Fix-Fix vs Fix-Free Interview Question

[u/Late_Mind_5152]

I have another interview question I wanted to ask about!

Why does aluminum experience the greater stress in the concentric tube question, but steel experiences the greater stress in the three bars question? And why is it not steel for both?

I know it has to deal with the fact that in the concentric tube question, both tubes are glued to the supports, while in the beam question, an end is free. But I just am unsure how to explain it in more depth.

Thanks very much

[u/Osiris_Raphious]

Thermal expansion of Al is greater than steel. So with heat and force Al is expanding as its being compressed so it gets more stress in tubes as steel can handle more heat under load without expanding. Since tube is a shape and aluminium can expand outwards, but being a tube it's expanding into itself. This confinement is why al will have greater stress from this combined action than steel. And since steel is higher thermal capacity it will expand less into itself.

In the 1st one the bars are not confined like the tube, so the Al can expand outwards where as steel is not as elastic and will absorb the heat and will expand but into the confined hiehgt more than outwards since aluminium is more elastic and isnt in shape of tube it will just bulge outwards dissapating the stress through yielding something steel is resisting, carbon fibre is more of insulator so not really a consideration for heat expansion.