STEPS in fatigue design of aircraft lower wing surfaces, as presented here, are:
1.
Selecting a structure that permits easy fatigue-crack detection. X-ray inspection of blind areas is suggested.
2.
Setting a design quality standard. This is the important stress-concentration factor Kt, which is shown to have a lower practical limit.
3.
Choosing a design gross area stress rather than net section stress to give the desired life. The gross area stress is also a better basis than the 1.0g operating stress level for comparing fatigue life in similar aircraft.
4.
Step testing to check design quality. This preferred method of fatigue testing is discussed here.
