The mean stress effect:

For a component subjected to cyclic stresses, the mean stress is the average stress of the maximum and minimum stress throughout the cycle. If the maximum and minimum stress are equal but with opposite direction, then mean stress will be zero.

The mean stress effect is reducing the fatigue capacity due to the mean stress. There are several methods available to evaluate the effects of mean stress, including Goodman, ASME-elliptical, Gerber, Morrow and Soderberg. As the graph below shows, these methods are defining different relationships between the stress amplitude and mean stress.

σa = represents the stress amplitude, the peak value of stress variation.

σm = stands for mean stress, the average of maximum and minimum stresses in a cycle.

σe = endurance limit strength

σy = yield strength

σu = ultimate tensile strength

σf = true fracture strength

σFS = fatigue strength at a specific number of cycles