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Stiffness Vs Strength

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foam-n-fibre
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Re: Stiffness Vs Strength

Postby foam-n-fibre » Thu Nov 03, 2011 6:01 pm

Just saw this thread, and thought I might add something about comparing different weaves. I made up some test strips using 1/8" corecell, about 2cm by 20cm, with three different types of cloth I had around. I was mostly interested in stiffness. They were glassed on both sides with 1 layer.

I was comparing 12oz bi-axial E-glass, 8.9ox fine weave S-glass (style 6781), and a 7.3 oz mostly unidirectional E-glass with about 90% of the fibers aligned with the axial direction (style 7715). Stiffness was simply judged by flexing the strips by hand. The 7.3oz unidirectional E beat out the 8.9 oz S, and the 12 oz bi-ax came in last. I expected that in this direction the 7715 would beat the slightly heavier S, but was not as sure what to expect from the bi-axial heavier stuff.

Of course this does not measure impact resistance, I expect the resuls would be opposite.

As for torsion, the 7715 still feels stiffer than the S, but I think the bi-ax might take that, but hard to tell. I think torsion is huge for snowboard building, but with tapered tips on a typical twin tip, I don't expect torsional stiffness is nearly as noticable.

For what we are building where much more strength is needed in the long axis, it's worth exploring the different weaves like tri-ax and unidirectional, and not just different cloths like E, S and carbon. You might find good strength vs weight vs cost without having to go to carbon.

Peter

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Re: Stiffness Vs Strength

Postby plummet » Thu Nov 03, 2011 9:40 pm

foam-n-fibre wrote:Just saw this thread, and thought I might add something about comparing different weaves. I made up some test strips using 1/8" corecell, about 2cm by 20cm, with three different types of cloth I had around. I was mostly interested in stiffness. They were glassed on both sides with 1 layer.

I was comparing 12oz bi-axial E-glass, 8.9ox fine weave S-glass (style 6781), and a 7.3 oz mostly unidirectional E-glass with about 90% of the fibers aligned with the axial direction (style 7715). Stiffness was simply judged by flexing the strips by hand. The 7.3oz unidirectional E beat out the 8.9 oz S, and the 12 oz bi-ax came in last. I expected that in this direction the 7715 would beat the slightly heavier S, but was not as sure what to expect from the bi-axial heavier stuff.

Of course this does not measure impact resistance, I expect the resuls would be opposite.

As for torsion, the 7715 still feels stiffer than the S, but I think the bi-ax might take that, but hard to tell. I think torsion is huge for snowboard building, but with tapered tips on a typical twin tip, I don't expect torsional stiffness is nearly as noticable.

For what we are building where much more strength is needed in the long axis, it's worth exploring the different weaves like tri-ax and unidirectional, and not just different cloths like E, S and carbon. You might find good strength vs weight vs cost without having to go to carbon.

Peter


So just to clarify.

Your bi-ax was layed up fibres at 45 deg, fine weave 90 and 0, uni all at 0? Id so your results make sence to me.

Interestingly carbon also seems to be more springy, a faster rebound. So you will get different board characteristics using carbon v glass.

Other things to think about are at what part of the layup should you use uni or ddouble bias etc. for example uni layer work best under extension ie on the bottom of the board. but you can still get good torsional sttiffness with double bias at 45 in compression (on the top of the board).

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Re: Stiffness Vs Strength

Postby grisen » Thu Nov 03, 2011 10:20 pm

As mentioned in another thread, I have broken three boards this season. Untill now I only had focus on stiffness and weight. This mean I used sligthly thick cores, a combination of ligth glass and carbon fiber weaves. But now I know I also have to take of the strength:-))

When comparing carbon and glass fiber, it is important to remember that the density of carbon is around 1700kg/m^3 and E-glas around 2600kg/m^3. This means on a 240g/m^2 carbon weave will have a thickness of around 0,14mm and a 240g/m^2 glass will have thickness of 0,09mm. Both MOE and tensil strength a measured as N/mm^2 (MPa) This means that not only does carbon have higher MOE than glass, but you also have "more" materiale if you compare they weave by weigth. Regarding tensile strength, carbon and glass fiber are pretty much the same (but if you compare same weigth on weave, you have more "material" on the carbon) The big difference is on the MOE, by whch "standard" carbon are approx three times higher than E-Glass (all values are before the resin is added)

