Numerical investigation of fatigue strength of grain size gradient materials under heterogeneous stress states in a notched specimen

•Competition between grain size effects and stress gradient effects could occur.•Local stress based fatigue criteria seem inappropriate to reflect grain size effects.•Aggregate calculations are a good tool to review critical distance approaches.

A possible consequence of forging in common steels is the apparition of a grain size gradient in the width of the component. For railway axles in service, this microstructure gradient is superimposed to the stress gradient introduced by the external load of rotatory bending imposed on the axle. To investigate the combined effects of these gradients on the fatigue lifetime of a forged railway axle, a numerical investigation of the effects of microstructure gradients inserted in a notched specimen is proposed, with respect to different fatigue indicators. In particular, the predictions of an approach based on the theory of critical distances seem to be very promising in this case.