Composite Properties

[canyon]

I was wondering which is stiffer, carbon fiber composites or steel.
I always thought carbon fiber was stiffer but after looking online it seems as though I am wrong
Steel has a modulus of 200 gpa but the fibers (Toray t700) I'm using have a modulus of 230 gpa. When laid up according to Toray the modulus is 135 GPA.
I'm at a loss here. If composites aren't stiffer then why do people buy them then? Are they stronger? Lighter?
I'm asking because I'd like to double check my knowledge.
Thanks!

[TET]

Carbon/epoxy composites are stronger and stiffer than steel for the same weight.

[canyon]

So the comparisons are made on a per weight basis? I tested some carbon fiber samples I made and I was surprised to see that their modulus was lower than that of steel's.

[fleisch]

It is very important to realize that stiffness is not only effected by modulus. Stiffness increases by the cubed of thickness. So if you want to minimize weight (which is usually a prerogative when choosing carbon fiber composites) you can make a thicker lighter part that is much stronger than steel. This is also why honeycomb and other core materials are used. They increase this thickness without adding much weight and the maximum stress in bending is concentrated at the skins.