It's a while now since I was in fatigue monitoring, but IIRC, the normal SN curve is almost asymtotic to the N axis, and as long as S (or a combination of major and minor cycles) is below the asymptotic value, it can be repeated an infinite number of times.
Oh! I glad to hear this kind of words. That's correct. Such a design is called "boiler-proof." Normally it cannot be used for airplanes because the structure is way too heavy. Nevertheless, for fighters in a level flight in many components the stresses are below the material fatigue or endurance limit.
So, the question is what are the maximum values of + or - G that can be
pulled without exceeding that stress.
Absolutely! The devil is in details.
And we know the answer. The local stresses in fastener holes or notches will be many times higher than "that stress". They will be way above the material yield limit, approaching or even exceeding FTU.
Up to those levels, there is no need to tell the aircrew to back off. As
aircraft age, it's normal for the G limits to reduce specifically for that
purpose.
Unfortunately, "those levels" are exceeded in every flight. This thread was started because Flight Manual are silent about this problem. To give you an idea about stress levels I show the picture for approximately F-15 situation. I hope you know the difference between "nominal" (reference, remote) stress and "local" (peak) stress.
