Steel has a definite 'fatigue limit' : below a certain cyclic stress (N/mm^2) it can withstand an infinite amount of flex cycles.
Above this limit, the amount of cycles it can withstand drastically drops as stress increases.
Aluminium does NOT have this lower fatigue limit (or rather, the limit is 0 N/mm^2); at ANY stress, even small cyclic stress, aluminium WILL fatigue! It can NOT withstand an infinite amount of stress cycles, even at low stresses, as steel can.
There's no safe threshold that, if you stay below it, it won't fatigue, as in steel.
Conclusion: aluminium WILL fatigue if subjected to stress cycles. Steel (and some other materials) may or may not, depending on whether the stresses stay below the fatigue limit.
Guess that in the case of your antennas you just haven't had enough stress cycles to cause catastrophic failure. Then again, have you -really carefully- checked for fatigue cracks in your antennas, with e.g. dye penetration checks or ultrasound?...
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All this is pretty much documented, btw, as Ron has said. You do what you think is best. But I feel it's important to have the details out here in the open so everyone can make an informed decision about their course of action.