How much stronger is carbon fiber than steel & aluminum?

How much stronger is carbon fiber than steel & aluminum?

The simple comparison between carbon fiber, steel and aluminum can be understood using common forms of the high-strength versions of these materials.   Compare the mechanical properties, the modulus of elasticity (stiffness) and the tensile strength (strength under tension) of these three materials:

1. Carbon fiber T700S from Toray, a standard modulus high-strength fiber, in an epoxy 250 F-cure composite

2. Alloy steel AISI 5130, a low hardenability alloy steel with moderate strength and good toughness

3. Aluminum alloy 7075-T6, a standard aerospace aluminum alloy

Note that all properties are at room temperature.   Environmental conditions have a great effect on carbon composites.   Once the temperature goes up above 150 F., carbon fiber epoxy composite properties will be reduced somewhat whereas the steel and aluminum properties will remain essentially the same.   High humidity with heat will have an even greater effect.

The more complex question you ask is, “How many times lighter is it?  And how many times stiffer is it?”   You can see that the carbon fiber is the lightest and the lowest density and has intermediate stiffness.  However, if you just look at the properties above, which are for a single-direction (unidirectional) laminate of carbon and consider the density of the materials you will get a misleading answer.

Generally, carbon fiber needs to be layered in multiple directions to be useful (somewhat like plywood).   If not, all the fibers will be going in one direction and the material will be prone to splitting because there will be no strength in the cross-wise direction, just like wood veneer.   If you layer carbon fiber in such as way as to have most uniform properties (a so-called “quasi-isotropic” laminate) you end up with overall properties somewhat like aluminum .    In fact, if you do this, you will end up with a laminate that’s a bit heavier than aluminum.   

So, after all that, how much lighter and stiffer is it?   Well, the better answer is:  if you have a structure that can take advantage of the directional properties of carbon fiber, like a bicycle wheel or frame tube, or a helicopter blade, where certain pieces of it can be aligned with the flexing or stress that the part will see, you can get up to about a 30% weight savings over aluminum.  (For more detailed analysis of this concept, see, “Composite Airframe Structures”  

by Michael C. Y. Niu, Chapter 1.0, Section 1.3, Composites vs. Metals (aluminum alloys).