[Flieger]

Well I know my professors use the example of the wings flexing on taxi and in the air when talking about fatigue. When those are made of Aluminum the life span must be calculated. This requires lots of data to know what stresses and at what frequency the metal is subjected to, then the lifespan is predicted based on high cycle fatigue SN graphs which reflect a curve fitting of experimental data, with modification factors for the type of loading and such that the actual part experiences.

Engineering is partially about acknowledging that nothing is perfect and is about knowing when enough is enough. You can't make money if you are replacing parts all the time for tiny cracks that are not hurting anything.

Even aircraft quality material is known to not have defects above a certain size. They use high powered methods to find tiny defects, but atomic sized cracks and slip planes which are cracks in waiting cannot be detected without even more expensive means such as scanning electron microscopes.

Aluminum and most other non-ferrous engineering metals do not have a fatigue endurance limit and even if steel is used, to design a part for infinite life adds cost of fuel to haul that weight around, so the designers want the lightest aircraft that meets the specs.

Cracks are expected to form and grow. The size of crack that necessitates replacement of a part may be quite small in a structural component and larger on others, but cracks are still expected to form after a certain number of cycles.

Granted, the published life span is more like 20 years for an airframe, with extra safety margin since fatigue is not based so much on first principles and is subject to statistical errors and such. So these failures on the Airbus wing are not normal, but cracks in general are.

Not even Boeings, good as they are, are immune from metal fatigue, unless they are made out of composite like the 787. At which point there are other things to worry about like delamination.