How to learn about metal-fatigue - do
Familiar theme - you have to "play" with the phenomenon to usefully know it enough to act upon it effectively.
From my "Career" section...
The effect of "playing" with fatigue-testing: leading for Hyundai's construction division on fatigue-resistant design for the 3rd Bosphorus Bridge project in Turkey, 2015 (1.4km span - a massive bridge).
This is an extensive writing on various topics. If you are interested in fatigue you might jump to the subsection "The story of fatigue-resistant weld design".