By MegaNoob - 8/28/2018 11:13:03 PM
I cant understand what makes carbon fiber stiff.What kind of magic happens between carbon fibers and cured resin that makes the resulting material so stiff?
To be more specific,I mean stiffness in the perpedicular direction to the surface.
Carbon fibers alone are stiff,but only when the force is perfectly parallel to the direction of the fibre and the fibre itself must be stretched.Except the stretching scenario,it have practically zero stiffness,its really just a wire.
If we imagine a sphere made of CF,how would it be any stiffer than just resin when the applied force is perpendicular to its surface? Like if the sphere had vacuum inside so the outside pressure would try to compress/squeeze/flatten it.
I cant think of anything that would make it any stiffer in such scenario than the resin matrix alone.Carbon fiber ball alone without resin is about as stiff as socks,the resin is stiffer and will hold its shape but its still just polymer,its stiffness is low.
Why when we combine woven fiber ball with zero stiffness with little bit of resin with low stiffness,we get high stiffness? I asked this before and the answer was that the resin holds the fibers,but that doesnt make any sense to me.The resin is weak and low stiffness,if it holds the fibers,that have zero stiffness perpendiculary to the surface,the force will just be transfered to the weak and soft resin and as result it cant be any stiffer than just the resin alone.
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By oekmont - 8/30/2018 11:10:31 AM
You are juggling with technical terms here while obviously meaning something else, wich makes it kind of hard to get the answer right, but I'll do my best.
What I understand you want to know: how is it possible that crp can endure pressure forces parallel to fibre better than the plastic matrix, while the dry carbon fibre is a cloth, wich can't take any pressure loads parallel to the fibre at all.
Here is why: crp composites combine the propertys of a resin matrix with those of a much stiffer and stronger fibre. Now think of the composite in a very small scale. If you take a very short piece of fibre (about the length of the fibre diameter) it's obvious, that the fibre can take huge pressure forces, before failing. This remains true if you extend the Fibre piece, but only to a point, where the fibre is so long, that it bends outward under the force. The engineering term is stability failure. Here comes the resin into play. It glues together multiple fibres, which makes it much harder to bend outward under the load. Think about two wooden boards. One alone is not strong enough to walk over, and two placed on top of each other are still not strong enough. But if you glue them together beforehand, they become as strong as a board twice the thickness, wich might be easily strong enough to walk over. The difference is, that you have to think about the boards standing upright, and you are standing on top. But the critical property is basically the same: the resistance against bending.
same goes for our fibres. Together, they are saving each other from stability failure. And the resin is needed to combine their strength.
However, you never reach the strength the fibres can take under pulling loads. and thicker fibres perform better under pressure than thinner ones. This is why some more basic carbon types are almost as good under pressure, than the higher grade types. And this is why glass fibres perform quite good under pressure/bending, too.
There is a difference however for aramid. The chemical structure of the fibre itself can't take much pressure along the fibre, and therefore aramid composites are weak when it comes to bending.
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