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Massimiliano
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Hi, what is the recommended maximum fabric thickness for a one shot successful infusion, given a 250 cm2 square area?
Thanks
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Lester Populaire
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+xHi, what is the recommended maximum fabric thickness for a one shot successful infusion, given a 250 cm2 square area? Thanks Depends in the fabric architecture and fibre type. I have done plates of 20mm thickness in one go without issue, and have done layups of 2mm in several steps to avoid problems with porosity.
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Massimiliano
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+x+xHi, what is the recommended maximum fabric thickness for a one shot successful infusion, given a 250 cm2 square area? Thanks Depends in the fabric architecture and fibre type. I have done plates of 20mm thickness in one go without issue, and have done layups of 2mm in several steps to avoid problems with porosity. I need to build a couple of swim platforms for my boat. Layup will be fiberglass + carbon + Kevlar, two skins with inner pvc foam core (3mm skin on each side + 10 mm foam core) I need them to be as light as possible, therefore the use of carbon and kevlar. I would like to infuse it in one shot if possible.
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DD-Compound
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That’s no problem. I have infused monolithic UD Carbon laminates with 120mm in one shot. Boat hills with much more layers and core also in one shot. But a 250cm2 platform might just be enough for one foot.
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Chris Rogers
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What they said! If you're using common loose weave or stitched material you should be fine. Make sure the holes in the core are pretty close - like a 50mm grid. What are you thinking for a laminate schedule? Do you really need the Kevlar? I'd highly recommend a quick test panel before you layup the real part!
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Hanaldo
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Like Chris, I'd question if you need the Kevlar. Kevlar is only useful if the laminate is very likely to break, in which case the Kevlar will hold it together. In something like a race car nose cone, it is very likely if not virtually certain that this will get damaged and you don't want bits of carbon flying all over the track when it does, so you would use Kevlar to avoid that. If the idea is for the laminate to be strong enough to resist breaking, then you don't want Kevlar in there because it isn't as strong as the carbon.
I also question the use of carbon and fibreglass together. You use carbon because it is very stiff, so you can use less of it to achieve a desired rigidity. Because of this rigidity, it is more brittle. Fibreglass has a higher elongation, which means it is tougher than carbon and can take more to break it. What happens when you use the two of them together is under load, the carbon will break first because it isn't as flexible. This means that the fibreglass is really carrying all of the load, and the carbon doesn't need to be there.
The only purpose fibreglass serves when used with carbon is as a 'bulker' to make the laminate thicker to increase it's rigidity, essentially serving the same purpose as a core - except it is heavier and more expensive than a core. You are already using a core, so just increase the core thickness and make the laminate skins entirely out of carbon or if you need to include some Kevlar to hold things together then a combination of those two, but leave the fibreglass out.
And finally, why do you want 3mm skins with a 10mm core? It isn't an unreasonable layup, but I just wonder why you have decided on that layup?
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Massimiliano
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+xLike Chris, I'd question if you need the Kevlar. Kevlar is only useful if the laminate is very likely to break, in which case the Kevlar will hold it together. In something like a race car nose cone, it is very likely if not virtually certain that this will get damaged and you don't want bits of carbon flying all over the track when it does, so you would use Kevlar to avoid that. If the idea is for the laminate to be strong enough to resist breaking, then you don't want Kevlar in there because it isn't as strong as the carbon. I also question the use of carbon and fibreglass together. You use carbon because it is very stiff, so you can use less of it to achieve a desired rigidity. Because of this rigidity, it is more brittle. Fibreglass has a higher elongation, which means it is tougher than carbon and can take more to break it. What happens when you use the two of them together is under load, the carbon will break first because it isn't as flexible. This means that the fibreglass is really carrying all of the load, and the carbon doesn't need to be there. The only purpose fibreglass serves when used with carbon is as a 'bulker' to make the laminate thicker to increase it's rigidity, essentially serving the same purpose as a core - except it is heavier and more expensive than a core. You are already using a core, so just increase the core thickness and make the laminate skins entirely out of carbon or if you need to include some Kevlar to hold things together then a combination of those two, but leave the fibreglass out. And finally, why do you want 3mm skins with a 10mm core? It isn't an unreasonable layup, but I just wonder why you have decided on that layup? Hello Hanaldo and others, thank you for your contributions. Please see attached pictures. The stuff outlined in red is what I want to make. The purple line is where it is fastened to the transom. The blue dots is where the weight of a person makes the maximum lever. The second picture show how the swim platforms are supported on the bottom side, with two diagonal pipes fastened to the transom (the two steel pipes of course will stay and will be attached to two steel brackets fastened to the underside of the new platforms). At the moment the swim platforms are made with 10mm thick aluminum plus the underside ribs weighting 40 kg each! I need them to be as strong as possible (they need to support 100 kg on the blue dots) but also the lightest possible. The Kevlar is because it they break with a person on them, I would like to see the fracture without the broken part fall apart and leave razor sharp edges exposed. I already have the pvc foam core and the kevlar, the other materials have to be bought so I will follow your advice. Given the above, what would be your suggested layup?  
