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 Post subject: Re: Strenght and flexibility
PostPosted: Fri Feb 15, 2013 4:38 pm 
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The rocker does not do anything for the stiffness or strength in the laminate. It might pre-stress some of the layers if the layers are cut to fit a straight plank and then the plank is bend into shape in a press after the layup and impregnating is finished.

The stiffness is closely related to the height of the sandwich construction since the height of the beam is elevated to the power of three. The stiffness is of course based also a product of how stiff the materials you are using are, however, since very low Vf's are reached with hand layup the most dominant factor is height.

Adding a shape to the surface of the laminate can give you added stiffness, think of a corrugated plate vs. just a sheet of the same material and thickness.

The strength is related to the orignal strength of the two components and the amount of layers / cross sectional area.

So if I was building a TT with some medium density closed cell foam, I would add that to the area between the pads and then taper that down towards the edges with a long slope towards the tips. I think torsional stiffness is most important so a +-45 degree layup will probably promote easy riding characteristics. That will give you most flex along the length of the board and minimize the twist.

The outline is very important for the riding characteristics as well so look at some of the good freestyle boards of today and use that as the base. Measuring and matching the flex characteristics of a production board will be very hard, it is most likely faster to just try different layups and core materials.


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 Post subject: Re: Strenght and flexibility
PostPosted: Fri Feb 15, 2013 6:48 pm 
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Concave affects flex.


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 Post subject: Re: Strenght and flexibility
PostPosted: Sat Feb 16, 2013 1:16 am 
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FrederikS wrote:
...

The stiffness is closely related to the height of the sandwich construction since the height of the beam is elevated to the power of three. The stiffness is of course based also a product of how stiff the materials you are using are, however, since very low Vf's are reached with hand layup the most dominant factor is height.
...


YEP -- stiffness increases by the Heigh cubed !!

And This is where choosing a material like Carbon or spectra is a Must !
Place fiberglass on a thin core ; and Ya might want to stay close to shore
so the swim back in won't be so tiresome.

You can adjust the flex-rate by adding or subtracting the fibers on each side
of the core, after the thickness is determined. but unless your fibers can stand
up to the rigors of thousands of cycles, than it will probably just break in half.

plummet wrote:
for a strong flexible board you want to choose a thin core and use more reincforcement. eg fibreglas


He "Meant" Carbon or spectra !!

Ideally , a well constructed board that you want to Last
would NOT contain even (1) gram of that SH*T called fiberglass.
Fiberglass is Good for row-boats that you plan to use on a flat
windless lake. Carbon is GOBS stronger for the same weight,
and if purchased on the roll and un-woven , it can be had for about
$25 a pound.

Seriously -- Carbon is (5) times Stronger than Steel.

Bille


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 Post subject: Re: Strenght and flexibility
PostPosted: Sat Feb 16, 2013 10:27 am 
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Bille wrote:
FrederikS wrote:
...

The stiffness is closely related to the height of the sandwich construction since the height of the beam is elevated to the power of three. The stiffness is of course based also a product of how stiff the materials you are using are, however, since very low Vf's are reached with hand layup the most dominant factor is height.
...


YEP -- stiffness increases by the Heigh cubed !!

And This is where choosing a material like Carbon or spectra is a Must !
Place fiberglass on a thin core ; and Ya might want to stay close to shore
so the swim back in won't be so tiresome.

You can adjust the flex-rate by adding or subtracting the fibers on each side
of the core, after the thickness is determined. but unless your fibers can stand
up to the rigors of thousands of cycles, than it will probably just break in half.

plummet wrote:
for a strong flexible board you want to choose a thin core and use more reincforcement. eg fibreglas


He "Meant" Carbon or spectra !!

Ideally , a well constructed board that you want to Last
would NOT contain even (1) gram of that SH*T called fiberglass.
Fiberglass is Good for row-boats that you plan to use on a flat
windless lake. Carbon is GOBS stronger for the same weight,
and if purchased on the roll and un-woven , it can be had for about
$25 a pound.

