Fatigue is caused by cyclical stresses. If the force is a constant torsion due to torsion in one direction, fatigue should not be an issue.
For a better understanding of failure modes, you could check out:
If there are any side loads on the shaft (such a bearing misalignment, etc.) then it is quite easy to get totally reversing bending loads and Adel's question is completely valid. It doesn't take much under certain conditions and I don't know of any perfect situations in real life.
Hi best friends
The only external load that i think :
1- A constant torsion force
2- permissible unbalance = G2.5 that cause F=m x R x omega power 2
(suppose no bearing misalignment is there)