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I'm curious about the carbon fiber and wanted to start a different thread
since the topic is a bit different than the others currently running.

I emailed Jim Watts about working with it and he wrote back saying that,
[i}acts like a spring, so when you bend it, it will want to straighten back
out.

How then does it hold the arch of the back? Wouldn't it be trying to pull
itself back flat?

Rick, I read in another thread that you use superglue to glue it and used
1/4" x .021" layers around the linings, which created an incredibly stiff
rim. I imagine that the answer to the first question will tell me how it
creates that stiffness, but how do you bend it? Does it bend on its own
like linings do, or do you need to introduce the arch some other way?
Would laminating it around the linings have the same effect as using
double-sides to create a stiff rim?

I emailed Jim Watts about working with it and he wrote back saying that,
it acts like a spring, so when you bend it, it will want to straighten back
out.

James buddy I believe that Jim was referring to CF rods and shapes and not the cloth CF that is .021 thick when he indicated that it acts like a spring.  There have been discussions here prior that CF rods, if to large, in neck can negate a truss rod's ability to work.

So the laminations that Rick is using on the linings and back strip only really become structural members when they are laminated to something else to provide the shape and stiffness.  .021 is the thickness of 3 layers of poly/cotton bias tape - not very stiff in and of itself. 

But like fiber glass cloth, once laminated to something with form it's incredibly strong.  One of the benefits of these laminations is that the CF helps minimize the tendencies of wood to move as it will.  Laminate CF on a back strip on a domed back and the back retains it dome but the back strip is now much stronger with very little weight added.

Rick is the expert here so hopefully he will weigh in.

The .021" I have bends quite easily.  And like anything made from bent laminations, unless the glue creeps, it will hold its finished shape quite well.

Maybe what James was talking about as a "spring" is the fact that CF has more "memory" than wood.  It will always want to go back to its original shape even if deformed under load for a long time.  So if the glue breaks down in a bent lamination of CF, yes, it will want to spring back to its original shape.

_________________
http://www.chassonguitars.com

Kent, that's where my confusion is coming from. If it bends easily, how
does it become so strong?     

Does bending this around linings produce the same effect as using
double sides?

This is the paragraph from Jim's email. He was referring to all CF. He's
very helpful and said I could ask him anything I wanted. I wanted to try
here first.

The stiffness of the carbon fiber is between steel and aluminum, it also
acts like a spring, so when you bend it, it will want to straighten back out.
So you're not going to bend anything with much thickness to it. If you
want to experiment with it on linings I'd use mulitple layers of .01.

Very interesting and at a few dollars per strip, not cost-prohibitive.

The carbon fibers are small like threads and flexible on their own. As noted by others, the strength comes when it is imbedded into a hard matrix, such as epoxy, etc. Carbon fiber is very resistant to elongating. When you try to flex the matrix, the carbon fibers on the outside of the flex will not elongate, and so the matrix will not flex, hence it is very stiff. Probably you are not familiar with reinforced concrete design, but if you are, the analysis is very similar. The steel bars in the concrete are flexible, but they resist elongation. When imbeded in concrete, the concrete keeps the bars in place, so that attempts to flex are converted to elogation of the steel, which does not happen. The same with the cabon fibers in the matrix. Attemps to flex are directed by the matrix into elongation, which basically won't happen.

You have to understand that the nature of "stiffness" is not the same as "strength".   As Bill says, the issue is using different materials where their inherent properties of resisting elongation or shortening work to advantage.   In the reinforced concrete analogy, you have steel which resists elongation and concrete which resists compression.   

In a neck, for instance, you have compression on the fingerboard side and tension on the back side, so the stiffest neck construction would use different materials on each side as close to the surface as possible. That's why when I designed the all carbon fiber neck, it was hollow.   In fact the CF is pretty good in compression and it's great in tension.   This is "I" beam theory...one flange is in tension and one is in compression, and the flanges are separated by a web of which the only function is to keep the compression and tension components apart.    

The compression strength of CF can be improved in exotic applications by mixing carbon nanotubes into the epoxy matrix used to bind the fibers.   The nanotubes take up the space between the fibers and are great in compression. It's probably massive overkill for what we do, though.   The approaches you're seeing from Kent and myself simply work well enough.

Thank you for the fantastic explanations---it makes sense now.

When the carbon fiber is glued to a surface, it's stiff because the carbon
fiber resists the compression and pulling. Because the ends can't push
out or pull in, it holds whatever curve or form it's fixed to.     

Is this the concept?

When used over the back strip, I'd imagine that the side taper needs to be
linear from head block to tail block because once the arch is set from
your radiused dish, you won't be able to bend the back lengthwise.

    James, it's much like plywood boat building. The individual plies are formed into a contour as the glue between the plies sets. Each ply resists the compression and expansion of the other layered plies and maintains the form.

    Airplane fuselages used to be formed in the same fashion. The same principle not just applies to CF but to any back strip.

    If a brace wasn't so thick in relationship to the back or the top, the brace wouldn't even have to be sanded to radius, but the reflex of the thicker brace would overcome the resistance of the top or back.

_________________
Billy Dean Thomas
Covina, CA

"Multi famam, conscientiam, pauci verentur."
(Many fear their reputation, few their conscience)

When I glue in the back strip, the cross grain cedar or spruce is there acting like the center web of an I-beam or the core of a stressed skin panel as well as acting in the conventional manner of helping to hold the center glue seam together.   It is absolutely amazing how well just that one thing helps to hold the back curvature.

I'm building a huge baritone 12 string on which I've made the back strip very tall...it's like the keel of a boat. It's way overkill, but it's fun...