Cross sectional strength of CFRP vs steel

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#1

So I’m playing around with some numbers, and I keep on running into a problem that I want to ask you guys about. I’m looking to make a drop in replacement carbon tube who’s circular cross section must match both the ID and OD of the steel tube currently in use to be a true drop in replacement. All stresses for the tube are essentially going to be down the axis of it, so it’s prime to be replaced by uni carbon. The commercial component is currently made out of various grades of steel.

My question is: even though CFRP (assuming the correct fiber orientation for the loads encountered) has a specific strength (kN.m/Kg) much higher than steel, it’s not actually stronger on a by-volume basis, right? Would the part would have to be thicker just because CFRP is so low density? To end up with a component that was as strong as the steel version (but lighter), would its cross section have to be thicker? CFRP’s specific strength is 3-7x stronger than steel (depending on the grade), yet in certain cases its also more than 7x as sparse (lower density), meaning that cross section for cross section, it might not be as strong as a steel component. Is my line of reasoning correct here?

I appreciate any input, thanks!

#2

Yes it is normally, depends on the grade of course but tensile strength or compressive, depending on your loading type is higher than that of steels: eg 700MPa for steel and upwards of there for carbon, eg 1256MPa here for one we have used recently:
http://www.gurit.com/files/documents/se-84lvv16pdf.pdf

Perhaps you are thinking of stiffness. Volume for volume basis, steel can certainly trump carbon on a stiffness basis.

See here for example
http://www.netcomposites.com/guide/material-comparison/4

#3

Practically speaking, you would need to assume it is made well as a poor manufacturing process and high porosity could drastically reduce compressive strength

#4

Thanks for those links! Very informative. We’d be making them with vacuum infusion so we’d hopefully be able to get excellent resin saturation. I’ve never infused a stack of Uni as thick as what this would require though, so hopefully it’s possible. Might have to switch to an even thinner resin formulation…

Stiffness is indeed an important characteristic for the part. It’ll need to have a high resistance the bending, but I think it’ll still be possible to make it out of carbon and work in the correct way.