I am trying to work out fatigue life of a part using Miner's rule, however I am having some trouble working out which stress values to use. I have seen some examples but there seems no rhyme or reason to the values they have used in the accumulative Miner's rule, where fatigue failure occurs when :
Where $n_1, n_2...n_k$ represents the number of cycles at specific overstress levels and $N_1, N_2...N_k$ represent the life (in cycles) at these overstress levels.
The example below just used the maximum positive values, with no regard for negative values, even if they were nearly the same as that maximum positive value. Surely this negative stress would have some impact on fatigue, more so than a zero based loading?
So if I have a stress vs time plot (shown below) that varies from positive to negative quite randomly, where do I get these overstress levels from? Are they the alternating stresses, mean stresses, or max/min stresses that are over the endurance limit of the material?