Re: [Harp-L] Carbon Fiber Reeds, 3D printing, plastic metal adhesion technology...?

[Zombor Kovacs <zrkovacs@xxxxxxxxx>]

Hi Folks, 

Reading the discussion about carbon fiber reeds, I feel I could add something having spent some time at Zoltek Corporation in one of the world's biggest carbon fiber factories as an engineer (I graduated as a materials science engineer, but not practicing it just have the basic education). 

First of all, I have a piece of carbon-epoxy composite sheet which I intended to make reeds of to see how they work. I haven't done it yet, I had more important things to do, but seeing the interest I will probably do it anyway and let you know in a couple of weeks (I am in a workshop reorganisation process). 

Carbon fiber is an interesting material used to create composites. Carbon fibers are made of a plastic fiber called PAN (polyacrilonitril) which is gradually carbonized in ovens while it is stretched. The molecules in the PAN fiber gradually straighten out, everyting burns away, only carbon remains with extremely strong covalent carbon-carbon bonds. The inner structure created this way gives the fibers an extreme stiffness and a very high tensile strength.
To exploit the extreme stiffness of the fibers it has to be embedded in a resin matrix, typically epoxy. As carbon is much lighter than iron (the basic element in steel), a carbon fiber has a much lower specific gravity than steel, in everyday language it is much lighter. And it is not only lighter, but much stiffer. Adding this up, we get a material with extremely high weight to stiffness ratio, up to 10 times higher than steel. 

Regarding reeds, I don't want to say anything before I have tried it, makes no sense to guess. What I can say is that there are different grades of carbon fiber, with different tensile strength and modulus values and the resin used for the matrix also makes a difference. Carbon fiber only works as a structural or spring material infused in a resin matrix, the matrix material and fiber volume fraction will (fiber to resin ratio basically) will determine the properties of the composite material. A great benefit is corrosion resistance. But because carbon fiber is extremely stiff, a reed would need to be made much thinner than a copper or brass reed, otherwise the pitch will be too high. If fibers only run in lengthwise direction in the reed, the torsional stiffness will probably not be very high, so the reed will twist easily. And here is another thing to know. The inner structure of a carbon fiber composite can be engineered and it not only CAN BE but it
 IS engineered in industrial applications. This is the whole point. You can design the material properties of a composite. This is done by laying plies of fiber on top of each other at different angles. So first for example you put a layer down in 0 degree angle, and put anotherone on top in 90 degree angle, and anotherone in 45 degree angle, or whatever your load on your part is. So in our case, we have the problem of torsional stiffness in the reed. What we need to do, is run a layer of fibers lengthwise in the reed, a layer crosswise, but we need two more layers at a calculated angle (eg +-45 degrees)  which will counteract torsional load. This way we can design a reed, which will only flex well in one direction but resists flexing in any other directions. We could probably design the best reed ever, which would never create dog whistles and other undesired noises during an overblow for example. It would only deflect in one direction, as it should.
 Now there are some problems here. As you see, you must design the ply order and use the appropriate ply thicknesses to get the right reed stiffness. This itself needs a high tech engineering software, but this is not the biggest problem. The biggest problem is the size of the reed. How do you make it in such a small size? It is extremely difficult if not impossible. You need a mould. Then you need to lay those fibers parallel to each other in each ply with a determined ply thickness - into the mould. Then you close the mould and infuse the plies with resin. For this you need tooling, resin mixing apparatus and finally you have to cure the whole thing in an autoclave to get the right composite properties. There is no way anybody wants to do this, it is too complicated and costs too much.
This is the biggest problem. Otherwise it has great benefits. As I said superior corrosion resistance, extreme fatigue resistance the longest lasting reeds could be made out of carbon fiber. It is not true, that a carbon fiber composite cracks. It only cracks if you have not designed your part with the right ply order and not used the right resin. Otherwise it will never crack. So a carbon fiber reed in theory could be just as well riveted, or bolted down as bonded. Another problem then is tuning. You would need to attach metal weights to the end of the reed, which makes it even more complicated (we don't want that to fall off any time ever). 
To summarize, it is a nice material but to make a good reed out of CF composite is beyond what is worth the effort. I will still try making one using CNC machining to cut it out of an already existing carbon composite sheet, but this will not be healthy, there will always be some loose fiber on the reed surface and the whole reed - because it is machined - will not have the correct integrity. Still enough to have a feeling about how it works and sounds though. 


Zombor
X-Reed Harmonicas