For many applications the number of stress cycles experienced by a component during its service
life is between 10 ^{3} and 10 ^{6} cycles. For these applications it is economical to use
appropriate fatigue strength as opposed to using the endurance limit. The strength value between the low cycle stress value
S'_{l} and the endurance limit value S'_{e} can be plotted on
a graph of log_{10} Fatigue strength (S) vs log_{10} Number of cycle (N'_{t}) as a straight line
following the equation.

b = the slope of the line
C = the intercept of the line with the vertical cycles ordinate
N^{'}_{t} = Number of cycles
S'_{f} = the fatigue strength value
S'_{e} = the endurance limit at 10^{6} cycles
S'_{l} = the fatigue strength at 10^{3} cycles

At the endpoints of the high cycles finite life rang (10^{3})cycles and (10^{6})cycles
...
The endurance strength values are

subtracting the equation for S'_{e} from S '_{l} results is the following

To obtain C it is simply necessary to substitute the above into the equation for S'_{e}

Therefore knowing the low level fatigue strength (S'_{l}) for a metal
and the endurance strength (S'_{e}) and the number of operating cycles it is
possible to obtain the relevant fatigue value using the equation.

If the fatigue strength is known and the number of cycles to failure is needed then the following formula applies

Example: to determine the strength at 10^{5} cycles when the strength at 10^{3} and 10^{6} are known.

Useful Links..

MATWEB ... Vast source of material data including some fatigue values