Here is the stuff from Marks on combined bending and torsion.
I would use this as an introduction and investigate it further
as it seems to be very complicated. 
Sorry about the file
size as lots of fine print. Since it is so complicated I tend
to err on the side of the over size. If I had a PSRU with
less than aircraft engine sized shafts I would magna flux 
them frequently.

Paul Lamar
 
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