[Roadster]

When I had a similar problem, my fan blades were hitting the alternator housing in different spots., they were not forcing the blades out. And it was making quite a racket. I tried bending the blades out myself to eliminate the noise. Little success for a very short time. And that was with a factory pulley. That's when I figured out that the replacement alternator did not have the factory spacer, it didn't have any spacer at all. My lack of the spacer did not cause the belt to fray in anyway. Not saying this could not be the cause, just from my experience on my engine.
When you use a replacement alternator, just make sure you use your original pulley and spacer as mentioned, (that's if they are still on the vehicle, which in your case they may not be) and you won't have any problems.
Just thinking out loud, IMO, your alternator fan blades would have to be so out of round and distorted to cause a blade to break and the belt to fray. I would tend to agree with Dave, that it may be the belt tensioner that could be causing this type of problem, to where higher or even normal rpm operation would be causing the belt to be interfering with something that would cause it to fray.
It would be interesting to see what your alternator fan blade looks like, post some pics if you can....
And was or is your alternator charging properly during this time???