STEPS in fatigue design of aircraft lower wing surfaces, as presented here, are:
Selecting a structure that permits easy fatigue-crack detection. X-ray inspection of blind areas is suggested.
Setting a design quality standard. This is the important stress-concentration factor Kt, which is shown to have a lower practical limit.
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.
Step testing to check design quality. This preferred method of fatigue testing is discussed here.