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I have noob question,I dont understand why are parts made from woven carbon fiber composite stiff.

I understand that carbon fibers are strong and have high Young modulus,its stiff and strong when the applied force is tensile type parallel to the direction of the fibre,basically its strong and doesnt stretch when we use it as rope and hang some weight from it,but otherwise,its very flexible yarn.

The woven twill cloth without resin is not much stiffer when it comes to holding complex 3D shape,like car mirror for example,than t shirt or woven shopping bag.Its still massively strong and stiff under tensile load in the direction of fibre,but if force is applied in any other direction,its extremly weak and soft.

Even when epoxy resin is applied,and its cured properly,I dont understand how the carbon fiber can improve strenght or stiffness over the values provided by the epoxy which have lower tensile strenght and Young modulus than carbon fiber,but is isotropic so its strong and stiff more equaly in all directions.

For example when making car mirror,there is not much tensile load,I dont understand how could carbon fiber improve it,why not just make it from epoxy resin only and skip the CF.When car goes fast it blows air on the mirror,its pressure/compressive force perpendicular to the woven carbon fiber surface,when it comes to perpendicular compressive force,as when crushing in hand carbon fiber cloth in 3D shape,its about as strong and stiff as pajamas.

I dont understand how this carbon fiber twill cloth that is soft and weak in every possible direction except when trying to stretch it becomes super stiff once resin was cured.I cant imagine reason why it would be any stiffer in ability to hold complex 3D shape than the stiffness of the epoxy resin alone,since when force is applied perpendicular to fibers,the fibers have practicly zero strenght and stiffness,so they cant contribute to the stiffness of the part.

Maybe car mirror is vague and unclear example,sphere is better.If I imagine carbon fiber composite sphere,I dont see how it will be stiffer against crushing force than cured resin alone without CF.Even if you tried to stretch it by grabbing left and right side of sphere,the carbon fiber will not begin to be loaded in tensile kind of way until the sphere collapsed into pancake disk shape.

Is there some "mechanism",some fundamental factor that I dont understand, that helps the cured resin to "translate" the high tensile stiffness of carbon fiber to other directions,so the cured part,such as mirror,sphere or car wheel are stiff and hold their shape against crushing,twisting forces?

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  • $\begingroup$ See this question & answer ... engineering.stackexchange.com/q/19889/10902 $\endgroup$
    – Solar Mike
    Commented Mar 17, 2018 at 19:28
  • $\begingroup$ you really need to learn how to ask a specific question. $\endgroup$
    – agentp
    Commented Mar 17, 2018 at 21:19
  • $\begingroup$ Solar Mike I dont understand why I should see that question.I am its author,its not duplicate of this question... agentp Why should I? I dont see anything wrong with this one,sure its really long,but its not badly written.I have experience here on stackexchange of people not underetsnding what I mean,so I decided to hammer the point down with many words so its crystal clear what I meant.Its not ideal or most elegant solution,but it works. $\endgroup$
    – wav scientist
    Commented Mar 17, 2018 at 22:26
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    $\begingroup$ "why should I?" Answer: to get good answers. I'm probably not the only one who got lost half way through your rambling. $\endgroup$
    – agentp
    Commented Mar 20, 2018 at 0:31
  • $\begingroup$ How would I get "good answer" by revisiting MY OWN question that is about completly different topic? In that question I ask if triaxial weave pattern is scam,and answer I get is,no,its not scam.How does that have anything to do with this question? $\endgroup$
    – wav scientist
    Commented Mar 21, 2018 at 5:48

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a fiber-composite like carbon & epoxy works as follows:

The tensile strength is furnished by the fibers and the shear strength is furnished by the resin. Furthermore, since the resin encases the fibers and prevents them from buckling in compression, it also enhances the compressive and bending strength of the part.

Fine-diameter fibers possess a lot of exposed surface area to which the resin can bind, and since adhesion between the fiber and the resin is key to obtaining the benefits of a composite, this enhances the strength of the part as well.

Nature uses this same scheme in wood, where the fibers are cellulose and the resin is lignin.

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  • $\begingroup$ But is it any stronger under compression than if there wasnt any carbon fiber and the part was made entirely of resin? $\endgroup$
    – wav scientist
    Commented Mar 17, 2018 at 22:51
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    $\begingroup$ it will be stronger in compression than resin alone. by how much i do not know. $\endgroup$
    – niels nielsen
    Commented Mar 17, 2018 at 23:15
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    $\begingroup$ There's two failure modes due to compression. There's reaching the ultimate compressive strength (imagine crushing a piece of fruit) , or bucking, where the object deflects sideways under compression. This happens when a long, thin object is compressed in the long direction (imagine a piece of spaghetti). Individual carbon fibers or weaves will usually buckle on their own, but the resin matrix means that they remain aligned. The matrix and all the fibers share the same stress as long as the fibers remain bonded. That's why the composite should be stronger in compression than the resin alone. $\endgroup$
    – C.R
    Commented Mar 18, 2018 at 8:40
  • $\begingroup$ niels nielson Thank you for your answer.I would ask you if you could describe in greater detail how exactly does the resin + carbon cloth become stiffer togerther than either of resin or cloth alone.And by stiffer I mean in that direction that that carbon cloth is soft alone,like a sphere or car mirror thats being squeezed in hand,I hope you know what I mean. $\endgroup$
    – wav scientist
    Commented Mar 21, 2018 at 5:54
  • $\begingroup$ @wavscientist, see the response by cr above. $\endgroup$
    – niels nielsen
    Commented Mar 21, 2018 at 7:47

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