Carbon Fiber Reinforcement

[manirban73]

I was just wondering if anyone had any experience with carbon fiber reinforcement? As some of you all may be aware, I'm working on refurbing a musical instrument with a curved neck, and I was thinking about reinforcing it with a carbon fiber strip. My basic question is if a carbon fiber strip (3/8" wide / thick) can be bonded (epoxy) to a curved surface (is it malleable enough)? If so, does it still offer the reinforcement properties as would a straight piece.

Any thoughts would be greatly appreciated.

Thanks,

Anirban

 

[CarvedTones]

The short answer is "no" - the CF rods you will find at StewMac or other vendors are straight and want to stay that way. But depending on the arc and the available depth, you may be able to cut a straight channel in a curved neck to reinforce at the points where it wants to bend most.

 

[manirban73]

Hmmm.. not going to work then... the curvature is probably too much to get a strip in there with a channel (which, is exactly what I was planning to do if the CF strips would work). OK, on to Plan B then.... just have to figure out what Plan B is.

 

[ptt49er]

You can get regular fiber glass and use it, or you can find the carbon cloth used for carbon fiber and get the epoxy.

It'll take some research, but you can make your own carbon fiber. And I doubt it's cheap.

 

[Gofor]

I worked a lot with composites in my last job. Carbon fiber, just like fiberglass cloth, provide directional strength, depending on the weave (or lack thereof). A straight flat sheet of fibers running the length of the neck parallel to the face would provide side to side warpage resistance, but do nothing for top to bottom. 90 degree crossed fibers will provide resistance to it cupping across the width as well as side to side, but will still allow top to bottom flexing of the neck over the longest dimension.

In essence, the fibers, be it carbon or glass need to be parallel across the direction of stress, so to prevent flexing of the neck along the length, the fibers would have to be perpendicular (stacked from the top face to the bottom face and running the length of it.) Kind of like running a steel spline down the center of it through the thickness.

If the back of the neck is curved, you could stiffen it some by wrapping the cloth around the radius of the curve on the back, (basically giving you a half tube),.

To relate this to wood:

Think of a thin piece of pine 4' wide cut along the grain. It will bow and cup easily, but because of the width, does not warp much side to side. Take two pieces and glue them together with the grain running perpendicular (i.e. plywood). It still bows and cups but there is no warp across the flat dimension. It stays square. The more layers you add, the stiffer it gets. Add a piece glued on edge vertical to the surface, and it stops the bow. Add another piece vertical to the first but perpendicular to the second, and you also stop the cup (ie torsion box). Same principal applies to fiberglass and composites. With Fiberglass, you also have matte mold, or short sections of fiber sprayed into the resin at all angles. This is more like particle board or MDF..


Not sure on what "stiffening" you are trying to achieve.

Go

 

[Alan in Little Washington]

Carbon rod, etc. is a composite of carbon fibers in a matrix of resin, usually epoxy. You can buy woven graphite fabric with various weaves one of which is "uni-directional" with 90% of the fibers running in one direction. You can also get roving or tow- bundles of strands of carbon fiber. These could be formed and used for reinforcing.

Many forms of carbon fabric, etc. are "pre-preg" meaning they are already impregnated with resin. This is done to ensure the proper amount of resin for maximum strength and partially because carbon fiber can be difficult to "wet out" . It can be more difficult to wet out than glass roving and woven cloth. To get them to harden, pre-pregs must usually be cured (heated) in an oven or large autoclave.

You can find non-impregnated carbon fiber roving or fabric, and possibly use these with epoxy resin to make your reinforcing parts. Here is one source. Check out the Carbon Tapes and Tow. Another option is to buy larger pieces and machine them to desired shape. Don't breathe any of the dust.

 

[manirban73]

So, hopefully the picture shows up above... otherwise, here's the link -

http://picasaweb.google.com/anirbanmukherjee73/SarodProject#5249791406353404786

Anyways, this is just a simple diagram of the stresses on teh neck of the instrument. If you scroll back a couple of images to this image -

http://picasaweb.google.com/anirbanmukherjee73/SarodProject#5243461378317703314

You will see the inside of the neck. In my instrument, there is a rib that runs longitudinally along the neck and offers reinforcement. But in my refurbishment, I'm taking that rib out, so the neck area is essentially fully hollow. This will undoubtedly add stress to the area from playing the steel fingerplate above, and also some torsional element as well.

My thought was to cut a channel along the length of the neck at the base and bond a CF strip to it, but I'm open to other ideas.

Thanks,

Anirban