Saturday, 31 December 2005 17:00

Carbon fiber, the composite material of the future?

Written by I-CAR Advantage Online
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Lighter, faster, and stronger. These three words sound like a description of the main character in a super hero movie. But they can also be used to describe what vehicle engineers are striving for when they design vehicles. Carbon fiber is one of the materials being used to do just that; make a vehicle lighter, faster, and stronger. 

As this new material enters the vehicle production lines, there are a few questions rising from the collision industry. Can this material be repaired? Can the same techniques and repair materials used on other composites be used to repair parts made with carbon fiber? Are structural parts made with carbon fiber repairable, or are they removed and replaced with a new part? To answer these questions, first look at what carbon fiber is, then, at some of the repair issues.

What is carbon fiber?

Carbon fiber reinforced composite (CFRC) is classified as a composite - which is a combination of two or more materials that, when combined, make up a stronger material. Other examples of composites include sheet-molded compound (SMC) and fiberglass. Traditional fiberglass parts are generally made using a polyester resin.

Carbon fiber is a mixture of fibers made from carbon (see Figures 1 and 2) and a resin, usually consisting of an epoxy. When the resin is hardened, the carbon fibers reinforce the material, making it extremely strong. The process is very similar to the processes used to make SMC. Composites are made by using a mold and glass fibers combined with a resin.

With CFRC, the strength comes from having the correct concentration of resin to the amount of carbon fibers, as well as the orientation of the carbon strands (see Figure 1). The excessive strength due to the strength of the carbon and the orientation of the strands allows vehicle engineers to build a lighter and thinner part, yet maintain the strength and rigidity of using traditional materials.

Applications

Most vehicles that have CFRC exterior parts are high-end sports cars where weight savings from using lighter materials has a positive effect on the horsepower-to-weight ratios. These vehicles are the proving grounds for new materials that may eventually enter the mainstream of vehicle production.

General Motors first used CFRC for the hood on the 2004 Commemorative Edition Corvette Z06. This created a weight savings of 56% over the traditional SMC hood, while maintaining or exceeding the strength of the part.

GM currently uses CFRC for the front fenders, the outer portion of the front wheelhouse (see Figure 3), and the floor boards of the 2006 Chevrolet Corvette Z06. The fenders are 1.2 mm thick, as opposed to the traditional reinforced reaction-injected molded (RRIM) plastic fenders from the previous Corvette Z06 that are 3 mm thick. The weight of each front fender is a mere 1.2 kg (2.7 lb).

The rigidity of carbon fiber for the fender application also reduces the need for reinforcements required for the thermal expansion characteristics of RRIM, thus creating a secondary weight savings. The floorboards are made with a balsa wood core encased in carbon fiber sheets (see Figure 4).

Some other vehicle makers using CFCR include the:

• 1997-2006 McLaren F1 for the complete body and chassis.

• 2003-2006 Mercedes-Benz SLR McLaren, which has a front structure made from CFRC (see Figure 5).

• 2003-2006 BMW M3 CLS, for the roof outer panel. Models with the carbon fiber roof are not available in the U.S. at this time.

• 2004-2006 Acura NSX-R, for the hood and spoiler.

• 2004-2006 Aston Martin Vanquish for the structure of the vehicle.

Repairability of CFRC

CFRC does offer some repairability, but the application, as well as the vehicle maker's recommendations, must be followed. When CFRC is molded into shape, the mixture of the resins and carbon are considered the primary bond. It is within this state that the part will be the strongest. All repairs to CFRC are considered secondary bonds, meaning the integrity of the repair depends on the adhesive properties of the repair resins or adhesives.

The application of the part is probably the most important consideration. CFRC parts made for structural purposes are generally replaced when damaged. There is no way to guarantee that the repaired part will have the same strength as the original part.

Parts used primarily for cosmetic purposes may be repairable. GM allows the front fenders on the 2006 Chevrolet Corvette Z06 to be repaired, provided that the damage does not extend to an edge.

Damage such as a puncture or tear that protrudes through the part should be repaired using a two-sided repair. This repair requires making a large taper to maximize the amount of surface area the repair materials can adhere to. When making the taper, the edges should be rounded without any sharp edges or angles. A backing plate should be made on the inner panel, either from a piece of SMC or a piece of CFRC. If a backer plate is not available, an alternative is to construct a backer from fiberglass fabric and repair adhesive.

The front side is repaired using a pyramid patch technique, where layers of fiberglass reinforcement fabric are sandwiched with repair adhesive in the taper from the smallest to progressively larger (see Figure 6), until the taper is filled with the repair materials. A roller is used to force any air out of the repair (see Figure 7), and heat is applied to cure the adhesive. After the repair has cured, it is sanded to contour and prepared for refinishing.

Conclusion

As vehicle makers continue to seek out weight-reducing materials for increased performance and greater fuel economy, CFRC is sure to be one of the materials that is looked at.

The reparability of CFRC depends largely on the vehicle maker's procedures and recommendations. Some adhesive product makers have developed procedures for repairing CFRC used on cosmetic panels, but structural parts made with CFRC will require replacement if damaged.

Although presently the use of CFRC is limited to high-end sports cars, technological advancements in the forming processes will make the product more available for vehicle parts. In the not-so-distant future, CFRC repairs may be as common as repairing SMC or sheet metal is today.

This Advantage Online article first appeared in the I-CAR e-newsletter, which is published and distributed free of charge. I-CAR, the Inter-Industry Conference on Auto Collision Repair, is a not-for-profit international training organization that researches and develops quality technical education programs related to collision repair. To learn more about I-CAR, and to subscribe to the free e-newsletter, visit http://www.i-car.com or contact I-CAR Marketing Communications Specialist Brandon Eckenrode at Brandon. Eckenrode@i-car.com.

 

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