Carbon fibre repair methods

Shift over strength steels and make room for the latest automotive trend – carbon fibre. II-Car CEO Richard Pratt considers the repair techniques needed to restore vehicles which use this material.

For many years vehicle manufacturers have struggled in an attempt to reduce vehicle weight without compromising structural rigidity and safety. We have seen a huge increase in the use of more exotic steels, and we are now facing a future where more and more composite materials will make their way into the production lines, and eventually to the repair industry.

Carbon fibre is just one of the materials used to make vehicles lighter, stronger and faster.

As this new material gains greater usage, there are a few questions rising from the collision industry. Can the same techniques and repair materials used on other composites be used to repair parts made with carbon fibre? Are structural parts made from carbon fibre repairable, or are they removed and replaced with a new part?

The make-up of carbon fibre

Understanding carbon fibre itself means delving into the sciences that support
its manufacture.
Carbon fibre reinforced composite (CFRC), like all composites, is a combination of two or more materials that when combined make up a stronger material. The most common composite would be fibreglass, where a polyester resin is used to bind together the fibreglass matting.

Carbon fibre on the other hand, has a much stronger base fibre, where around 80,000 individual strands make up a woven strand, which is then matted together and bonded using an epoxy resin. The strength comes from the use of carbon, and the correct concentration of resin to the amount of fibres.

The manufacture process of carbon component manufacture is carried out by either pre-preg laminating, rtm (resin transfer laminating) or vacuum infusion.

Carbon fibre composites are used in construction for both closure panels and mudguards, as well as structures like the chassis in the McLaren Mercedes Coupe and the Lexus LFA sports car. Generally these structural components are a combination of carbon fibre in association with a honeycomb monocoque, and any damage to the structure would mean a new part replacement.

When to repair or replace

Closure panels (guards, door skins, turrets) bear little or no structural assistance and are generally repairable. They come as open visible weave, which, due to the look of the weave and the impossibility of accurately re-aligning the fibres, are not repairable. For non-visible weave, i.e. where the weave is covered by resin, either coloured or a covering colour coat, are normally repairable by use of adhesive repair. The only time this non-visible weave is considered too difficult to effectively repair is where the edge of the panel is damaged and it exposes some of the fibre ends.

The repairable panels are normally reclaimed by the use of adhesive product applied to the backside of the panel after preparation of the surface in accordance with the manufacturer guidelines. This reclamation process requires grinding out some of the material/resin and reapplying a new mixture of the relevant chemical adhesives. The re-application process is quite specific in terms of how the resin is laid down and built up.

Careful use of adhesives

The use of chemically activated adhesives requires knowledge of the chemical process. Both Urethane and Epoxy adhesives have a number of ‘windows’ in which they must be applied to achieve the maximum strength (yield).

These windows are temperature (ambient, product, and material) and humidity. If applied outside those windows, the rate of chemical activity will change with a negative effect of the bond. All reputable adhesive manufacturers clearly identify these parameters.

The other important features are the times the repairer must be familiar with, and these include the shelf life of the product, the wet time (the duration during which the product can be out of the tube and during which the panel can be placed in position), and clamp time (the duration under which it should be clamped, and not disturbed).

I-CAR delivers courses such as PLA03, Plastic and Composite Repair, and also delivers a qualification program, ADH-QT which provides accreditation on the ability of the repairer to correctly complete a series of bond applications.