The original question was about whether a steel tubular frame would be prone to fatigue failure and the answer is "not likely" because of the high endurance limit of steel compared to aluminum.

A tubular structure is designed for stiffness which is a function of modulus of the tubes, steel at 30MSI and aluminum at 10 MSI. The stress level at which the structure is designed to operate is kept far below the yield point or proportional limit. But if you keep design stresses so low that fatigue never can happen, the plane will be very heavy or not fly. Most commercial transports have a warranted life where no primary structure will suffer fatigue cracking issues, somewhere around 60,000 hrs or more. To verify this, periodic inspections are done on critical areas using ultrasonics, dye pen or X-rays.
On a GA plane, the design stress levels are much lower so fatigue of aluminum structure is not an issue unless there are localized high stress areas or poor workmanship which allowed stress raisers to initiate cracking. This is why edges of detail parts must be smooth and blended and holes deburred.

If you have a steel frame and are concerned about fatigue, clean the frame down to bare metal, examine the tubes with a 10X loupe for cracking, check closely at welded joints for crack initiation, and if all looks good, prime and paint the frame to inhibit corrosion/rust.
Bob H