Stronger than an equivalently sized glass layup?
The same strength as an equivalently sized glass layup but lighter?
Strong enough to be lighter AND use less material

And bonus question:
is a carbon strand weaker or stronger than an equivantly sized glass strand?

What's your definition of stronger? people use this word in a variety of contexts.
Carbon is miles stiffer, so if it's stiffness your after, you can use much less.
Carbon has little impact resistance, so it it's going to be hit, you need lots of it.
As far as breaking strain goes, I don't think there's much difference to glass.

Carbon fibres have a higher tensile, (stretch), strength than glass. Almost double. This I believe to be its only positive. It will make a stiffer skin when mixed with resin because the fibres won't stretch as much, not allowing the resin to stretch and crack. This allows you to use less for the required strength you want. Fibres only work when being stretched. As a negative, carbon has lower malleability, (the amount it can bend before snapping). Carbon also has a lower shear strength, (at a single point, breaking the fibres with a sideways motion). The weight of the fibres are close.
Thats the simple science of the fibres.
However.....
It gets very complicated when using fibres in a windsurfer. How do you want your strength? Stiffness of the board, twist, its ability not to snap, ding, heel dints, price, weight? They are all different characteristics of different fibres, which there are many. Then you use combinations, which has to be done correctly. For example, if you had the same weave in carbon and glass and laminated them together it would not be good because all the force will be taken up by the less stretching carbon. The carbon will break first then the glass will take all the load. This combo will first only have the strength of the carbon, then the strength of the glass. Not strong for its weight. But,... they are used very well in combination, but you have to know what you are doing. Different types of weaves, angle of the weaves, it goes on and on. This is stuff I don't really know enough about, so I just stick with glass. I do know that a full carbon board can be a very harsh ride because it ends up too stiff. So, combinations of glass bottom and carbon deck can be a good combo. One of many, many combos.

Imax is onto it. High tensile strength, but not great so far as compressive strength goes.

We have done a lot of testing on breakage of the fibres as a part of our R&D as we supply custom cloths to the surf industry and for them its all about the cloth and force breakage - cracking decks and full snaps

We have access to a machine which is like a heel shape which puts force down onto a blank - (being surfboard focus the blank is always at 32kgm3 - higher density blanks and molded instead of block foam )

Here are some of the results (all samples were tested at least 3 times so this is the av for 3 tests (i have rounded it up so we dont get bogged down on too much details etc - so you get the jist of where these all sit ) the foam is EPS and using epoxy and all were woven into a standard cloth configuration

Imported 4oz - force 320kg breakage
Colan 4oz - force 340kg breakage
imported 6oz - force 335kg breakage
Colan 6oz - force 350kg breakage
S glass 4oz - force 360kg breakage
Carbon 4oz - force 350kg breakage
carbon 6oz - force 360kg breakage

A lot of this was published on Swellnet.com when we launched basalt cloth to the surf industry- as for force breakage, the Basalt was way better than any of these listed - though for stiffening up a shape, Thats where carbon is where it comes into its own.

Again this is surfboard focused - so more layers and different fibres added into the matrix would give different results

Imax - the tensile strength advantage starts really getting much smaller when u look at good glass

FYI too non heat treated glass from Colan as sold by Sanded is way stronger and S glass is of course better
if you just buy fibreglass you could be getting anything. I had a very big supplier tell me they had never heard of non- heat treated and it is all news to them but if it wasn't heat treated it would go fluffy and be useless. I had to show them the letter on the end of the product code and what it means
so that's what you get trying to make a good product in Oz unfortunately. Luckily Sanded put that effort in for us to supply good stuff with a true description of the *source* which tells a lot

However the dent test is not the whole story - stopping the flex with stiffness will help - hence carbon under heels etc. .. repeated flex causing delam etc.

my take on it is if you just want light and strong, yes just carbon as most of our board is a tensile strength issue and only some areas are pounded on. A 6oz carbon bottom done right will be way stronger in tension than a couple layers 4oz glass but then we get into what weave to minimise warp/weft crossing and resin uptake ... that can be more helpful

Also more helpful can be where u put things for light and stronger
it's not just a couple layers all over and a foot patch. but not giving away that stuff after 20yrs of deconstruction to figure out some tricks

Everything is write above. Which stengh? When we speak material strengh in engineering world (my day job is engineering teacher) is mostly tensil strengh at elastic limit because it's easy to measure but it's often means nothing about useful strengh you want for your use. Boards (wave/jump) mostly break because of skin buckle. Local bending instability under compressive forces. Compressive stiffness is one main factor to fight against buckling... Dent is an other way of skin destruction where low velocity impact strengh, toughness, isotropic bending stiffness play...

