A straight line joining '$S_e$' on the stress amplitude axis and '$S_{yt}$' on the mean stress axis is called soderberg line.

According to the soderberg criteria, the triangular region below this line is considered to be safe. Hence, any point laying in this triangular region will ensure the safety of the component for infinite line. The equation for the soderberg line is,

$\frac{S_m}{S_{yt}}$ + $\frac{S_a}{S_e}$ = 1 ......(1)

OR

$\frac{\sigma_m}{S_{ut}}$ + $\frac{\sigma_o}{S_{e}}$ = $\frac{1}{N_F}$

where, $S_{yt}$ = yield strength of the material,

The equation (1) and (2) represent the soderberg line.

Soderberg diagram gives the most conservative/safe design.

Soderberg diagram is used for ductile materials.