Carbon Fiber Goes Mainstream Automotive
Improving performance in sports cars is quite simple — just increase the power-to-mass ratio of the vehicle. Getting there requires either raising the engine's output (horsepower and torque) or reducing the vehicle's weight. If successful, the results include faster acceleration and higher top speed, advantages sports car buyers desire. For the Dodge Viper SRT-10 convertible's first top-to-bottom redesign since its introduction in 1992, DaimlerChrysler engineers pursued both options: The new Viper boasts a huge powerplant, a 505 in3/8.3L V-10 that produces 500 hp and 525 ft-lb of torque, up 11 and 7 percent, respectively, over the previous engine. Engineers set a vehicle weight reduction goal of 91 kg/200 lb, to be achieved through innovations in design (e.g., parts consolidation) and use of lightweight materials. An impressive 21 kg/46 lb of that goal was reached through what DaimlerChrysler product engineer Mike Shinedling calls "the first use of carbon fiber sheet molding composite in a production automobile."
The Viper team looked at many material options to save weight. Aluminum stampings and welded brackets were ruled out due to tooling costs, Shinedling explains. Viper marketing personnel were asking for carbon fiber components for their "high-tech" impact on customers. But the aerospace prepreg technology used in certain "supercars," produced in a few hundred per year, was not considered practical for the Viper volume of 3,000 units. Carbon fiber prepreg would require different processes and equipment. By contrast, carbon fiber sheet molding compound (CFSMC) relied on familiar technology that DiamlerChrysler knew could be extended to much higher volumes, emphasizes Doug Denton, senior materials specialist. "It allowed us to extrapolate conventional SMC design concepts and processing methods without having to make a huge jump, thereby reducing risk."
CFSMC totaling 8.0 kg/17.5 lb per car is used in the doors, the windshield surround, and the six components that make up the fender support system. All were designed using DaimlerChrysler's standard CATIA software (IBM PLM, Dallas, Texas) and molded by Tier 1 supplier Meridian Automotive Systems (Dearborn, Mich.) at its facility in Shelbyville, Ind.
HYBRID COMPOSITE BALANCES PERFORMANCE, COST
The doors and windshield surround are principally fiberglass SMC; carbon fiber SMC augments each structure in critical load areas and is co-molded with the fiberglass SMC.
On the doors, styling features — in particular the large gill opening between the front fenders and the doors — limit the height of the hinge pillar to each door's lower half. The weight of the door creates a sizable moment load on its hinges and on the door's inner panel. In the original Viper, large steel panels were attached to the doors and hinge pillars to attain the required stiffness. For that reason, critical performance requirements for the 2003 vehicle included a reduction in door sag, or the maximum deflection due to a load applied to the edge of the open door, and a reduction in permanent set, or the residual deflection after the load is removed. The 20 percent of the inner door panel that is nearest the door hinge, where the loads are highest, is made from AMC-8590 carbon fiber SMC from Quantum Composites (Bay City, Mich.) while Meridian's low-density fiberglass SMC covers the remaining 80 percent of the surface. AMC-8590 is a toughened vinyl ester resin reinforced with a 55 percent loading of chopped 12K PAN-based carbon fiber in a random orientation. (The door outer panels are fabricated from conventional Class A fiberglass SMC, supplied by Meridian.)
Working with Meridian and Quantum Composites, DaimlerChrysler developed an insert joint technique, overlapping the CFSMC on each side of the low-density glass SMC in the charge pattern. This results in a double scarf joint and eliminates "knit line" effects which compromise the integrity of the structure. A total of 0.45 kg/1.0 lb of carbon fiber SMC is used in each door. After molding, a steel hinge reinforcement much smaller than that used on the original Viper, is bonded to the door inner panel, and the inner and outer panels are bonded together using Pliogrip two-component urethane adhesive supplied by Ashland Specialty Polymers and Adhesives (Dublin, Ohio). The assembled door has a 206 percent improvement in door sag stiffness and a 350 percent improvement in permanent sag deflection. The smaller steel reinforcements contribute to a door system mass reduction totaling 3.0 kg/6.5 lb per vehicle.
The new Viper's windshield slope is more acute and its A-pillars, the sloped sides of the windshield frame, are considerably longer than those on the original model (610 mm/24 inches vs. 490 mm/19.3 inches). Recent federally mandated head-impact requirements drove the addition of an energy-absorbing interior trim cover to each pillar. Styling requirements forced a decrease in the cross-sectional area of the structural portion of each A-pillar to accommodate its non-structural thermoplastic cover.
The combination of a longer pillar and reduced cross-section posed a considerable challenge for the Viper engineering team. To answer the challenge, DaimlerChrysler turned to a patented two-piece windshield surround introduced in 1997 on its Plymouth Prowler. It consists of an inner and an outer panel molded in fiberglass SMC and bonded with structural adhesive. On the Prowler surround, unidirectional glass fiber SMC is molded along the length of the A-pillars in combination with random glass SMC, to provide additional stiffness. Unidirectional fiberglass, however, did not provide sufficient stiffness for the Viper surround, so the team opted to incorporate unidirectional CFSMC in combination with 50 percent-reinforced, random fiberglass SMC.