Starting to sound like the FOUR layers of 200gsm carbon I put over my last repair was a touch over the top.

Yeah lol
and I know it's harder and all bloody Gucci - but a well done vac bagged spread-tow would prob be almost as strong with one layer - certainly would be with two- than a hand layup of plain weave with all its crossing over points. Less crimped fibre =stronger. Less resin = stronger
vac pressure = better bonding to substrate

so maybe four is not a bad idea if it's not a new $4K board and u wanna just go sail

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Mark _australia said..

so maybe four is not a bad idea if it's not a new $4K board and u wanna just go sail

Right under my heel.

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sanded said..
We have done a lot of testing on breakage of the fibres as a part of our R&D as we supply custom cloths to the surf industry and for them its all about the cloth and force breakage - cracking decks and full snaps

We have access to a machine which is like a heel shape which puts force down onto a blank - (being surfboard focus the blank is always at 32kgm3 - higher density blanks and molded instead of block foam )

Here are some of the results (all samples were tested at least 3 times so this is the av for 3 tests (i have rounded it up so we dont get bogged down on too much details etc - so you get the jist of where these all sit ) the foam is EPS and using epoxy and all were woven into a standard cloth configuration

Imported 4oz - force 320kg breakage
Colan 4oz - force 340kg breakage
imported 6oz - force 335kg breakage
Colan 6oz - force 350kg breakage
S glass 4oz - force 360kg breakage
Carbon 4oz - force 350kg breakage
carbon 6oz - force 360kg breakage

A lot of this was published on Swellnet.com when we launched basalt cloth to the surf industry- as for force breakage, the Basalt was way better than any of these listed - though for stiffening up a shape, Thats where carbon is where it comes into its own.

Again this is surfboard focused - so more layers and different fibres added into the matrix would give different results

Interesting, only 12% gap between each fiber. More or less same gap as fiber thickness...May be because in this test skin bend and foam under compress, and bending stiffness is mostly function of thickness. Do same test with lighter foam and sandwich skin, same overall sample weight but i bet need more load to break this way

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lemat said..

sanded said..
We have done a lot of testing on breakage of the fibres as a part of our R&D as we supply custom cloths to the surf industry and for them its all about the cloth and force breakage - cracking decks and full snaps

We have access to a machine which is like a heel shape which puts force down onto a blank - (being surfboard focus the blank is always at 32kgm3 - higher density blanks and molded instead of block foam )

Here are some of the results (all samples were tested at least 3 times so this is the av for 3 tests (i have rounded it up so we dont get bogged down on too much details etc - so you get the jist of where these all sit ) the foam is EPS and using epoxy and all were woven into a standard cloth configuration

Imported 4oz - force 320kg breakage
Colan 4oz - force 340kg breakage
imported 6oz - force 335kg breakage
Colan 6oz - force 350kg breakage
S glass 4oz - force 360kg breakage
Carbon 4oz - force 350kg breakage
carbon 6oz - force 360kg breakage

A lot of this was published on Swellnet.com when we launched basalt cloth to the surf industry- as for force breakage, the Basalt was way better than any of these listed - though for stiffening up a shape, Thats where carbon is where it comes into its own.

Again this is surfboard focused - so more layers and different fibres added into the matrix would give different results

Interesting, only 12% gap between each fiber. More or less same gap as fiber thickness...May be because in this test skin bend and foam under compress, and bending stiffness is mostly function of thickness. Do same test with lighter foam and sandwich skin, same overall sample weight but i bet need more load to break this way

Yes mate the more layers and foam sandwich does help.. The interesting one was basalt due to the elongation with the epoxy resin. It bends and stretches as one with the epoxy so it breaks as one instead of normal composite matrixes that the resin or cloth fails first. The basalt samples force breakage is a lot higher in the same test. Each cloth sample went over 400kg plus with some of them beating the machine (maxed out at 450kg) These tests are a few years old as we were helping in developing a cloth for one of the big manufacturers in the industry - they are still using it in their models of boards under their "volcanic" range.

I'm building a basalt version of a board for myself at the moment - (getting back into windsurfing after a 25year break) i'll do a build thread once finished.

This is an interesting thread, I love the accumulated knowledge here.
With carbon decks providing tensile strength, wouldn't a glassed bottom benefit from a layer of wood for added resistance to compression?
When Starboard used it, it only showed in the deck (I don't know if they used them in the bottom laminate as well) but the bottom sounds like the obvious choice for me.
Am I mistaken?

^^ Advantage of a carbon deck is the buckling resistance much more than tensile strength
There is not much compression on the bottom (only water from chop strike etc which is nothign compared to heel hits) so wood deck better than wood bottom

Wood over carbon or CK is the gold standard in WS board deck.

