Whether we are using Soderberg or Goodman line; it is two dimensional Cartesian coordinate system in which x-axis is the mean stress and y-axis is stress amplitude. For them to calculate we have two values one is maximum stress and other is minimum; I found on Quora.com that they are not principle stresses by nature. Even in books that I read in graduate level, they not mentioned about their origin. One more thing is that; to calculate endurance limit every specimen goes through the same testing method i.e. rotating beam machine that means maximum and minimum are always maximum tensile bending stress and maximum compressive stress respectively. Mean will be always zero as specimen is made always symmetric. Pls help what is maximum and minimum stress in the fatigue analysis.