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thin wood cores...

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thin wood cores...

Postby BWD » Sun Jul 11, 2010 9:42 pm

I have been thinking about how to use 1/4 inch cedar strips left over from a boat project for kiteboard cores...
Normally people claim success for cedar or poplar cores 11-15mm thick with 4-6oz glass laminate top and bottom.
More laminate is needed for a thinner core, but how much more?

The typical ~12mm wood strip board is ok with 6oz glass skins.
The typical ~12mm foam core board seems to have 18 to 20+ oz glass skins.
So as a rule of thumb (and oversimplification), it seems for glass skins, you need 1/3 the laminate for wood strip core compared to foam core.

Also know a few years back folks made boards with ~12-15mm foam core and 12oz carbon skins.

I don't know too much about more sophisticated layups for thin cores, but know from the website that the company, "Brokite" makes 1/4 inch thick foam core boards supposed to be durable and reasonably flexy with 6oz S glass skins over essentially 22 oz carbon -this might be sort of equivalent to approximately 45-50 oz glass skins in terms of overall strength, but it is more complicated because the brokite boards are heavily stringered....

Does all this suggest that a 15-18 oz skin for a 1/4inch cedar strip core might be right (following the pattern of ~1/3 the laminate required for wood compared to foam)?
Or am I off base and extrapolating way too much?
I have Larsson & Eliasson to fall back on for basic composite sandwich design equations, but how well that translates to kiteboards in the first place I am not sure, so I thought I would ask here, if anyone has the experience....

I know gbleck just made a 1/4" poplar core board with 10oz glass skins, as posted here, but I think I would want something stiffer.

Was kinda thinking of making my board like this:
6oz foot patches
9oz S glass
6oz Eglass
1/4 cedar core
6oz E glass
9oz S glass

FWIW the other cloth I have around besides 9 oz S and 6 oz E is 6.6 oz carbon uni....
Anyway, suggestions appreciated, especially from experienced wood strip builders (zfennell,. gbleck, etc.)!!!
And thanks to you guys and others for the info you have shared before!

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Re: thin wood cores...

Postby zfennell » Mon Jul 12, 2010 2:02 am

hi BWD,
if you wish to keep the core at 1/4 and are only conserned with stiffness there are a few guidelines that you may find useful.
bear in mind that stiffness and strength are closely related but definitely not equal.
so, it's possible to make your board extremely stiff, only to have it snap like a twig ( a brittle twig :)

for simple beams ( boards) : stiffness is proportional to E*I / L^3
.....more or less of the above will alter the stiffness accordingly
E= elastic modulus of the material used for reinforcement
I = moment of inertia of cross-section of board and is dominated by the boards thickness
I= (b * h^3)/12 ( for a rectangular x-section)
b= board width ( a const for this example)
h= board thickness (our variable of interest and a huge factor)

L= board length ( another big driver but i'm assuming a constant in this example)

The modulus of your core certainly is a factor, especially when comparing cedar to any of the foam cores used. cedar planks will even be a lot stiffer than any plywood of similar thickness due to amount of grain aligned with the boards long axis.

but as the board gets thinner, the h^3 term dominates (or fades to nothing, in this case )
and the core material is less of a factor compared to the extra glass you'll need to compensate for reduced x-section geometry.

one way you could increase I, is by adding concave to the board (artificial thickness)
personally i think 1/4 inch planks are ideal for making 1/2 inch boards.
rocker and concave become very easy on the table once you have 2 layers for laminating.

but, back to the beginning:
if everything else is equal and you are comparing the required glass thickness to maintain stiffness as you reduce overall core thickness , i think it would all boil down to this....
(c+g)^3 - c^3 =const
c=core thickness
g = glass thickness ( assuming the same flavor again) 6 oz is approx .010" thick


(c1 + g1)^3 - c1^3 = (c2 + g2)^3 - c2^3

if you are more like me, trial and error is an easy path in this case.
take your best guess and glass yourboard.
let it cure and compare to the board or numbers that you'd like to hit.
if the results are too soft for your taste a light sanding and another glass lam will quickly make you the expert in this matter. with no real penality for the extra steps.

if you want to make it more like a science project, a little bit of math would let you scale smaller sample sections to get all of this before committing to the full scale version, as well.

i know this is mostly a long winded non answer.
but i hope it helps and let me know if you need something more concrete.

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Re: thin wood cores...

Postby gbleck » Mon Jul 12, 2010 3:08 am

My poplar core layup was 16oz glass with 4 + 6 + 6 on each side. The last two inches is 4 + 6. I'v riden it in chop and kind of liked the flex. Flexed then poped. I may still add some carbon uni at the center. Board isn't to heavy but not exactly light. If you are baging the board you can leave the top rough with a nylon fabric finish you can bond to and add more if you need to adjust the flex.

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Re: thin wood cores...

Postby kitehok » Mon Jul 12, 2010 3:03 pm

Flex will be challenging. Why not use 3 layers of 1/4" cedar. The middle layer can be partially hollow, just a frame with edges and a couple stringers. Then you probably only need 1-2 layers of 4 or 6oz.

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Re: thin wood cores...

Postby BWD » Mon Jul 12, 2010 4:44 pm

Thanks y'all.
I do have those equations floating around but thought I would ask for anecdotal info by way of benhmarks, since I am lazy with math and a kiteboard doesn't in fact approximate a simple sandwich/beam all that well...
I should probably "calculate" some layups that way anyhow just for insight....

kitehok I have though of building like you mention, but with ply skins to save labor and add impact resistance, but right now am looking for a shorter, faster build, so this board will not be hollow.
Also, in the past I've built plyboards that were hollow, but feel I got little advantage from it.

I guess the outcome may be /sigh/ ripping some 1/2 inch strips (I am also lazy when it comes to ripping!) or maybe a 2 layer core after all, I do like my concaves....

Thanks again!

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