Fatigue Analysis: Maximum & Minimum Stress

[Ravi Singh choudhary]

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.

 

[Illmatic1]

Is it principal stress?No, the maximum and minimum stresses used in Soderberg and Goodman's fatigue analysis are not principal stresses. These stresses are simply the maximum tensile bending stress and the maximum compressive stress, respectively, encountered during a standard rotating beam machine test. The mean stress used in the analysis is always zero, as the specimen is always symmetrically loaded.