electrical damage to bearings
I read somewhere that there are frequent cases of minor lightning strikes to turbo-prop aircraft which result in current flow through the main reduction gearbox (presumably as the prop blades are exposed pointy thingys that attract strikes?) Because the contact face of a ball or roller bearing is so miniscule, the resulting current densities are very high & cause micro-pitting to the bearing faces. This pitting leads to premature failure of the bearing, many hours after the strike that caused it. I assume it cannot be directly detected, without dismantling & inspecting every bearing - clearly impractical.
I guess pure turbines do not attract strikes through their bearings as the engine is fully contained in the pod?
The same thing can occur with large electric motors running with variable speed drives - stray circulating currents of low magnitude find their way through the bearings and cause premature failure.