NWA,
You are right to say that changes in load can have an effect on the progress of a fatigue crack. And you rightly note that the maximum axial load on the engine is at the front where the fan is since it is the part which is producing most of the thrust. I had a feeling these big fans which are really ducted propellors would have a big influence on the stresses at the front of the engine. That fan is literally dragging the whole aeroplane behind it. We had it easier on the low bypass engines like the Conway (0.6 bypass ratio). But what do I know - it is a long time since I designed the turbines and thrust reversers on the Conway and I left RR before the RB211 debacle. Stewart Miller the man who rescued RR from that catastrophe worked on the next board to me in those long gone days.