Sorry brokite bros. I wasnt clear with my statement. There is no use for carbon Fibre s running at 90 deg on a wood core board. The core supports its self in this direction. Foamcore boards will need Fibre s running at 90 deg.brokite wrote:The fibres running a 90 are almost useless for board construction. They offer no torsional or longitudinal stiffness. In effect a waist of material adding weight but not performance.... Thus making the use of carbon moot.... Stupid.
The fibres need to be going in the direction that the stiffness and strength is required. That's is at 0 and between 60 and 45 degrees for board construction.
I have to respectfully disagree with the bro kite men regarding core material. You need to match the core material with the desired application. Foam cores indeed give light weight performance with good stiffness and fast rebound properties.
But they have a tendancy to shear and fail sooner. Also the flex response maybe too fast! I for example don't want all the quick flex response that carbon provides. I want a more rebound dampening ride. in this regard the types of core that is used will affect the flex and rebound properties of the board.
ood cores definitely have a place in board construction. Look across many different types of boards , skateboards,wake boards, snowboards.... You will see wood core used extensively.]
Our experience tells us that there are a couple of problems in the quote above.
First wood cores are used extensively by everybody because they are cheap, and do not require much craftsmanship or skill. To use a foam core which is lighter and allows a board to be more responsive, a great deal of engineering must take place. This costs money, and takes a lot of time.
Secondly when a foam core is used it should not be used as a structural component. The foam core should be used as a space filler. All the strength and flex properties come from the carbon. The foam core is a neutral material that allows a box beam to be formed around it by the carbon.
Thirdly the statement is correct about the carbon fibers should run in the direction needed for flex and strength, but with a foam core the foam does not add strength so the carbon fibers must be used in both directions. This is where the warp weave carbon material comes in handy. You need some carbon running at 90 degrees, this is a good thing, it is stronger and lighter than wood which it replaces. It also adds structural integrity to our boards that are made with 5 box beams that must be held together. You can't impose simple wood core/glass construction methods on foam core/carbon boards, believe me we tried.
Foam cores fail because of poor engineering. Most builders use wood core/glass technology when experimenting with carbon and the results are disastrous. In effect we have replaced wood with air and glass with carbon. All of our work has been done to create a light, lively board that can feel input and react instantly to it. This has greatly increased pop, carving, control, and put a big grin on our faces. Be careful, we don't want a more rebound dampening ride, our boards may be too fast for you.
No It was on a flat concrete floor.zfennell wrote:
I'm assuming your panels were tested as a 3 pont bend to evaluate flexure properties.
Q.E.D.zfennell wrote: "My conjecture is the test results would also be similar if the panel was laying flat on the floor".....
....."This may be the best way to identify and correct impact failures under the feet.
Certainly this is where most board failues occur,"
From the buckled (surf)boards I've cut up to understand what happened (and then fix it for the person...no charge) I think it is mostly related to compression after impact. Composite panels can greatly lose compression strength after impact even if there is not visible damage. The phenomena is well known to occur to the point where composites industry has developed a test for this (ASTM D7137). Which basically goes like this. Perform a drop test, then compression test the panel till failure.zfennell wrote: but my personal belief is that it is more related to buckling of the skins as the entire board flexes.
I agree....for twin tips....if you're using carbon....but in using carbon with a foam core you have to have enough laminate thickness on the board to keep from cracking it underfoot (heavier than glass) or use a heavier core...(unique brokite like solutions excluded), if the strength weight and toughness are merely the same or worse than an equivalent glass board it's hard to say what the carbon is accomplishing other than adding cost.zfennell wrote: You mentioned that because carbon is so stiff, the skins need to be made thin...
"unique thing with kite boards is that using them with carbon we need them to have VERY thin skins to retain some flexibility,"
This may be where we do differ. I think the best way to use carbon in a flexible application is to increase the laminate
thickness while reducing the overall thickness of the structure
Your short, thick panels certainly highlighted what carbon cannot do.
Although the first paragraph is not relevant as I wasn't doing 3point bending tests,zfennell wrote: If your test panel were full length as well as full thickness..... is it possible that considerably more of your momentum would be absorbed by the carbon sandwich , before the core was crushed and the skins buckled? It seems reasonable the design can allow for energy to be absorbed because the structure deforms, as opposed to the fibers reaching their individual strain limits.
I say this because buckling is not typically linked to allowable strain of the material. The limiting forces associated are generally proportional to modulus and area moment of inertia (F= E*I ). Once the skin becomes unstable, the strain limit is pretty much guaranteed to be exceeded.
We use Divinycell 100H Plain for our cores, this is a foamed PVC product which is a closed cell foam so water can not penetrate or be retained. Of course it is never good to smash full pace into a boulder with any board and if the damage has compromised the structure of ours, it's game over, but isn't that true with most TTs? We repair minor impact damage to our boards with a thick mixture of epoxy and mico-ballons.Sorry brokite bros. I wasnt clear with my statement. There is no use for carbon Fibre s running at 90 deg on a wood core board. The core supports its self in this direction. Foamcore boards will need Fibre s running at 90 deg.
A good way to tell if you have a poorly designed woodcore board is to look for fibres at 90.
Can you comment on the durability of your boards regarding impact resistance? One of the factors keeping me from making a full foamcore boardis the fear of punching through the skin into the foam. Ps im in a rocky wave location so its very easy to smash full pace into a Boulder thats boiled up out of the water
i have no wish for you to eat your hat nor change your mind regarding carbonJohnny Rotten wrote:
If it's from buckling, I will eat my hat and indicate carbon has a well suited place on thin wood cored twin tip boards in a unidirectional fabric (kinda sounds like plummets design here)
thanks Frederik SFrederikS wrote:There are many issues in providing a proper evaluation of GF vs. CF.
If we assume that two parts are created equal in terms of porosities and their distribution and have a similar volume fraction of fibers, the laminates will have almost the exact same thoughness (k1c) of around 21.
What upsets the pictures here is that GFRP usually has a volume fraction of fibers of around 40-45 where CFRP is 60+. Also the distribution of porosities within the laminate is usually worse for CFRP parts. Since the polymer part has a much higher ductility than the fiber part a laminate with a laminate with a higher volume fraction of it will perform better in normal impact test scenarios.
The most important aspect for durability is usually the fiber/matrix interactions and not the fiber or the polymer properties viewed seperately, at least for high performance laminates.
One of the greates influences on the long term durability is the volume fraction of porosities, as these work as crack initiation sites.
Was a bit messy sorry. If you take high strength carbon and compare the specific toughness to the glass fiber it will be the same. From what I have seen the failures originate at porosities and ill conceived lay-ups. The porosities have a tendency to distribute themselves in a more favorable way in GFRP which could be why it usually is better in kite boards?zfennell wrote:thanks Frederik SFrederikS wrote:There
i dont completely understand your position.
you may need to walk me thru this one more time.
do you believe the 'fracture' toughness of different fibers in a similar Vf polymer matrix does not properly predict the overall toughness of the resultant laminate?
(johnny's test panels appeard to have the same volume fraction for both carbon and glass)
do you think there is practical method to build a carbon laminate with toughness propertites similar to e-glass or s-glass?
any opinions are appreciated.
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