Binding with Carbon Fiber?

[dsgoen]

I was just watching an interview with Rick Turner (https://www.youtube.com/watch?v=o5D0BThSZIk) where he shows an acoustic guitar that he used carbon fiber binding to increase the rigidity of the sound board.

Any thoughts on the validity/usefulness of this? I don't yet know enough to make any determination on my own.

Thanks,
David

[kiwigeo]

I would have thought increasing the rigidity of a soundboard is not where you should be going. Increasing stiffness yes but rigidity...no.

[Woodsy23]

"Carbon fiber (CF) "strands", which consists of thousands of CF strands of about 7 microns diameter, only has stiffness (high elastic modulus) in tension in its longitudinal direction. When formed into a laminate by embedding the fibres in an epoxy matrix, the stiffness of the laminate parallel with the fibers is dominated by the elastic modulus of the CF strands and it therefore very high (230,000 MPa). In any other direction, the stiffness of the laminate is effectively the stiffness of the epoxy matrix. The elastic modulus of epoxies used for bonding CF laminates (such as Sikadur -30) is 12,900 MPa so it is similar to the long grain modulus of many types of wood.

To effect the stiffness of the joint between the top/back and sides, stresses in the binding are across the grain so the relevant stiffness of a CF laminate binding is around 12,900 MPa whereas the cross grain stiffness of timber binding is the Ec of the relevant timber (possibly 5 to 10 times lower??).

While the binding may end up stiffer I doubt that this small piece has much effect at all on the transfer of flexure between the top and sides . Also, the carbon fibres have no effect in this direction so you may as well just use the epoxy for the binding.

He also mentioned that he applied CF to reinforce the marriage strip of the back although I'm not sure if it is for stiffness or strength or both. The same principal applies to that, if he applied the laminate along the length of the strip, because the strengthening required is in the transverse direction. The strength of the CF laminate in the transverse direction is only the strength of the epoxy matrix. I don't know what that is but I doubt if it is significantly different to Sitka Spruce in its longitudinal direction (which is oriented transversely in the marriage strip). I don't think it would be any better that a spruce marriage strip and would look much worse.

[Trevor Gore]

I think I might have fallen asleep before he got to saying why he used CF in the bindings (been a long day...) but otherwise, what Richard says. To make any useful contribute to stiffening the sides/soundboard interface he'd need two layers of CF; one inside the linings and one in the binding channel. Easier to do laminated linings or reverse kerf linings and make them as thick as you want to get the stiffness you want. Or use a Smallman style tone-well...