Mean stress effect on Ga{\ss}ner curves interpreted as shifted W\"ohler curves

Abstract

A criterion for the mean stress effect correction in the shift factor approach for variable amplitude life prediction is presented for both smooth and notched specimens. The criterion is applied to the simple idea proposed by the authors in a previous note that Ga{\ss}ner curves can be interpreted as shifted W\"ohler curves. The mean stress correction used has been proposed by Smith, Watson and Topper and, more in general, by Walker. By applying the correction, a new expression for the shift factor G is obtained and, through the application of the theory of the critical distances in its point variant, surprisingly G is demonstrated to be valid for both smooth and notched geometries since it does not seem to depend on the geometry, but only on the fatigue exponent and the loading history. Finally, a comparison with the SAE Keyhole test program data is added to substantiate the findings.