where s is the standard deviation for fatigue lives in logarithmic form.Through the regression analysis of the fatigue test data taking thestress range as the parameter, the fatigue S-N curves of M30 highstrengthbolt are shown in Figs. 7 and 8, where the logarithmic equationfor the corresponding part of the oblique line of the S-N curve in Fig. 8can be developed as Eq. (4).