carbon-carbon (carbon fibres reinforcing a carbon matrix) since their introduction by Brabham in 1976, operating at an average temperature of 650 C and peak temperatures of around 1000 C. This material offers suitable properties for the exhaust system, but unfortunately its production method is not viable for the complex geometry.
This has meant that Formula One exhausts have remained metallic, fabricated from a superalloy called Inconel. This metal was developed for aerospace applications, and retains its high strength at over 1000 C. It has a higher density than steel (0.00844 g/mm3 compared with 0.0078 g/mm3) and to save weight it is used in thinwall sheets between 0.7 and 1.2 mm thick. This exposes the exhaust system to high stress levels and fatiguing, ultimately leading to the formation of cracks which can result in retirement from a race.
There are alternative composite materials available, or in development, that could supersede this superalloy, reducing weight and increasing the life of the exhaust. Metal matrix composites (MMCs) and ceramic matrix composites (CMCs) are offering engineers superior specific mechanical properties in high-temperature environments.
CMCs currently offer the best potential for use in a Formula One exhaust system. There are various materials available, including alumina matrix reinforced with ceramic oxide fibres and glass-ceramic matrix reinforced with silicon carbide (SiC) fibres, with densities between 0.00185 and 0.0028g/mm3. These can be laminated using predominantly conventional polymer composite techniques in complex geometries, and can resist long-term exposure to 1000 C and brief exposure to even higher temperatures. They also offer improved thermal management properties over Inconel, with a thermal conductivity an order of magnitude lower (0.9 W/mK compared with 9.8 W/mK for Inconel 625).
There is still development to be done before CMCs will replace all highly loaded high-temperature metallic components such as the exhaust, and Formula One will need to gain confidence in these materials. Access to CMCs can be restricted, with some designated as strategic materials, but demand from the aerospace industry is high, where it is believed they will deliver considerable benefit to jet engine performance. This will ultimately drive development, making CMCs a practical material choice for engineers.
Fig. 1 - Comparison of mechanical properties of metallic and composite materials
Fig. 2 - Prost Grand Prix AP04 2001 Formula One Championship car’s fabricated Inconel exhaust system
Written by Dan Fleetcroft