Modelling of fatigue thresholds for small cracks in a mild steel by ‘‘Strip-Yield” model

The fatigue of mechanical components can be explained by the growth of very small fatigue cracks, which lead to the final fracture of the component. It is, therefore, essential to try to describe the growth and the thresholds of small cracks. In this paper, the Strip-Yield model is used to analyse the fatigue thresholds of a mild-steel widely used for manufacturing railway axles. Furthermore, the need for a precise definition of the constraint factor, under cyclic, non-linear response of the material, is addressed, together with the choice of an appropriate constraint formulation. This results in good fatigue threshold predictions at R = 1 for long cracks (obtained from SE(B) specimens), whereas the description of the Kitagawa diagram (obtained by fatigue tests on specimens with artificial defects) is relatively poor. The analysis of the cyclic stress ahead of small cracks enables us to compare the results with the threshold models for small cracks by McEvily, Usami and Shida.