Depends on how you define it. Certainly the SRB failure with
Challenger had a catastrophic result, regardless of
how "catastrophic" a failure it was.

Yes the SRB caused the 51-L accident, but if the same failure occurred
with a CEV atop it, hypothetically, it would be a survivable abort
situation. I don't recall reading about any "explosive" type of
failures of the STS SRB in testing.

Not really. More like waivered away.
Again, no one really knows what "human rating" means. I wish that
we could purge the phrase from our vocabulary.

FWIW:

"Human-Rating Requirements, JSC - 28354"
http://www.hq.nasa.gov/office/codea/...documentd.html

Jon