Which of the following methods can improve the fatigue strength of a circular mild steel (MS) shaft?

A machine component made of a ductile material is subjected to a variable loading with $${\sigma _{\min }} = - 50MPa$$ and $${\sigma _{\max }} = 50MPa.$$ If the corrected endurance limit and the yield strength for the material are $$\sigma {'_e}$$ $$100MPa$$ and $${\sigma _y} = 300MPa,$$ respectively, the factor of safety is __________.

For the given fluctuating fatigue load, the values of stress amplitude and stress ratio are respectively.

Which one of the following is the most conservative fatigue failure criterion?