-----Lets try another simple analogy.
When you use graphite as an overlay to a glass based material it's like
pulling on two ropes tied to the same post.  Lets say each rope has a
breaking strength of 100 lbs.
as you start pulling, the graphite gets tight (it's stiffer) while the glass
doesn't (it's more flexible).
As you get to the breaking point of the carbon (100 lbs), the tension in the
glass is much less, Lets say 50 lbs.
As you increase the pull, the carbon will fail and then you will have the
one glass rope left.
What you have, is twice the weight ( two ropes) but only 50% more strength.
If the two ropes had been bonded together with epoxy during this test, the
shear stress between them would have been another  problem.
If the graphite rope were smaller than the glass rope, like using a smaller
percentage in the layup, then the failure of the graphite would occur much
earlier in the loading,  (like under normal flight conditions.)

This is a simple analogy and nothing in the aircraft is so simple.

Mark Ravinski