I'm assuming your panels were tested as a 3 pont bend to evaluate flexure properties.
No It was on a flat concrete floor.
From the (surf)boards I've had to repair, I haven't seen any clean failures in tension, I've seen a few creased and buckled top sheets but they are all surrounding a heel dent of significance and would be consistent with compression after impact
The vast majority of failures are either
a) cracking/buckling under foot
b) debris impact
c) Rail damage from rolling down the beach in >30knots
d) hole from harness hook
So I feel the weaklink on surfboards is that they need improved impact toughness at minimal weight. This was my goal.
"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,"
but my personal belief is that it is more related to buckling of the skins as the entire board flexes.
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.
As to the drop test. ASTM 7136 http://www.youtube.com/watch?v=lGRPwDf4Nzg
results of this are well published and consistent with what I've found.
According to Fiber-Reinforce Composites by P.K. Mallick
In drop weight tests containing 10 ply the results were (kN/mm)
The testing I did was to see if this still applied to the panels used for surfboards and a "hammer" the size of your heel.
and for the most part it was.
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.
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.
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.
Although the first paragraph is not relevant as I wasn't doing 3point bending tests,
You brought up a good point that I didn't consider with regards to compressive buckling.
When there is no chance for denting, or failure underfoot carbon should improve the compression buckling. (It's stiffer thicker and as you stated buckling is not related to the allowable strain). So for wood cores it seems to make sense, It allows a thinner core which saves weight and should reduce compressive buckling, particularly if unidirectional fibers are used. I have been advised by a VERY reliable source that uni perform enormously better in compression due to the lack of waviness that will promote earlier buckling.
So question is how are thin, wood cored twin tips breaking....(I haven't actually had to repair any)
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)
If it's from tensile failure....it's got no purpose as a glass equivalent will absorb the energy of landing better delaying failure.
For the most part twin tips are easy enough to make light enough and tough enough with the desired flex as thickness is not really a constraint.
My very thin skins comment was mostly directed towards surfboards which I feel are seriously lacking for kiting.
For surfboards you can't just "make em thinner" you need some volume or the board will bury it's rails badly negatively affecting the way the board carves and how you have to ride it. (tried this already).
I like my boards flexy as do many other surfers who rave about the feel of a traditional 3lb polyu/ board with an 8oz polyester/glass top and 4 oz bottom. Trying to get this flex out of glass on a kiteboard and not put your heel through it is challenging....using carbon puts this goal even further out there I also like my boards LIGHT....as it's easier for the wind to lift it and keep it on your feet when you point it into the wind on strapless airs. It's also much nicer to swing around a light board. I'm still trying to determine the best way to get there....but am pretty sure it won't involve carbon.
Great thoughts and input from everyone, thanks for the interesting discussion. Sorry for the long winded posts
zfennell thanks for taking an interest in some of my testing. your wise comments and well thought out posts.