Agree with mark.. re decks. wood is used on the deck to prevent compression perpendicular to the surface.

Additionally tho, Adding timber veneer to the bottom would improve compressive loads, but it's compression due to buckling from torsion and usually happens at the rails and not from impact like on the deck

A carbon/veneer/carbon layup would improve strength because the carbon helps extend the veneers compressive strength longitudinally by preventing the wood fibres from breaking apart. The veneer also acts as a core so you get the benefits of a sandwich. Even better is to lay carbon at 45deg. There is lots of testing around timber beams with carbon layers and different layupdirections. Typically you want the core at 0 and the outside layers at 45 for strongest layups.

veneer is heavy and expensive so foam sandwich construction is used to save weight and money. the cloth is also orientated to best strengthen against torsion

making the eps blank thicker also improves torsional strength and along with it buckling and compressive strength, so does using a higher density eps blank. Lots of ways to skin the cat

^^^ although it kinda isn't heavy
0.6mm pine at an average of 500kg/m3 is same weight as 3mm PVC at 80kg/m3 (ignoring resin uptake which isn't heaps....)
that why I love it - strong and easy math

I think it's counterintuitive thinking buckling is a compressive force. Well it is , but no fibres have compressive strength. All compressive force is put onto the poor resin to not crumble.To buckle , it first has to fail with a bend. It has to crack the resin. To crack resin in a folding way is indeed tensile force . See , this is where it gets real funky. Not so simple.

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Mark _australia said..
^^^ although it kinda isn't heavy
0.6mm pine at an average of 500kg/m3 is same weight as 3mm PVC at 80kg/m3 (ignoring resin uptake which isn't heaps....)
that why I love it - strong and easy math

Kinda. Maths alone we're talking 3-400 grams for a 120lt board not including resin uptake for a full wrap.

Most likely the 3mm thick core trumps the .6 veneer core for strength due to thickness. That aside I prefer veneer too

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Imax1 said..
I think it's counterintuitive thinking buckling is a compressive force. Well it is , but no fibres have compressive strength. All compressive force is put onto the poor resin to not crumble.To buckle , it first has to fail with a bend. It has to crack the resin. To crack resin in a folding way is indeed tensile force . See , this is where it gets real funky. Not so simple.

Yeah full deep dive needed for sure.

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Imax1 said..
I think it's counterintuitive thinking buckling is a compressive force. Well it is , but no fibres have compressive strength. All compressive force is put onto the poor resin to not crumble.To buckle , it first has to fail with a bend. It has to crack the resin. To crack resin in a folding way is indeed tensile force . See , this is where it gets real funky. Not so simple.

It's simple
the stiffness of carbon resists the bending moment required for buckling at any given location

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Mark _australia said..
same weight as 3mm PVC at 80kg/m3 (ignoring resin uptake which isn't heaps....)

That really depends on the PVC you get. I've ordered from the same supplier in the US three or four times over the years, and what you say used to be true. But the PVC I got in my last order was much more porous than in previous orders, and needed a lot of resin. That caused me a few problems when I did not realize quite how much more, and used similar amounts to what I had used before.

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Imax1 said..
I think it's counterintuitive thinking buckling is a compressive force. Well it is , but no fibres have compressive strength. All compressive force is put onto the poor resin to not crumble.To buckle , it first has to fail with a bend. It has to crack the resin. To crack resin in a folding way is indeed tensile force . See , this is where it gets real funky. Not so simple.

With a sandwich layup, the fibers on one side of the sandwich have to elongate (or break) for any bending to occur. Higher tensile strength should resist this elongation and bending more. I remember a conversation with an experienced board builder a few years ago. He stressed that thicker sandwich layers really increase stiffness and robustness. He'd actually build some hollow boards, where that really mattered.

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Mark _australia said..
^^ Advantage of a carbon deck is the buckling resistance much more than tensile strength
There is not much compression on the bottom (only water from chop strike etc which is nothign compared to heel hits) so wood deck better than wood bottom

Wood over carbon or CK is the gold standard in WS board deck.

There is plenty of compression on the bottom once you add a foil.

Buckling is compressive forces in lengt direction not perpendicular to skin. Buckling is deviation bend. By weight dry wood is strong against buckling.

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boardsurfr said..

Mark _australia said..
same weight as 3mm PVC at 80kg/m3 (ignoring resin uptake which isn't heaps....)

That really depends on the PVC you get. I've ordered from the same supplier in the US three or four times over the years, and what you say used to be true. But the PVC I got in my last order was much more porous than in previous orders, and needed a lot of resin. That caused me a few problems when I did not realize quite how much more, and used similar amounts to what I had used before.

Pvc quality has been falling for years. Seems to have foam fillers in the mix now..