When I heard these things I was a Junior and afraid to question what I was told. They just stuck with me. I remember he was specific about short life of aluminum.

As for submarines, the diving and surfacing results in hull expansion and contractions which are the same a work hardening. The same as bending a piece of wire back and forth and it will eventually break.

I wonder how they determine metal fatigue other than looking for cracks? metals investigatins tend to be after the failure. I am under the impression that the old planes are less in number flying every year. Crashes or failed frame annuals? Not my area of expertise.