Each windshield surround contains 0.92 kg/2.0 lb of Quantum's AMC-8595, produced using the same toughened vinyl ester resin as the AMC-8590, but reinforced with a unidirectional, cross-stitched 12K continuous carbon fiber mat at 55 percent loading. For the outer panel, 0.23 kg/0.5 lb of AMC-8595 is sandwiched along each pillar between layers of fiberglass SMC, which fully encapsulate the stiffening carbon during molding. The inner panel features 0.23 kg/0.5 lb of unidirectional carbon on the inside surface of each A-pillar, which is covered by the thermoplastic trim panel — the balance of the inner material is fiberglass SMC.
Following molding and trimming, the inner and outer panels are joined, using urethane adhesive. The 8.9 kg/19.6 lb assembly demonstrates a 45 percent lower deflection in bench testing than the original Viper windshield surround.
FRONT STRUCTURE PARTS CONSOLIDATION
The new Viper's six-piece fender support system replaces 15 to 20 metal parts. Compression molded exclusively from random carbon fiber AMC-8590, the system not only provides fender attachment points, but also supports the headlamps and serves as the principal dimensional reference for the entire front end of the vehicle, emphasizes Shinedling. This reference is the basis for locating the majority of the underhood components and the hood. Initially conceived as single left- and right-hand molded structures that would attach to the vehicle frame, the design evolved into three components per side — a fender support, headlamp support and a sill-to-fender bracket.
Breaking each side into three separate molded items reduced the tooling complexity and increased serviceability of the components. Structural analysis of the fender support structure, performed using NASTRAN finite element analysis software, showed that carbon fiber makes a significant contribution to global vehicle stiffness. The front-of-dash and A-pillar stiffness is increased by 22 percent, and the first bending mode by 3.1 Hz. Shinedling also says headlamp flutter is significantly reduced.
The fender supports, weighing 1.93 kg/4.25 lb each, contain molded-in inserts that provide attachment points for 34 underhood components. The 0.80 kg/1.75 lb headlamp supports are attached to the fender supports at the Meridian facility, using urethane adhesive, and this assembly is subsequently secured to the vehicle frame at the assembly plant with urethane adhesive and mechanical fasteners. Each 0.34 kg/0.75 lb sill-to-fender bracket is attached afterward to the fender support with three rivets. With an average thickness of 2.0 mm/0.080 inch, the 6.14 kg/13.5 lb assembly reduces vehicle weight by approximately 18 kg/39.6 lb compared to the metal alternative.
ACCOMMODATING MATERIAL DIFFERENCES
Although there are considerable similarities between carbon and fiberglass SMC, there were some techniques that had to be developed to ensure complete fill and acceptable moldings, notes Dan Dowdall, director of composites engineering for Meridian. Though the tooling is very similar to that for fiberglass SMC (constructed of P-20 steel with hardened chrome), mold design is affected because there is near-zero mold shrinkage in the planar directions with carbon, explains Mike Kiesel, quality assurance and technical services manager for Quantum Composites. Additionally, the molded parts are thinner than those molded from fiberglass SMC and the 55 percent carbon fiber SMC flows less easily than fiberglass SMC — factors that required some "science" in charge placement and coverage, plus higher molding pressure, says Dowdall. Mold coverage of 70 to 90 percent is necessary to completely fill the carbon fiber parts. New part handling techniques and cooling fixtures were developed to improve the dimensional capability of the molded parts. Deflashing, drilling and routing are more difficult, as carbon fiber is much stronger than glass — Meridian's Shelbyville operations use waterjet, laserjet and drill fixtures to cut openings in the parts.
Kiesel envisions eventual use of CFSMC in floor pans and other inner panel structures where weight savings must be accomplished without sacrificing structural strength. Shinedling agrees, noting, "Almost any structure using glass fiber today could benefit from carbon fiber." This includes the underbody and cross-car instrument panel backing structures, where the two- to three-fold increase in material stiffness could result in part weight savings of as much as to 50 percent.
Some industry observers believe the Viper's use of carbon fiber SMC is the first step toward eventual integration of carbon fiber into automobiles made in larger volumes — including its use in exterior body panels. David White, vice president of sales and marketing for Meridian Automotive Systems, sees significant implications arising out of the Viper program, especially as U.S. Corporate Average Fuel Economy (CAFE) mandates play a larger role in new vehicle design. An enabler will be lower material prices that come with higher volumes.
"Even though the Viper is a relatively low-volume vehicle, this still represents the first use of 100 percent carbon fiber components on a U.S. production automobile," White sums up. "Historically, such platforms have served as the introduction point for new materials and new technologies that later become much more widely applied in high volumes."