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 Post subject: Re: Strenght and flexibility
PostPosted: Mon Apr 29, 2013 12:20 pm 
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Location: Ottawa, Canada
Think about what a steel I beam looks like in a bridge or other structure. To bend that beam one side has to be stretched out, and the other side has to be compressed. In the middle of the beam there is what is called a "neutral axis" which is not stretched or compressed. You don't see I beams with a third level of horizontal steel in the middle, and that's because it will do very little. A layer of glass between foam layers would add a very small amount of stiffness. It would add much more stiffness if you just use the glass on the outside.

Peter


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 Post subject: Re: Strenght and flexibility
PostPosted: Mon Apr 29, 2013 1:14 pm 
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peter is right and matthieu is wrong ...sort of.

if you really need to sandwich your core in order to build up the thickness,
buttering both halves of the core with epoxy can add lots of weight.

one way to minimize the amount of epoxy in your glue joint is to build your own 'film adhesive' laver.
wet up a single layer of 2 or 4 oz on a sheet of plastic or glass.
squeegee off excess epoxy.
transfer glass to the blank and clamp.
the glass is just something to 'hold' the desired amount of glue in the right place.

-bill


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 Post subject: Re: Strenght and flexibility
PostPosted: Mon Apr 29, 2013 2:02 pm 
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thanks, that i beam story makes sens, and i feel like a d..b.

i'l stick the glass on the outside of the board !


maybe keep the sandwitch thing for nwxt board using carbon fiber tow in the center ( to hide them as they look dificult to plane out on the surface of anything !! )

other question here : IS glass and epoxy stronger in traction ( extention ) or compression ?

meaning, where should i out more ( or ad reinforcements ) glass on the board ? top or bottom ? why ?

i get the part where heel area get's more pressure and in order to avoid the foam to compress in you have to ad layers, ok, but, flex and STIFNES wise, i vould put extra layers on the botom, seems to me that, when you flex the board, this layers would be getting traction and resisting would prevent to mutch flex ? WRONG AGAIN ??


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 Post subject: Re: Strenght and flexibility
PostPosted: Mon Apr 29, 2013 6:26 pm 
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stronger in tension.
Quote:
top or bottom ?

Top. Boards fail by buckling the deck.
The thinness of the laminate can't keep the skin in column to take the level of compression one might expect from a spec sheet on the glass or resin, and the deck is subject to point loading from bindings/straps.
Extra layers of glass on the deck help the board survive, and also help deal with "springback" if you are bending a stiff core onto a rocker table.


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 Post subject: Re: Strenght and flexibility
PostPosted: Tue Apr 30, 2013 12:10 am 
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ok i get that !

but, once put 3 layers on top and 3 botom, IF bord is still to flexy, adding one layer on botom would do ??


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 Post subject: Re: Strenght and flexibility
PostPosted: Tue Apr 30, 2013 9:30 pm 
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mattthieu wrote:
ok i get that !

but, once put 3 layers on top and 3 botom, IF bord is still to flexy, adding one layer on botom would do ??


For reference e-glass has a higher modulus (stiffness) in tension than compression
I believe by a factor of 36/25... I'm going off an old calculation work sheet and don't know the original source so take it with a grain of salt. Regardless of the exact numbers, if you need more stiffness add it to the bottom sheet.

That stated if a board with 18 oz of glass isn't stiff enough you shoulda made the core thicker and a single ply isn't going to do a whole lot. Carbon would make difference here just understand it is AWFUL for impact toughness, So for something designed to be jumped upon (like your top sheet) mounted onto a semi compressible e core (like foam) there are far more suitable materials available.


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 Post subject: Re: Strenght and flexibility
PostPosted: Thu May 02, 2013 2:45 am 
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what is your sugestion ?

i have just enought glass for 3 layer top and 3 botom ( 6 oz glass ) and i have 9oz 3 inch tape for an x patern, plus 9oz 4 inch tape for reinforcement at the hells !

i was thinking addingone layer of carbon ( closer to the core ( let's say, foam, glass, carbom gla and glas )). maybe an x patern of carbon tape also, and thinking gettin unidirectional stiched or so carbon

question, where could i get the carbon cheap ( i'm in canada ) and, what carbon could i use ??

i know i should have made the core thicker but, that's to late, core's already grinded !!!!! tapered, 14mm center to 3-4 mm tips, 8mm rail on the sides. or so..

i plan huge : 20mm concave and so it should be mor rigid due to the shape itself !!

where to get ( and wich ? ) carbon ?

thanks


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 Post subject: Re: Strenght and flexibility
PostPosted: Thu May 02, 2013 4:42 am 
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carbon VS glass

question, cause carbon sems hard to work with and is more expensive !

but, comparing

one layer of carbon 5.7oz is how many times equal ( stonger ) ( ads the same rigidity ) then plain 6 oz cloth ? let's compare 45 degré biax carbon and 45 biax glas ?

if usin one layer of unidirectional carbon cloth, how mutch standard glas layer could i replace with ?

you se my point here, as carbon is like 35 the yard or more and glas 10 to 15 ? maybe less


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 Post subject: Re: Strenght and flexibility
PostPosted: Thu May 02, 2013 5:18 am 
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Location: The Naki
my experience is 30-50% material to get the same stiffness.

So 300 grm carbon will do the job of 450 to 600gm glass.


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 Post subject: Re: Strenght and flexibility
PostPosted: Thu May 02, 2013 6:09 am 
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so, if carbon price is 35 the yard and glass 15, then, carbon is a good choice !

say, is it really do hard to wet ?

what dificultys can i await from carbon Vs glass ?


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