Seriously -- Carbon is (5) times Stronger than Steel.

Bille


A finished composite using carbon fiber woven mat is approximately as stiff as normal industrial steel with a Young's modulus of around 65 GPa (Assuming HT CF and a Vf of 50%). The yield strength is around 250 MPa. If you look at specific strength or stiffness CFRPs is approximately 4x better than steel due to the lower density of 1.8 vs. 7.8 of steel.

I think the real issue for DIY will be getting the impregnation done right. Using VIP/VARTM process managing voids and getting good wetting is easy. For hand layup it has a lot to do with skills of the manufacturer. Therefor carbon fiber, which is hard the hardest to impregnate, should not be the first choice. Besides that you are approaching marginal returns if you are building a fun TT kiteboard. Getting the layup right with glass and taking care to get bubbles out will be a lot easier with glass due to the much higher permeability. For DIY I think glass is perfect, get something that is coated for your matrix material and you are good to go. Buy some samples first and do some test layups, some of the mat properties are ill defined such as drapability so you wan to get some hands on experience before purchasing enough for a board.

Cheers,
Fred


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 Post subject: Re: Strenght and flexibility
PostPosted: Sat Feb 16, 2013 12:29 pm 
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Location: scott klandl burlington, VT
Lol
Just because carbon has better strength to weight ratios doesn't mean much on paper. S glass is better than e glass. But that doesn't mean much either. Look to the largest industry of sandwhich const, the ski and snowboard industry. E glass is used for 99% of the builds, that 1% that's not is sales gimick. A properly built board with e glass will last for years. Carbon is not worth the time, money, or the effort to work around its faults, and the performance gains are limited to select applications none of which involves any board sport.

As public enemy put it yeArs ago Don't believe the hype


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 Post subject: Re: Strenght and flexibility
PostPosted: Sat Feb 16, 2013 3:43 pm 
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gbleck wrote:
Concave affects flex.


That's for sure. I made two board identical materials and layup, but one had massive concave (like OR Mako) and the other was quite flat. The flat one was really flexy and the concave one was very stiff.


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 Post subject: Re: Strenght and flexibility
PostPosted: Sat Feb 16, 2013 5:36 pm 
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For your carbon & tight weaves satins etc use a lower viscosity lam resin than the typical surfboard epoxy, then it's not that hard to get good results by hand or vacuum.
I want to do one with a 2 layer wood core with unis between some time...
Anybody tried that?


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 Post subject: Re: Strenght and flexibility
PostPosted: Sat Feb 16, 2013 5:36 pm 
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For your carbon & tight weaves satins etc use a lower viscosity lam resin than the typical surfboard epoxy, then it's not that hard to get good results by hand or vacuum.
I want to do one with a 2 layer wood core with unis between some time...
Anybody tried that?


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 Post subject: Re: Strenght and flexibility
PostPosted: Sun Feb 17, 2013 1:59 pm 
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BWD wrote:
For your carbon & tight weaves satins etc use a lower viscosity lam resin than the typical surfboard epoxy, then it's not that hard to get good results by hand or vacuum.
I want to do one with a 2 layer wood core with unis between some time...
Anybody tried that?


That would not make sense as it is the skin layer that will take all the load. Better wrap the sandwich core with some UD carbon if you want.


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 Post subject: Re: Strenght and flexibility
PostPosted: Sun Feb 17, 2013 7:32 pm 
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Skin only takes all the load when it's way stiffer than the core.
I'm talking about building something besides a standard composite I-beam...
The wood core can control some aspects of flex (how much, damping etc...), while unidirectional fibers near the neutral axis add some stiffness and reflex but are away from impact, point loads and supported against buckling, the primary failure mode for kiteboards.
I don't envision this idea mainly for light foam or balsa but maybe it could be good with spruce, larch, fir, cedar poplar etc. It could apply with various fibers. Carbon for example, likes to be unkinked and well supported, and most efficiently used used in flat unidirectional sheets and tows...
I would try this with cedar or paulownia with a light bias e glass on the outside, and E S or CF unis at the core.
Just an idea...


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