Ned Simmons wrote:

You mean they split longitudinally like a sausage? I'm just trying to
figure out the stresses in my mind. Probably the corners/ridges are
important too.


Sounds like a textbook torsional fatigue failure.
http://school.mech.uwa.edu.au/~dwrig...e/fatigue.html
http://school.mech.uwa.edu.au/~dwrig...urfaceBIG.jpeg

I was thinking a torsional failure should have a 45 degree helical
fracture surface, but I guess the ridges change the course of the crack.

Chris