Carbon Fiber

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Epoxy reinforced with carbon fiber can be a great way to add needed strength to broken or weakened furniture. The high tensile strength and the ability to spread the load evenly over a relatively large area (compared to the point loading of fasteners) are often crucial qualities needed for a successful repair.

There are some situations where epoxy seems to be the most conservative choice. Though the glue itself is not reversible, the techniques are nearly so. And when compared to the alternatives, they are often more so. To increase reversibility, the surface can be sealed with animal glue, before applying epoxy.

In situations where a point of stress could be fatal, carbon fiber spreads the load and can be feathered out to decrease the strength gradually, something impossible with fasteners and often difficult with other techniques. These are structural repairs, where a weaker alternative could well lead to more breakage, which is inherently irreversible.

Carbon fiber types

Carbon fiber rod, round tube, and square tube

Carbon fiber is available in many different shapes and forms, such as tape, cloth, rods, tubes, and strips

Unidirectional carbon fiber tape

Carbon fiber tape (West System Episize Unidirectional Carbon Tape)

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Carbon fiber rod

Carbon fiber rods are great for those situations where you need strength but don't want to remove more wood or be more invasive. Using epoxy makes more sense since the carbon fiber is bound in epoxy resin. Unfortunately reversibility is only mechanical. As an alternative, one can drill a larger hole, isolate it with hide glue and epoxy the carbon fiber dowel in place. Additional testing on this technique for strength may be necessary.

Multiple thin carbon fiber dowels can be used in place of the traditional larger wood dowel to repair cross grain breaks. They are stronger and much less compromising to the broken part.


Application

High load across the grain

An extreme example of a load across the grain is the bottom leg of a very large neoclassical (or Duncan Phyfe-style), single pedestal, four legged table. The red line on the left indicates the break along the grain of the wood. Lengths of carbon fiber can be epoxied along the bottom of the leg (as indicated by the blue line).

One general type of problem that can best be repaired using epoxy/carbon fiber is where there is high load across the grain of the wood, causing it to split. In this situation, it is good to start by carefully putting everything back together with hide glue and/or wood patches.

An extreme example of a load across the grain is the leg of a neoclassical single pedestal, four legged table. Carbon fiber is exceptionally strong and rigid under tension, and running on the underside of the legs, can provide the necessary reinforcement. Optionally, one can rout a shallow groove to accept the carbon fiber in the underside of each leg starting from the point where the grain ran parallel to the leg, out to the tip where the grain was running across the leg. While the epoxy itself is not reversible, this repair (without the groove) is more reversible and less invasive than doweling and certainly no worse than drilling screw holes for an iron strap, and it is far stronger than either.

Compromised joinery

A good example of compromised joinery that may benefit from a carbon fiber-epoxy repair is the leg/column joint of a pedestal table. The traditional repair for this problem is a fitted metal strap, looking a bit like a three or four legged spider, which is screwed to the bottom of the column and the undersides of the legs. But there is little strength gained in the horizontal plane, since there usually is hardly more of the leg lying in the horizontal plane than the dovetail itself to screw into. A screw hole here greatly weakens the dovetail and with the sideways, that is horizontal, load on the screw it acts as a lever to break the dovetail off. The other screws along the downward sweep of the leg do little or nothing unless the fasteners and the metal are utterly rigid—a dubious assumption.

The carbon fiber strand that was inserted into a hole in the leg can be fanned out a little and epoxied to the bottom surface of the column. The light grey areas are where the epoxy/carbon fiber is sanded smooth.

A small, more or less horizontal, hole can be drilled into the underside of each leg at the level of the column base. The hole and a shoelace-sized strand of carbon fiber can be wetted with epoxy, after which the strand is folded and pushed all the way into the hole, leaving both ends of the carbon fiber sticking out (perhaps 3-5 inches depending on the size of the column). The carbon fiber can subsequently be fanned out a little and epoxied to the bottom surface of the column.

The hole in the leg needs to be no more than a quarter inch in diameter, so it has almost no effect on the strength of the leg. In contrast to the metal spider the hole for the carbon fiber is into the main body of the leg, not the dovetail. There is a sufficient amount of bonding area between the carbon fiber and the wood of both the leg and column for the repair to be quite effective.

As a reinforcing band around weak spots

Another important use of epoxy/carbon fiber can be as a reinforcing band around a weak spot. Previous repairs of inserted dowels or insect damaged parts are examples of this situation.

A less common situation is a previous repair with a short scarf joint with insufficient strength. The joint can be wrapped with a band of epoxy/carbon fiber. The only alternative can be to replace the old patch with a new, longer scarf joint. That would mean removing even more wood from the original piece. The epoxy/carbon fiber was certainly the lesser of evils and a more effective repair too.



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