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Can't let Bill take the blame for that; it was I that posted the
original message.
My choice of words was pretty poor. In my mind I was expecting the
resonant stress to be included in the stress calculation. My attempted
point/question is that rolled threads don't help if the bolt is excited
at its resonant frequency (accelerating the fatigue rate).
You just missed the mark by just a little.
If the bolt is strong enough, it will never break due to fatigue. That is, as long as the peak stress everywhere on the bolt is below the "endurance limit", the bolt will last forever. For steel, the endurance limit ranges from 35% to 60% of the tensile strength.
Therefore, if
you avoid the resonant frequency are cut threads really that big a deal
if the bolts are 15 or 20 times the needed strength?
The cut threads have two separate problems. First, the sharp bottoms and rough surface finish really concentrate the stress and invite fatigue cracks. Second, the rolled threads are "work hardened" and the steel in that area is much stronger, right where it needs to be.
The bolts are long and slender. Thus, they will vibrate, much like a guitar string. If they are shorter, the resonant frequency will be higher (if everything else is the same.)
The trouble is, we really don't know what frequency the stock 13b bolts are vibrating at. (Maybe someone knows, but I certainly don't) If the problem happens to be at the second harmonic of the resonant frequency of the stock 13b bolt, then the short bolts may worse off since they will be excited at their primary resonance. Conversely, if the excitation frequency is at the primary resonant frequency of the long bolts, the short bolts will not have vibration problems at all.
Since we don't really know the excitation frequency spectrum, we can't make any assumtions about the vibration of the short bolts. Thus, the smart thing to do is simply use bolts that are as strong as the stock bolts. Then it is a very good bet that they won't fail. Not a certainty, but a very good bet.
If it were a car, there would be little risk involved in trying easy to find low strength bolts or cutting threads on shortened stock bolts. It is in an airplane, however, and the risks are much greater.
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