Understanding carbon fiber repairs

Sept. 1, 2016
As more OEMs use more carbon fiber material in car manufacture, there will be more damaged carbon fiber vehicles needing repair, and a knowledgeable repair technician should be ready to make the repairs properly.

Carbon fiber material for car manufacture is popular due to its superior strength and light weight even though it is more costly than metals, such as steel and aluminum, and traditional composite materials. Carbon fiber is made of thin carbon filaments bound together with a plastic polymer resin to form a composite material. The material features a “woven” design that is exposed for use on exterior vehicle components such as door and roof panels, fenders and hoods. Carbon fiber can also be painted and used on exterior or interior surfaces.

While carbon fiber is becoming more prevalent as a substrate in car manufacturing, it is still pretty much relegated to high-end automobile models. The more complicated production techniques and molding requirements for manufacturing carbon fiber parts keep the costs high. Carbon fiber bodies and/or parts can be found on super-expensive race cars and sports cars, and more recently on “less-expensive” models from BMW, Chevrolet Corvette, and Alfa Romeo. As advances in manufacturing technology bring down the costs of using carbon fiber, the material will find wider acceptance in more mainstream vehicles.

Carbon fiber consists of thin carbon filaments bound together with a plastic polymer resin to form a composite material.

According to a report by Lux Research, “Scaling up Carbon Fiber: Roadmap to Automotive Adoption,” carbon-fiber reinforced plastics (CFRPs) “will be poised to gain widespread adoption for automotive lightweighting by 2025, driven by a faster-than-expected pace of technology development.” The report also notes that the adoption of CFRPs is prevalent on whether “they can become affordable enough for use in mainstream vehicles.

Cosmetic vs. Structural Repairs
It is important to understand the difference between a cosmetic and a structural repair. Most of the repairs will be structural repairs to the cosmetic panels, such as mending a hole in a carbon-fiber panel. Although the cosmetic carbon-fiber panels add some strength to the car, they are not structural to the integrity of the whole vehicle. The majority of the carbon-fiber panels in use now are mechanically fastened to the car, although there are some panels, such as Tesla’s, that are bonded to the base structure.

As for choosing the proper adhesive for making a repair – bonding will be done with a urethane or epoxy adhesive, while repair work is always done with an epoxy. The decision centers on the benefit the repair accomplishes for the life of the vehicle. If the damage to the vehicle requires a cosmetic repair to a structural element, then epoxy is the choice. Epoxies are not flexible and will form a solid attachment; urethanes are too flexible for this type of repair.

A repair is considered to be cosmetic when the carbon fiber is not damaged – such as a surface scratch or pitting to a panel. This basic type of repair involves hiding the imperfection and painting the repaired portion. An epoxy filler can be used to make this repair, since it is as rigid as the panel. When damage has been done directly through the carbon fiber part, such as a hole, the damaged fiber must be replaced with a suitable repair fiber and an epoxy.

Carbon Fiber Repairs
When the actual structure of the vehicle is composed of carbon fiber, repairing damage takes more skill and the repair process is defined by the OEM.  Due to the configuration of the carbon fiber material, repair work must take into account how many layers of carbon fiber are involved, along with what type of carbon fiber cloth needs to be installed and at what orientation.

Carbon fiber cloth has a directional weave. The various layers of carbon fiber cloth are rotated – 30 degrees, 45 degrees, 90 degrees – because the cloth has more strength in one direction than another direction. The final part can have as many as 12-to-14 layers or more.

Due to its strong lightweight properties, pre-impregnated (pre-preg) carbon fiber us finding uses as an alternative to fiberglass and sheet molded compound (SMC).

Taking the composition of the carbon fiber material into account, this type of repair is even more critical. To make a repair, you perform a process called “scarfing.” In this process, the repair technician uses a tool to grind outwards to expose each layer of cloth and then begins to build the repair from the low center. Each layer of repair carbon fiber must be oriented to match the original structure.   

