Spectacular structural collapses sometimes happen due to inadequate strength of the material used to make the structure. But machinery components, subject to stresses from rotational motion and/or vibrations, usually break due to fatigue.
What does this mean exactly? Metal and plastic parts do not “get tired” in the same way humans do. For humans and other animals, being in a state of fatigue implies that we are as good as new after resting. But metal components subject to fatigue do not get their strength restored. They are permanently impaired unless they are somehow repaired.
Fatigue loading is repeated, cyclic loading. The loading may be uniform, random or some other pattern. A truck going down a smooth highway may experience fairly uniform loading on the axles once it is loaded with whatever it is carrying. It may carry a heavy load for one trip and a lighter load for another trip. A vehicle on a dirt road with many potholes will experience a non-uniform load, even if the exact same weight and distribution of cargo is hauled every time.
What is unique about fatigue and what confused early railroad engineers trying to understand why locomotive axles were breaking is that a fatigue crack initiates and grows at stress levels below the nominal yield strength. Why would a crack initiate and propagate at such low stresses?
Check back next time to find out.