DAAquamaster wrote:
3AA steel tanks are over engineered enough that they effectively have no fatigue life and the only thing that kills them is rust.
Even rental tanks that are filled daily (or even a few times a day) don't experience enough cycles to make a dent into fatigue life curves. The fatigue life curves are normally plotted on a logarithmic scale.
Fatigue failure is a big issue with the number of cycles experienced in rotating machinery or a vibrating object, not so much for a recreational scuba cylinder.
Also most fatigue life curves are based on reversing stresses. In other words stresses that cycle in both direction (positive and negative cycles), like bending a beam back a forth. A pressure vessel only sees positive tensile stresses, which is less of a fatigue issue.
The statement that "3AA steel tanks are over engineered" is not really a true statement. A cylinder is just a very simple geometry and it is very predictable. The stresses can be easily calculated for a theoretically perfect cylinder without imperfections. It just happens that the fabrication methods also produce a fairly reliable, uniform wall thickness cylinders.
Yes, rust pits is what will destroy a steel cylinder.