>I'll go ahead and predict that the strength of that tube will have materially decreased after 1 year in the elements. With steel, the real question is not the

>I'll go ahead and predict that the strength of that tube will have materially decreased after 1 year in the elements. With steel, the real question is not the weakening from Year 0 -> Year 1, but from Year 4 -> Year 5. After an initial period of weakening the strength of steel levels off.[1] Anyways, another user pointed out that a cheap foam layer can reduce the daily flexing to arbitrarily low amounts, and give an arbitrarily amount of time for the tube temperature to equalize circumferentially.[2] Cheaper than either a temperature equalizing water jacket or an underground tunnel. I know you disagree in that other thread, but I now consider this problem solved. [1] https://en.wikipedia.org/wiki/Fatigue_limit https://en.wikipedia.org/wiki/Fatigue_limit [2] https://news.ycombinator.com/item?id=15460535 https://news.ycombinator.com/item?id=15460535