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DD-Compound
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In your case there is bending and torsion. A 10mm core will be way to thin and will flex. You can calculate a part by stress or stiffness. If you calculate by stiffness in most cases you are also safe for fatigue. I would suggest a 30-40mm core, can be a 50-70kg/m3 PVC foam. Top layer I would use around 2mm skin to avoid denting. I usually use a quadraxial as it is quasi isotroph. Bottom side you should be fine with 1mm but for symmetry you can also go with 2mm. Kevlar is not needed in my opinion, just make it strong enough that it won’t fail
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Chris Rogers
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I agree with DD Compound - you need thicker (and maybe higher density) core. That aluminum is rugged! If you're going to use Kevlar, put it in the top skin so it is loaded in tension. Not having any numbers and not being an engineer... the 25-40mm core with 2mm of carbon skin on each side sounds ok. You can probably glue the core together to make 20 or 30mm thick panels - 20mm will be pushing it without heavier skins. You can mix the glass and the carbon but ideally only if they are going in different directions for reasons Hanaldo points out. Glass biaxial with a carbon uni aligned with load or reinforcing a flange is fine and quite common. If you can - mold in the perimeter flange (50mm+) and put some uni in there so it gets to 4-5mm thick - this will close out the edge and stiffen the whole thing - got to make a mold up-stand though. It will also be good to reinforce the landing spots of those struts with some extra reinforcement or a G10 (or similar) plate. You can make the brackets out of composite or metal but metal is easier. Realistically these will (should) still be heavy - but maybe 20kg each instead of 40...
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Massimiliano
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+xI agree with DD Compound - you need thicker (and maybe higher density) core. That aluminum is rugged! If you're going to use Kevlar, put it in the top skin so it is loaded in tension. Not having any numbers and not being an engineer... the 25-40mm core with 2mm of carbon skin on each side sounds ok. You can probably glue the core together to make 20 or 30mm thick panels - 20mm will be pushing it without heavier skins. You can mix the glass and the carbon but ideally only if they are going in different directions for reasons Hanaldo points out. Glass biaxial with a carbon uni aligned with load or reinforcing a flange is fine and quite common. If you can - mold in the perimeter flange (50mm+) and put some uni in there so it gets to 4-5mm thick - this will close out the edge and stiffen the whole thing - got to make a mold up-stand though. It will also be good to reinforce the landing spots of those struts with some extra reinforcement or a G10 (or similar) plate. You can make the brackets out of composite or metal but metal is easier. Realistically these will (should) still be heavy - but maybe 20kg each instead of 40... Hi, thank you. Structures like cleats and ladder will have a wood insert in the foam and a metal plate in the underside. Brackets to fasten the supporting pipes to will be made of metal. Usually boat builders use 10mm thick solid fiberglass for swim platforms, with a 6-10cm deep border flange to add stiffness. My platforms will have for sure the border flanges as the aluminum ones have now. My idea to use a core is to try to save additional weight. I can glue together two layers of pvc foam to get to 20mm. Do you think adding ribs (v shaped foam covered with fiber) in the underside could help adding rigidity?
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