Carbon Fibre in boards??

[zfennell]

Thanks Johnny,
you've given me a lot to chew on and your point is compelling.
i seldom give S-glass the credit it deserves. the apples to apples comparisons suggest alot of attention to detail and highlight the relative diffferences in material properties.

i find myself arguing the benefits of carbon only to be contrarian. For the most part, i do tend to see your side of this.

The level of detail in your response has answered many kiteforum.com... thanks again.
and also raised some more questions.
please do not take the following as criticism. as i have not done my homework here.
just idle thoughts to ponder.


I'm assuming your panels were tested as a 3 pont bend to evaluate flexure properties.
yes/no?

I believe the low ratio of span length to panel height (6x) is close to that specified in a Short_Beam_Shear_Test.
typically, this config is used to challange the inter-laminar shear properties of the composite beam as opposed to the tensile/compressive limits of the skins.

I suspect that you would have observed similar results if your test had been more static in nature.
By loading up your 'foot-print' with an hydraulic jack and tracking load vs displacement. i think the carbon laminates would fail at higher loads and lower displacements than the S-glass counterparts.
That would definitely support your definition of toughness as the area under the stress-strain curve.
It seems logical that the S-glass should win.

My conjecture is the test results would also be similar if the panel was laying flat on the floor.
Do you think that your test effectively isolates all deflections to a specific area of the board?
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.

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 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 asssociated 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.

You mentioned that beacuse carbon is so stiff, the skins need to be made thin...
"unique thing with kiteboards 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.

Thats probably too much rambling for one post. again thanks. i'd like to continue as i get my thoughts in order.

finally, i appreciate you tracking the relative weights of panel components. I was surprised that the apparant density advantages for the carbon were negated by 'real life' resin contents of the lam.
maybe this is another limit for garage builds?
A lot of folks have resigned themselves to hand-lams as opposed to vacuum bagging, especially when they know fill coats are required

regards,
-bill

[plummet]

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.
brokite.com

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

[Johnny Rotten]

I'm assuming your panels were tested as a 3 pont bend to evaluate flexure properties.
yes/no?

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,"

Q.E.D.

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)

Carbon 1.07
Kevlar 1.16
E-glass 2.90

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.

[magic%20Ed]

Sorry for the long winded posts

Sorry? Great to have a good, informed discussion!

[FrederikS]

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.

My experience with repairing a North 69 LTD (2012) multiple times is that it fails because of insufficient skin strength and stiffness, coupled with horrible laminate quality. Mostly I have noted a high amount of porosities near the interface with the high density foam core and large parts that are improperly wetted. The carbon bottom layer is improperly bonded to the top glass fiber section and the bonding in the overlap between the two is not properly designed. Overall it shows very poor workmanship and laminate design quality.

Of course this is fixed now by adding more glass fiber around the foot pads and reinforcing the central portion of the board. It has put on 200 g but now it lasts.

[brokite]

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

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.

[zfennell]

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)

i have no wish for you to eat your hat nor change your mind regarding carbon
your tests are quite conclusive.
my concern is that the constraints imposed by your particular flavor ASTM D7137
have predetermined the failure mode.

we both agree that the region of failure is where you place your feet.
no doubt the loads normal to the board's deck create compressive and shear stress' that get resolved in the skins and cores.
However, to ignore the resultant compressive stress (that is aligned with the skins) due to bending seems a bit short sighted.
regardless of their relatives magnitude they both occur and should be accounted for.
perhaps you're way ahead of me and this was just a way for you to pick your favorite build method while design iterations happen down stream.

IMO, heels dents are compressive in nature while creating some vertical shear forces and local buckling in the skin. The best laminate to resist such a force would ideally be more isotropic in nature. which is probably why the combination of 0-90 and +/- 45 fibers was a good thing for you.
unfortunately, the 45 and 90 fibers are not particularly beneficial when resistang buckling due to bending. So, where do the trade-offs occur?

Everyone gets to choose for themselves.
but since your particular 'wish list' includes low stiffness, min weight, a specific volume, max toughness and ..?
you may have to make a few trade-offs to get there and still be happy.

another smart kid with time on his hands ran some tests and design calculations with simialar goals in mind , benjamin thompson (sp?)
even if you dont completely agree you will find his threads interesting
take a peek at the two below.
-bill

[ http://www.swaylocks.com/forums/tests-results-13-unique-constructions ]

[ http://www.swaylocks.com/forums/prevent-buckling-without-changing-flex ]

[zfennell]

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.

........

thanks Frederik S
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.
-bill

[FrederikS]

There
........

thanks Frederik S
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.
-bill

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?

Impact strength from what I can remember is dependant upon volume, E-modulus, and yield strength. So that would lead to the glass performing better, but being heavier its specific/per mass performance should be in the same ball park. Since the density of glass is around 55% more than that of carbon there must be something else accounting for the difference here.

I think Johnny Rottens stuff is pretty much on, was just trying to add some of my observations. There are so many factors in how laminates perform it is hard to single one thing out besides the few obvious things.

Was a bit later here and my mind is mess from writing all day

The high density performance foam besides being a sandwich material can also be used to supress buckling, don't know if that was said before in this thread.

[Jbrook]

How does the Carved Imperator 5 stand up to you guys with all the knowledge of carbon, do you know this board? First time I rode it I loved it. I was not really looking for a board at the time but sold what I was riding to purchase this one, I got the 130x39 for powered riding it is just great. It definitely has something special in the way it is made? Your thoughts?