ive been playing around a little with adding some uni carbon on to my eps epoxy stringerless boards. At present i have been doing it under the glass but i have fond it is easy to hit the cloth down the edge of the carbon ( i am using 6oz uni carbon tape down the center of the board but would also like to try it on the rails) the few threads i have read i got the impreshion it was being done this way around but wondered if it could also be done the other way way around ie doing the lam and then waiting a bit and doing the carbon on the top, i figure that way the edges of the carbon would blend in with the hot coat easier and no chance of going into the glass.
if i did the carbon on top i was planing to use a 4oz strip of glass a inch wider than the carbon over the top as i have found it does tend to help get the carbon to lay flater and you can squegee it out a little beter. im not woried about puctures of the carbon so much as it will still have the full lam shedule underneath.
Whilst building racing yachts I carried out a lot of destructive tests on glass / carbon laninates. They were very thin, over a foam core. 0.5mm glass and 1.5 mm carbon.
What I found was that if the carbon was left on top of the glass and the structure flexed or went into tension, the carbon broke away from the glass surface because it was so much stiffer than the glass. A thin layer of glass overlapping the carbon stopped the problem by holding the edges down and stopping them lifting.
In a tensile test the carbon tapes began to break away from the glass at 1500 kg load. With a layer of 200gm glass over the top to spread the stress the carbon never lifted and the structure began to break at 6000kg.
I can post pictures if anybody’s interested.
I would therefore put the carbon under the last layer of glass.
From how it was explained to me, graduate from the most elastic items to the least. That means Foam to Nylon to Fiberglass to Carbon. It has to do with shear failure. Carbon against foam would be the worst.
But if you put Carbon on top, you are going to need to deal with fairing it in. If the Carbon is your strength, you dont want to sand into it.
That means, if you are putting Carbon on the rails,
glass the bottom, then the top with a single layer of cloth.
Add whatever Carbon to the rails you want.
Laminate your last layer on the deck.
The other option would be to laminate the bottom only. Add your carbon to the rails over the fiberglass only, then laminate the deck with two layers. Not as good as the first option, but you can get away with doing both deck layers at the same time.
As for the carbon running flat along the stringer, cosmetic only, and really isn’t doing much.
Take a piece of paper. Which is stiffer, when it is flat and un wrinkled, or rolled into a tube? A 3 dimension shape is always stiffer. The deck is flat, while the rails roll.
Which is stiffer, a plate flat to stresses, or perpindicular to the load.
Which beam will take more load, a 6" high beam, or a 16" high beam?
Rails placement is more effective, because it is a 3D shape, oriented perpindicular to the load.
the trick of paper is right but not what happen in surfboards. When you roll your paper you drastically increase quadratic momentum by change shape. Quadratic momentum determinate most of flexural properties of structure. Surfboards quadartic momentum is fixed by shape, you don’t roll board to increase stiffness, but you can increase board stiffness to increase quadratic momentum (for exemple). For a given shape, in flexural, that’s material the further away from the neutral axis that take the biggest load. For a surfboard the further point away from the neutral axis is at the top of the deck (for the deck) and for the bottom it depend of the shape. So if the purpose is to stiffen the board it’s more effective to put carbon at those place. That’s way main plane wing stiffener (spare) is at thickest point of wing.
But is it good to stiffen? Here we speaks of overall behavior of board but local behavior can be leading for structural integrity of sandwich panels (because of buckling)…
Hi Lemat, does the carbon on the deck stiffen a board much? I was thinking that the deck is mostly in compression and from what I’ve gathered the stiffness of the fiber doesn’t really matter much in compression, or do I have that wrong? I guess it would be stiffer from the added thickness though.
I’m also wondering what the effect is of the carbon webbing type reinforcements used under the feet. I’m assuming it’s used to prevent compression dings, does this mean that the carbon fiber webbing is in (localized?) tension under your feet?
Carbon is stiff, strong in tension and compression and light. So for a given weight you can make a stiffer and stronger structure with carbon than with glass. But stiff is not tough.
There are no pure compression because skins are so thin that they buckle quickly. Buckling is a flexural stress, stiffness increase buckling resistance but it’s stiffness of resin that try to stabilize fiber the limite. For flexural stress, quadratic momentum is far more important than fiber stiffness, so you are right it’s thickness dependant.
When you say compression ding you thinck of dent or foot prints. You are right again, when the board is dent, the foam experiment compression the skin above is in tension+flexion that’s way carbon can improve dent resistance, and sandwich (thick) skin do it better.
Ahh Lemat, but the trick of paper is what happens in a surfboard, In the skin. The Fiberglass/ Nylon/ Carbon is rolled at the rails, just like the paper tube. Only with a surfboard, the paper tube is flattened in the middle.
So in the paper tube, which part resists most of the load, the area flat to the load, or perpindicular to the load?
