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

In a structure subjected to fatigue loading, the minimum and maximum stresses developed in a cycle are $$200$$$$MPa$$ and $$400MPa$$ respectively. The value of stress amplitude (in $$MPa$$) is ____________.

A rotating steel shaft is supported at the ends. It is subjected to a point load at the center. The maximum bending stress developed is $$100 MPa.$$ If the yield, ultimate and corrected endurance strength of the shaft material are $$300$$ $$MPa,$$ $$500$$ $$MPa$$ and $$200$$ $$MPa,$$ respectively, then the factor of safety for the shaft is __________.

In terms of theoretical stress concentration factor $$\left( {{k_t}} \right)$$ and fatigue stress concentration factor $$\left( {{k_f}} \right),$$ the notch sensitivity $$'q'$$ is expressed as