Mean stress and plasticity effect prediction on notch fatigue and crack growth threshold, combining the theory of critical distances and multiaxial fatigue criteria

In the present work, we propose a robust calibration of some bi-parametric multiaxial fatigue criteria applied in conjunction with the theory of critical distances (TCD). This is based on least-square fitting fatigue data generated using plain and sharp-notched specimens tested at two different load ratios and allows for the estimation of the critical distance according to the point and line method formulation of TCD. It is shown that this combination permits to incorporate the mean stress effect into the fatigue strength calculation, which is not accounted for in the classical formulation of TCD based on the range of the maximum principal stress. It is also shown that for those materials exhibiting a low fatigue-strength-to-yield-stress ratio sigma(fl,R = -1)/sigma(YS), such as 7075-T6 (sigma(fl,R = -1)/sigma(YS) = 0.30), satisfactorily accurate predictions are obtained assuming a linear-elastic stress distribution, even at the tip of sharp notches and cracks. Conversely, for any materials characterized by higher values of this ratio, as quenched and tempered 42CrMo4 (sigma(fl,R = -1)/sigma(YS) = 0.54), it is recommended to consider the stabilized elastic-plastic stress/strain distribution, also for plain and blunt-notched samples and even in the high cycle fatigue regime still with the application of the TCD.