Originally Posted by
avidflyer
I believe you are a commercial chemist or materials expert so you know far more than I on the subject but the way I understand it is the pressurize / depressurize cycles tend to act on an aircraft's skin (at the rivets) much like bending an aluminum can back and forth; eventually it breaks. I am sure this was just a surface (paint) crack but they do have to look at every one.
Aircraft (and engines and other items)are tracked both by hours and Landing / Take Off Cycles and in the case of engines, type and duration of power changes.
Some failure modes are driven by cycles like metal fatigue and others are driven by hours of use like corrosion and some even have combinations of both.
The FAA requires inspections and replacements based on both factors and the airlines sometimes have more restrictive limits.
As far as this incident reported by the OP, who knows what it was and the fuselage is not going to fall apart on the ground.
Cheers