Kind regards
Lars

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Re: Stiffness Vs Strength

Postby foam-n-fibre » Thu Nov 10, 2011 10:00 pm

Hey Plummet, yes you're right about the layup of my test pieces. Good point about the different spinginess of different materials. In the world of windsurfing, a carbon mast will rebound faster than one with a more regular glass. For masts, the more carbon the better, and you really can feel a difference. I wonder what speed of rebound is best for pop in a board???? Maybe we should be all making thin carbon boards like the Brokite guys(?) If anyone has not seen what I'm talking about check out their construction videos.
http://www.brokite.com/joomla/index.php ... &Itemid=63

Peter

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Re: Stiffness Vs Strength

Postby Bille » Fri Nov 11, 2011 3:21 am

foam-n-fibre wrote:...
I wonder what speed of rebound is best for pop in a board???? Maybe we should be all making thin carbon boards like the Brokite guys(?) If anyone has not seen what I'm talking about check out their construction videos.
http://www.brokite.com/joomla/index.php ... &Itemid=63

Peter


Excellent Find on the video there, Mr foam n.. !!

The speed for rebound is probably an individual thing
In the future, i see kiters asking for a Real Specific amount,
depending on their weight and riding style ;
kinda like solemn & down-hill skies have for snow.

((BTW)) when they wrapped the four sections with carbon, the
part that went up the thickness of the 1/4" PVC, (that WAS the sheer-web))
i was describing earlier. It keeps your foot from crushing the core.
It ALSO ties the bottom surface skins to the Top surface skins, otherwise
the only thing keeping the skins from separating is the glue bond to the core.

Wrapping your sections is one way of doing this, there ARE other ways !!

Bille

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Re: Stiffness Vs Strength

Postby plummet » Fri Nov 11, 2011 5:30 am

here's an idea that i've been mulling over in my head for a while.

hollow tubular carbon core.

essentially you lay up a the core with 1/4" greased plastic tubing. you space the tubing 1/4" apart lay the a couple of layers of carbon on. then put another layer of 1/4 tube on top and press them down into the carbon thus creating a cardboard style effect. when the resin is cured you pull the tubing out and end up with a hollow but bridged carbon core.

you then layup a few layers of carbopn top and bottom and you end up with a completly carbon board that is light and in theory very resilient. theres no foam to delam and the bridging should keep the board strong as.

Will it work? i dont know. but the idea intrigues me.

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Re: Stiffness Vs Strength

Postby Bille » Fri Nov 11, 2011 6:12 am

plummet wrote:...
you end up with a completly carbon board that is light and in theory very resilient. theres no foam to delam and the bridging should keep the board strong as.

Will it work? i dont know. but the idea intrigues me.


& each of those sides turn into sheer-web, the flex of the board can still be adjusted
with a constant tapering of the Uni-Carbon in lay-up.

Will it work ? i dono either
but
sure wish Ya didn't live on the
other side of the planet there --plummet-- i'd like to work on
that With Ya !!

OH Yea-- & instead of grease on your tubes ; Wax will suffice quite nicely !

Bille

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Re: Stiffness Vs Strength

Postby BWD » Fri Nov 11, 2011 3:58 pm

Or you could just use a sheet of something already corrugated for a mold :!:

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Re: Stiffness Vs Strength

Postby plummet » Fri Nov 11, 2011 8:12 pm

BWD wrote:Or you could just use a sheet of something already corrugated for a mold :!:


that defiets the purpose of making it all out of carbon. which is to have the same modulus through out the board. the same shear strength. the same flex characteristics.

if you used corrugated cardboard as an example yourd end up with a stiff yet brittle board the will fail when flexed hard.

Infact thats how most boards fail. Due to shearing between the core and the reinforcements. Thats why a foam board is generally weaker than a wood core board. the foam is weeker and shears alot sooner. The wood core is heavier more robust with greater shear strength.


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