Concept:
Modified Goodman line:
The shaded region represents Modified Goodman's Criteria

The equation for Modified Goodman line:
It depends upon the condition below,
\(\frac{{\sigma _a^}}{{\sigma _m^}} = \frac{{{\sigma _e}({\sigma _{ut}} - {\sigma _{yt}})}}{{{\sigma _{ut}}({\sigma _{yt}} - {\sigma _e})}}\)
If, \(\frac{{{\sigma _a}}}{{{\sigma _m}}} > \frac{{\sigma _a^}}{{\sigma _m^}} \Rightarrow Line;AP;will;be;chosen;\)
If, \(\frac{{{\sigma _a}}}{{{\sigma _m}}} < \frac{{\sigma _a^}}{{\sigma _m^}} \Rightarrow Line;PB;will;be;chosen\)
Where,
σa = limiting safe stress amplitude
Se = endurance limit of the component
σm = limiting safe mean stress
Sut = ultimate tensile strength
Syt = Yield strength
FOS = Factor of safety
Calculation:
Given:
σmax = 150 N/mm2, σmin = 50 N/mm2, Se = 200 N/mm2, Sut = 400 N/mm2 and Syt = 300 N/mm2
σm=2σmax;+;σmin⇒2150;+;50=100;N/mm2
\({{\rm{\sigma }}{\rm{a}}} = \frac{{{{\rm{\sigma }}{{\rm{max}}}} - {{\rm{\sigma }}_{{\rm{min}}}}{\rm{;}}}}{2} = \frac{{150 - 50}}{2} = 50{\rm{;N}}/{\rm{m}}{{\rm{m}}^2}\)
Now, check the condition,
\(\frac{{\sigma _a^}}{{\sigma _m^}} = \frac{{{\sigma _e}({\sigma _{ut}} - {\sigma _{yt}})}}{{{\sigma _{ut}}({\sigma _{yt}} - {\sigma _e})}} = \frac{{200\left( {400 - 300} \right)}}{{400\left( {300 - 200} \right)}} = 0.5\)
σmσa=10050=0.5
\( ⇒ \frac{{{\sigma _a}}}{{{\sigma _m}}} = \frac{{\sigma _a^}}{{\sigma _m^}} = 0.5\)
⇒ So, any of the among AP and PB can be taken as the boundary for modified Goodman's Criteria
The equation for modified Goodman line for line PB,
σytσa+σytσm=N1
30050+300100=N1
∴ FOS = N = 2