by Dale Brosius, Contributing Writer
High-Performance Composites Magazine
Text and Images: ©2004 High-Performance Composites Magazine
March 2004, pg. 33-37
Any list of the world's most highly regarded sports cars includes General Motors' venerable Chevrolet Corvette. An icon in American automobile history, the Corvette is in its 51st year of production. Since 1953, when only 300 of the fiberglass-bodied vehicles were produced, more than 1.3 million have been sold to sports car enthusiasts worldwide.
The Corvette also serves as GM's leading technology platform, notably in the areas of braking, engine technology and materials. The Z06 Corvette Coupe, the version equipped for highest performance, has a 5.7L, 405 hp powerplant that delivers a top speed of 171 mph/275 kph and goes from a standing start to 60 mph/100 kph in a seat-pinning 3.9 seconds. The 2004 model represents the final year of production of the C-5 (fifth generation) Corvette, with the next-generation C-6 entering production this summer as a 2005 model. To send the current design out in style, GM management developed a Commemorative Edition Z06, with a unique "LeMans Blue" paint and racing decor, a tribute to the Corvette race team's recent success at the famous 24-hour French endurance race (two teams placed 1st and 2nd in teh GTS class in both 2001 and 2002; and finished 2nd and 3rd last year). It comes complete with a lightweight carbon fiber hood -- the first application of the material on a painted, Class A exterior body panel offered as original equipment on a North American vehicle.
"We had several reasons for putting a high profile carbon fiber component on a production Corvette," explains Mark Voss, design release engineer in GM's performance car group. "From a marketing standpoint, new technology like carbon fiber creates enthusiasm among Corvette buyers. Further, this provided a perfect opportunity for GM to better understand and implement the technology and achieve weight savings in a key area, improving vehicle performance." Weighing only 20.5 lb/9.3 kg, the new hood shaves 10.6 lb/4.8 kg, or about 33 percent of the mass from the standard fiberglass sheet molding compound (SMC) hood, and is more than 50 percent lighter than an equal-sized steel hood. This translates into higher power-to-mass ratio, and improved fore-aft weight distribution. Between 2,000 and 2,500 Commemorative Edition Z06 Corvettes will be produced for 2004. To bring the hood to production, GM relied on aerospace autoclave and automotive SMC technologies and an intense development effort among molders and material suppliers.
Key to weight reduction in the hood is the unidirectional carbon fiber/epoxy prepreg used for the outer skin panel. While identical in form and quality to traditional prepreg tapes produced for the manufacture of aerospace components and sporting goods, the material brings a unique set of properties suited to production of large automotive panels. "Obtaining high thermal and mechanical properties, combined with fast cure and friendly processing characteristics, was a real challenge in product development," emphasizes Andrea Dorr, business development manager for Toray Composites (America) Inc. (Tacoma, Wash., U.S.A.), the prepreg supplier for the hood. "Of course the pricing had to be attractive, so that ruled out traditional aerospace-qualified materials," she adds. The prepreg selected and qualified for the hood is Toray P3831C-190, a combination of Toray T600024K carbon fiber, and Toray's G83 "quick cure" epoxy resin. The resin is formulated to maintain a traditional long shelf life and out life, yet fully cure in less than 10 minutes at 150oC/302oF, developing a glass transition temperature of 145oC/293oF (reported as onset of storage modulus loss by dynamic mechanical analysis). Aerospace epoxy prepreg systems typically require two or more hours to achieve the same degree of cure.
MacLean Quality Composites (West Jordan, Utah, U.S.A.) is the Tier 1 supplier of the carbon fiber hood to GM's Bowling Green, Ky., U.S.A. Corvette assembly plant. MacLean uses the autoclave process, adapted to achieve high-rate production, to mold the highly cosmetic outer skin panels, and also is responsible for the adhesive boding of the inner and outer panels and the surface quality of the finished hoods. Early in the program, MacLean and GM developed a 100 percent carbon fiber prepreg inner panel that, when assembled with the all carbon outer panel resulted in a hood weighing only 13.6 lb/6.2 kg, a 56 percent reduction over the baseline hood. While the performance and mass characteristics of the all-carbon solution were exciting, the complex inner panel, originally designed for the SMC process, was too expensive to produce in prepreg, and economics took over, notes Voss. The carbon fiber outer panel, at 1.2 mm/0.048 inch thickness, weighed 60 percent less than the 2.5 mm/0.100 inch standard density SMC outer panel, and was more practical to produce. Balancing tooling and piece costs against potential weight savings, GM elected to go with an autoclave-cured prepreg outer panel and a compression molded low-density SMC inner panel for the production version.
Supplied to MacLean by Meridian Automotive Systems (Dearborn, Mich., U.S.A), the inner panel is molded at Meridian's Shelbyville, Ind. facility, which also produces the standard SMC hood used on all other Corvette models. The inner for the Z06 Commemorative Edition uses the same low-density (1.3 specific gravity) fiberglass SMC employed on the all-fiberglass hoods and is molded in the same tooling, but with a novel modification - the peripheral bonding flanges incorporate AMC-8590 carbon fiber SMC, supplied by Quantum Composites Inc. (Bay City, Mich.). Approximately 1.0 lb/0.45 kg of the material, first used for structural components on the Dodge Viper SRT-10, is placed around the edges of the inner panel and co-molded with Meridian's low-density glass formulation.
"We found that the autoclave-cured carbon fiber bonded to the low-density SMC inner did not provided the overall system performance we expected, primarily due to the large mismatch in mechanical properties between the two materials," Voss explains. "Putting carbon fiber SMC in the bonding areas of the inner creates a transition zone, providing efficient transfer of stress loads between the panels, which was validated in performance testing." This testing matrix included thermal cycling, hood slam, deflection, torsion, durability and crash, among others. In every test performed, the hybrid carbon fiber hood met or exceeded the results of production SMC parts. Dent and hail testing showed the carbon fiber outer panels performed far superior to traditional steel body panels.