Also from Wikipedia: Fracture is governed by the hoop stress in the absence of other external loads since it is the largest principal stress. Note that since the hoop stress is largest when r is smallest (nope, when r is largest), cracks in pipes should theoretically start from inside the pipe (nope, the outside should fail first).

Rocketman: This was a direct quote from Wikipedia including the last sentence. It was not a conclusion I drew from it. My comment was it seemed contradictory.

So many guns, so little time!