Next up he'll switch to titanium and wonder why it deflects even more than steel, when everyone knows titanium is always stronger than steel. Which titanium isn't; it is actually only as strong as some steels, but is about 2/3 the density, 2/3 the modulus**. Titanium also maintains its properties to higher temperatures than steels do, which is the main reason it is used to cover the SR-71.
It's a shame not knowing the engineering definition of the word stress. Where in the Google could such information be found?
** A plot of density vs Young's modulus shows that many metals fall nearly on a line; that is, the ratio of density to Young's modulus is a almost constant in comparison to non-metals. One standout material is Boron, which is exceptionally stiff for its density. However Boron is also extremely weak, hence subject to brittle failure.
See http://en.wikipedia.org/wiki/Specific_modulus
Picture http://aluminium.matter.org.uk/content/html/eng/default.asp?catid=151&pageid=2144416669