I was listening to some co-workers talking about problems meeting stiffness requirements. Someone said that even with a solid metal rod (instead of the existing tube) we could not meet stiffness requirements.
I started daydreaming... and went back in time over a quarter of a century to some class I was taking in college. Things are hazy when you go back that far; but I am sure that someone with a Ph.D., or some other letters after his name, said that you could actually add stiffness to a solid rod by drilling out the center (and maybe by appropriately treating it, I forget). The reason, if I recall correctly, had to do with the added tensile strength of the inner surface. The reason I remember this from so long ago was that it was so counter-intuitive: I was stunned.
I'm not a stress/structures guy; so I asked a co-worker about it, and he said that the solid rod would be stiffer, because it had the greater (bending) moment of inertia. I agree with the latter part of the statement, but my hazy daydream keeps me from agreeing with the preceding conclusion. My money is still on a series of concentric tubes, appropriately processed and internally supported, being stiffer than a solid rod of the same O.D.
So, my question: Does anyone know of any references to this little structural trick (or engineering wives' tale). If so, can you quantify how much stiffness is gained?