This article was first published in 2006.
article first appeared in the I-CAR Advantage Online, 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
publication, visit www.i-car.com.
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 fibre 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 arising 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 fibre? Are structural parts
made with carbon fibre repairable, or are they removed and replaced with a new
part? To answer these questions, first we should take a look at what carbon
fibre is. Then, we can look at some of the repair issues.
What is Carbon Fibre?
Carbon fibre reinforced composite (CFRC) is
classified as a composite. A composite is a combination of two or more materials
that, when combined, make up a stronger material. Other examples of composites
include sheet-moulded compound (SMC) and fibreglass. Traditional fibreglass
parts are generally made using a polyester resin. Carbon fibre is a mixture of
fibres made from carbon and a resin, usually consisting of an epoxy. When the
resin hardens, the carbon fibres 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 mould and glass fibres combined with a resin.
With CFRC, the strength comes from having the correct concentration of resin to
the amount of carbon fibres, as well as the orientation of the carbon strands.
The greater 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.
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 power-to-weight ratios. These vehicles are the proving
grounds for new materials that may eventually enter the mainstream of vehicle
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....
...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 moulded (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 fibre 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 floor is
made with a balsa wood core encased in carbon fibre sheets.
Some other vehicle makers using CFCR include
- 1997–2006 McLaren F1 for the complete body and
- 2003–2006 Mercedes-Benz SLR McLaren, which has a
front structure made from CFRC (pictured)
- 2003–2006 BMW M3 CLS, for the roof outer
- 2004–2006 Acura NSX-R, for the hood and
- 2004–2006 Aston Martin Vanquish for the
structure of the vehicle (pictured)
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 moulded 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
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 fibreglass fabric and repair adhesive.
The front side is repaired using a pyramid patch
technique, where layers of fibreglass reinforcement fabric are sandwiched with
repair adhesive in the taper from the smallest to progressively larger, until
the taper is filled with the repair materials.
A roller is used to force any air out of the
repair and heat is applied to cure the adhesive. After the repair has cured, it
is sanded to contour and prepared for refinishing.
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 currently 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.
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