An empirical approach to a comprehensive damage-equivalent stress function for fatigue

This paper develops an empirical damage-equivalent stress function for fatigue. Classical analysis methods are used to 'fit' an equation to a number of S-N curves for various grades of carbon steel. The resulting equivalent-damage stress function is applicable to steels subjected to a wide range of heat treatments, from normalised up to hardened and tempered to 1900MPa. It is also applicable to a wide range of stress concentrations, unnotched up to Kt = 5.0 and typical of screw threads. A range of stress ratios and mean stresses are also considered. The function overcomes some of the limitations of existing methods of 'correcting' for mean stress. Existing methods are limited in that, while they may give good results over a range of conditions, there are some circumstances where the results are highly inaccurate. The damage-equivalent stress function is suitable for use in automated calculation procedures such as spreadsheets, MathCAD ©, and SMathStudio ©.