Although the C5 Corvette will be coming to the end of its production road soon, as the C6 will make its initial public debut in 2004 at the North American International Auto Show in Detroit, the C5 is by no means running out of gas so far as vehicle style and innovation go. To make sure that the car continues to be special, General Motors engineers developed a Commemorative Edition Corvette, one that stands as a tribute to the vehicle’s performance in GTS class racing. . .at Le Mans, in particular. Yes, there is special paint: Le Mans blue. Special badging. And a shale-colored interior, complete with specially embroidered headrests.
But there’s something more significant about this limited-edition Z06. Something that the causal eye won’t notice. But something that is race-oriented. Something that sets the Corvette apart from other vehicles in its class. Something that’s “go,” not just “show.” The hood above the 405-hp V8 on the Commemorative edition is special: It is made of carbon fiber.
According to John Remy, lead engineer for body structures and closures for the vehicle, this sets the Corvette well apart from all but the limited-edition vehicles. The carbon-fiber hood is the largest painted production component of its type being fitted to cars. In terms of volume, he suggests that it is “an order of magnitude higher than anything else being done today.” They’re making 2,000 or so of these special vehicles.
All of the aforementioned badging apart, the fact that the carbon fiber is being used is somewhat hard to discern. If someone looks very closely at the vehicle, they’ll notice that the hood is something out of the ordinary. That’s because there’s a red border that surrounds a silver graphic painted on the car, a border that consists of a woven pattern—woven like the carbon fibers below. But you’ve probably got to know.
WORKING HARD TO MAKE IT SEEM ORDINARY
One thing to note about the hood and its transparency vis-à-vis what it is made of presented a challenge to Remy and his colleges. Although the Corvette body is otherwise SMC, the carbon fiber composite is trickier when it comes to surface quality. “There were a lot of difficulties in the development,” Remy says. “The biggest was getting a Class A paintable surface.” This problem takes the form of what is known as “bond line readout.” That is, the inner panel—a compression-molded SMC component (it is a low-density SMC that includes a layer of carbon fiber)—is adhesively bonded to the outer, the carbon fiber-epoxy composite panel. (The inner is necessary because there is the need for such things as the latch/striker, hood blanket, under-hood lamp, and hinges to be mounted.) What happens is that when the pieces are painted, there is the potential—almost a likelihood, in many cases—for the trail of the adhesive to become visible through the outer painted surface.
One way that the bond line readout problem is sometimes solved is by using a thicker outer panel (e.g., the Cadillac XLR, which is built in the Bowling Green Assembly Plant along with the Corvette, has an SMC outer that’s 3-mm thick, which is about 0.5 mm thicker than the panel used for the conventional Corvette: the reason that the Cadillac engineers decided to go with a thicker panel was to minimize any bond line readout). However, as Remy explains, one of the reasons why the composite is being used for the Commemorative Edition is in order to have as thin a panel as possible (while, of course, still achieving the structural performance required). “We’re making the panel at 1.2 mm,” Remy says, adding, “It’s the thinnest bonded skin that I’m aware of.” One of the ways that they are getting around the bond line readout problem is by using a reduced amount of adhesive as compared with a comparable SMC component: “Because the carbon fiber panels are so strong, we don’t need as much adhesive as SMC,” he explains.
ASSEMBLING & CURING: A COMPARATIVELY LENGTHY
PROCESS (BUT IT COULD BE LONGER)
The outer panel is based on a unidirectional prepreg tape from Toray Composites. It is a multilayer ply pattern. This is a rather labor-intensive process, that combines laying some of the fabric directly into the mold, and some of the material onto a preform that’s brought into the mold separately. Unlike a typical SMC, the carbon fibers that are used are long strands, not chopped bits. In a chopped SMC, there’s, perforce, random orientation of the fibers, which provides strength. In the advanced composite material, the strength is a consequence of the specific orientation of the fibers. Speaking of the hood, Remy says, “We’re probably stronger than we need to be.” Another consideration that was taken into account is crash energy management: “We purposely stayed away from placing the fibers longitudinally,” he notes.
In addition to the lengthy hand lay-up of the materials, there is the use of an autoclave to cure the epoxy resin. The mold is placed into a vacuum bag, then into an autoclave. “We’re in the autoclave for approximately two hours, to heat up the mold, bring it to cure temperature, then to cool it so that it can be safely removed,” Remy explains. He adds with more than a bit of understatement: “The throughput is limited by that cycle: that ends up being the bottleneck in the process.” By way of comparison, the cycle time for compression molding an SMC part is 180 seconds. “The two-hour time limits us in terms of volume,” he says. “We could buy more tools, but then that drives the business case”—into an area that’s less amendable to someone actually being able to buy the cars.
According to Remy, of the total cycle time for the composite hood, the mold is at cure temperature for about 10 minutes, which leaves 110 minutes essentially wasted in heat transfer. GM engineers, along with those at MacLean Quality Composites (West Jordan, UT), which is actually producing the outer panels (that’s right: a company that’s at the base of the Wasatch Mountains that makes a variety of carbon fiber composite products including windsurfing masts and bicycle frame tubing is making the outers; Remy says that prior to selecting MacLean, which does have an automotive component to its business, they talked with a variety of possible suppliers, even including Boeing), are working toward reducing the processing time.
However, he notes, “I believe we have taken the process farther than anyone else has in productionizing it.” He says that (1) the molds they’re using are less expensive than those used by other companies, (2) other companies can have their molds in the autoclave for as many as 20 hours, and (3) “They spend a lot of time in hand-finishing. What they get is not a Class A surface, so they sand and prime, again and again. You can afford that on six-figure cars—and the first figure is not a ‘1.’”
Compared with a typical Corvette, the carbon fiber hood for the Commemora-tive Edition Z06 is lighter: 20.5 lb., or 10.6 lb. less than a typical hood. The hood was selected, Remy explains, in order to take mass out of the front of the vehicle.
Corvette has been something of a technology platform for General Motors since the first 1953s rolled out. Although he’s closed lipped about the further use of advanced materials, John Remy does admit, “We’re already looking into the future, to see how we can do this more affordably.”