Carbon Fiber and Wood

So why aren't carbon fiber and wood a good match?

Carbon fiber embedded in an epoxy matrix is light, stiff, strong in tension and compression, and expensive.

Wood is not nearly as strong, stiff, or expensive. Wood occurs naturally, is pleasant to work with, it can be beautiful, and wood is resistant to fatigue.

Lets say you designed a spar to resist a load of 600 pounds. You use enough carbon fiber to handle 500 pounds and enough wood to carry 500 pounds total strength 1000 pounds. Nice safety margin.

Now apply a force of 500 pounds to the spar. The carbon will flex very little say an inch. The wood will flex an inch readily resisting with say only 20 pounds.

Apply 550 pounds and you will snap the carbon. Now the entire load falls to the wood which will flex several inches before resisting with its 500 pounds. Then the wood will fail.

You would be better off designing and building the spar entirely of wood or carbon.

I did see an article in Woodenboat Magazine where they built a hollow wooden spar and lined the inside of the wood staves with carbon fiber and carbon rod.. Under load the wood fibers on the outer part of the stave will have to flex a greater distance than the carbon fiber laid on the inside of the stave. By adjusting this distance you could get the carbon and the wood to work more closely together.

But my spars have a carbon braid on the outside. The reason has to do with something called hoop strength.
If you design a hollow wooden spar with walls just thick enough to resist a certain load in tension and compression. The spar may fail prematurely because the walls buckle or splay apart under load. The spar needs more hoop strength.You could make the walls thicker but that would add weight. Or you could surround the hollow wood spar with a carbon braid to keep the wood squeezed into shape. The carbon fibers are running diagonally around the spar using their stiffness to keep the walls from bulging even a smidge. Since the carbon is not running straight up the spar the wood is free to flex and the two radically different materials work together to make a light and strong spar.