Hello all,
this is my silly question:
for a carbon fiber 1 1/2" wing tube with thick wall (0.078")

moment of inertia Jx is 0.088 inch^4 (3.68 cm^4)
stregth modulus Wx is 0.118 inh^3 (1.93 cm^3)

let's assume the following:

a 118" TOC plane weights 40 lb (roughly 20 kg)
at the end of a 10g dive it has a stalled wing
the plane now weights 400 lbs (200 kg) over a single wing.

The wing loading is triangularly distributed over half the span.
The center of loading is roughly at 1/3 half the span that is 20" (50 cm).

The bending moment on the wing tube thus results 8.000 lbs*inch (1.000 kgcm)
Under such a load the material stress results in 67.919 lbs/inch^2 (518 kg/cm^2).

This stress is less than 1/10 of the maximum stress for that material.

Now, why should we under-employ the material strength or, as counterpart, oversize the structural elements?.

Is there anything wrong in the calculations?
Are there other reasons I don't know?

I hope I have performed correctly the units conversions (I'm Italian, I know, nobody's perfect), and I apologize for my poor english.

Many thanks to all.
Enrico