The Z axis was recently behaving strangely. I noticed the belts were flopping around so I retensioned them. In a couple days, they were flopping again. I took them off the machine to see what was going on and instead of finding that the steel cores had broken, I found this:
The polyurethane belt had slid/stretched over the steel core (of course it did!) inside the belt clamp.
I have redesigned the belt clamps so the ends of the belt will fold over and the teeth will interlock. That should prevent this sort of problem from reoccurring.
I have used this folded self-locking type of clamp for years in the XY stage and in SoM, my other printer.