Graviman,
The concepts between the two metals (Al and steel) is quite similar. The value we use for the max stress to allow infinite life is called the "endurance" level, and is determined for each type of material by coupon tests, then adjusted for shapes and stress risers for any given design (K factors and knock downs). For critical parts, real data is taken by breaking parts, but airframes are not generally of that class, since they are designed with redundant load paths and a great deal of inspectability. Airframe stress gages compare flight loads to predicted loads to assure design adequacy, and beefups are applied if necessary during flight test.
There is a FAA AC on fatigue testing, and I believe some great AHS papers over the years. If you are building something commercial, I can recommend some expert friends!
Nick