STEPS in fatigue design of aircraft lower wing surfaces, as presented here, are:
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1.
Selecting a structure that permits easy fatigue-crack detection. X-ray inspection of blind areas is suggested.
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2.
Setting a design quality standard. This is the important stress-concentration factor Kt, which is shown to have a lower practical limit.
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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.
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4.
Step testing to check design quality. This preferred method of fatigue testing is discussed here.