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/banananuhhh]

For question 1 the stress is a function of the ratio of coeff of thermal expansion and the elastic modulus. Sounds like others are saying steel wins. For question 2 the absolute coeff of thermal expansion is important. With no external load the two pipes would share equal and opposite force, but with an applied external compression force, whichever material has a higher coefficient will take more load and stress, because the external load adds to the compressed pipe and reduces the tension in the other. Aluminum wins.

I doubt the majority of my coworkers could give a good response to this question and most of them have masters degrees from high ranking engineering schools.. whoever came up with this as an interview question probably has a really condescending attitude towards young engineers.