I copied this
A stator is nothing more than a laminated iron core with copper wire wound around it. That being said there is very little that can fail on them. Typically what happens is the coating on the wire burns off causing a short to ground, when this happens the stator can no longer produce current.
Often what causes this is heat or overworking the stator. Trying to charge a battery that is bad will cause the stator to work much much harder producing amperage that the battery will take, the end result is once the amperage gets high enough the coating melts off the coils causing a short..
Corrosion takes a part in this as well, as the stator has to work harder to push current into the battery again working hard.