As carbon fiber becomes more mainstream as a manufacturing material, it will be used in more areas of car design. Since carbon fiber is strong and lightweight, OEMS are using it as reinforcement in A or B pillars, rocker panels and roof rails on vehicles. Carbon fiber is also being used as an alternative to “fiberglass” or sheet molded compound (SMC). As an example, the hood on a current production car is comprised of 50 individual pieces of carbon fiber. The carbon fiber is pre-impregnated (pre-preg) with end resin. All the pieces are placed into a mold in a specific layering orientation. The mold is heated and the part is formed. With this procedure, the OEM obtains the moldability of SMC, and the strength and light weight of carbon fiber, in addition to a lowered cost suitable for high-volume operations. 

The crucial question that has to be answered when deciding to repair or replace a carbon fiber part is: “Will the repair be strong enough to survive the life of the vehicle?” If a damaged carbon fiber part has a hole in its center, that repair will be strong since it is surrounded by support. If a carbon fiber part is missing a corner piece, such as a corner broken off of a hood, can that part be repaired, or does a new hood have to be ordered? Because the repair area is hanging off the edge of the hood panel, will the repair be strong enough for the lifetime of the vehicle? The final decision rests on what the part to be repaired is going to be subjected to through its life, where it “lives” on the car, and how detectable is the repair.

You can use a generic carbon fiber cloth, or repair cloth, which can be kept in stock, for performing structural repairs to a cosmetic panel (such as a small hole). When damage to the carbon fiber part is more severe, or damage to the vehicle structure is being repaired, specific carbon fiber material must be used. This type of material, especially pre-preg versions, cannot be kept in stock. Pre-preg carbon fiber cloth must be stored at -40 degrees. It is shipped on dry ice and has a very short life span once it is removed from the shipping container. The material cannot be stored; it must be cut it to fit and installed as quickly as possible. These repairs are very manufacturer/vehicle specific, so check with the OEM before attempting repairs on carbon fiber parts that display significant damage. Furthermore, additional repair equipment and training will probably be required.

Choosing a Repair Product
Basic repairs to carbon fiber parts are not difficult to execute. No special equipment, tools or products are needed for repairing structural damage to a cosmetic part or panel. Most repair shops should have on-hand an epoxy for small repairs to a cosmetic panel, and urethane for bonding a carbon fiber panel to a vehicle. Repair technicians should be familiar with how to use these adhesives. There are no differences when repairing carbon fiber parts and SMC panels.

No special equipment, tools or products are needed for repairing structural damage to a carbon fiber cosmetic part or panel.

If a repair technician is not sure which adhesives to use, check with the OEM and the supplier. It is important that the repair technician understands the capabilities of the adhesive. Be sure that adhesives formulated for fiber-reinforced plastic and fiberglass repairs can also be used on carbon fiber material. Consult with the supplier for information on which adhesives are suitable for carbon fiber repairs.

When bonding a carbon fiber panel to a vehicle, rely on OEM information. You don’t want to
“overbond” a panel or make the attachment too rigid. Remember that the carbon fiber material is different from the vehicle’s steel or aluminum structure; it is important to accommodate for the expansion, contraction and twisting nature of the carbon fiber part. 

Carbon Fiber Education
Several training programs are available, such as Abaris Training and I-CAR Alliance courses, for those interested in learning more about carbon fiber repairs.

Abaris Training offers a variety of courses in advanced composite structures engineering, manufacturing and repair. The repair courses offer hands-on practice to provide students with the knowledge needed to conduct top-quality repairs in an efficient manner. Among the courses available are “Advanced Composite Structures: Fabrication and Damage Repair” and “Adhesive Bonding of Composites & Metals.”

I-CAR (Inter-Industry Conference on Auto Collision Repair) Alliance programs bring together technical training providers to optimize educational efforts in the collision industry. As an example, the Fusor 003 Composite Repair & Bonding course provides instruction on making OEM-approved repairs using LORD products. The course covers the techniques, procedures and safe use of adhesives and seam sealers; how to select the proper product for each type of repair; and surface preparation for the best repair results. Upon completion of the course and exam, the attendee can apply for I-CAR alliance credits.

Repairing damage to carbon fiber cosmetic damage is similar to fixing damage to any composite panel – just make sure to use a fiber-rich adhesive to ensure the repair will be long-lasting with no read-thru. As more OEMs use more carbon fiber material in car manufacture, there will be more damaged carbon fiber vehicles needing repair, and a knowledgeable repair technician should be ready to make the repairs properly. 

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