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Hello, I have been reading everything I can find about composites but I haven't found any type of equation/calculator for determining composite thickness requirements. I know different resins, etc. have a bearing on the strength but, there's got to be a "safe" relation between how much composite equals the strength of an object made of steel/aluminum. I am trying not to waste too much material and loose the weight saving of making parts out of composites. I'm also not looking to make parts that risk my life on (bike frames, control arms), just automotive body panels, intake manifold or a pickup tonneau cover. I think a formula wouldn't be too hard that would make parts safely 15-20% strong than OEM and still have a substantial weight savings. For example: How thick would a fender need to be if it was made out of e-glass or carbon fiber vs. OEM sheet metal? I know a wheel well would have to thicker to survive a rock from the tire tread at speed, but how thick? Would a layer or two of kevlar add to 2mm of e-glass be good or over-kill? Everywhere I see PSI tensile strength,etc. but does it mean that I would need half the thickness of an aluminum bracket if the fiberglass is twice the metal's strength rating? Thank you for your time, hope to hear back from you soon.
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For what it's worth, I'll show you how I would tackle the Fiat 300 bonnet that EC did in the tutorial if I needed to make it with fibreglass instead of carbon. Assuming that you want the fibreglass bonnet to be as similar to the carbon bonnet as possible. Also remember that different materials will have different properties, so it's difficult to give a definite 'X-material will produce Y-results' (which is inherently your problem, as Warren mentioned). With that in mind... If you look at the 'average' mechanical properties of carbon fibre compared to woven fibreglass, carbon is around 1.4 times stronger than fibreglass and around 5 times as stiff. We are using carbon as our base measurement, so let's say the specific tensile strength of carbon is 1 and the specific modulus strength of carbon is 1. Therefore, the specific tensile strength of fibreglass would be 0.71 and the specific modulus strength of fibreglass is roughly 0.2. Therefore, a fibreglass laminate of equal strength to a carbon laminate will be roughly 30% heavier, and will still be almost 4 times as flexible. So, the outer skin of the Fiat 300 bonnet was made using 1 layer of 200gsm carbon, 2mm bulker mat, and another layer of 200gsm carbon. The core complicates things slightly, so we will ignore that for now. If for simplicity we assume the bonnet is exactly 1 square metre and has a 60% fibre weight fraction (or if you prefer, a resin to fabric ratio of 0.4:1), then that would mean the laminate weighs ~333 grams for each ply, or 666g total. If an equivalent strength fibreglass bonnet is required then it would need to be 30% heavier, so 433 grams per ply. Take out the 40% resin content of that and you can work out that you would need to replace the 200g carbon fabric with a 260g fibreglass fabric. This will depend on what is commercially available to you, I know that the closest weight fibreglass I can get here is 280 grams. Now if you simply replaced the carbon skins with the heavier fibreglass, and still used the 2mm bulker mat, then you would end up with a laminate that is of equivalent strength, but still much more flexible than the carbon version. One could tackle this by using an even heavier fibreglass, however I would go another way. We know that the stiffness of a laminate is a function of the cube of its thickness, so x^3. Ie, if you had a laminate that is 2mm thick, and you doubled it so that the laminate is now 4mm thick, you will have made the laminate 8 times stiffer. If we apply that to the bonnet, then let's say the modulus of the carbon bonnet with a 2mm core is 1, so the modulus of our fibreglass bonnet made with the same 2mm core would be ~0.27. However if we used the next size up core, which is 3mm, then we are increasing the thickness of our laminate by a factor of 1.5, resulting in a laminate that is 3.4 times stiffer. Fortunately this is pretty much spot on for what we would require, so based on those calculations you could conclude that if you wanted to produce the carbon bonnet in fibreglass with similar properties, then your layup would be 1 layer of 280g woven fibreglass, 3mm bulker mat, and another layer of 280g fibreglass. The weight of this would then depend on the type of core you used, but say you used 3mm Soric then your fibreglass bonnet would weigh somewhere around 2.5 kg compared to the ~1.75kg carbon bonnet (excluding the inner skin obviously).
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Thanks for the reply Hanaldo, I've always had an engineering mind and like to know the why's & how's. I would have been one if life tuned out different, that's why I try reading everything I can find on a subject. I get that rigidity being affected by thickness/shape/resin/material, like with the hoods underside support and how greater spans need more thickness... just wish there was an equation. The main project I have planned is replacing the front sheet metal for my Street/strip truck project. I want to lighten the front end as much as possible, even thought of making it a 1 piece/tilt nose.
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