Comparison between defects and micro-notches in multiaxial fatigue – The size effect and the gradient effect

This paper attempts to improve the understanding of the multiaxial high cycle fatigue response of micro sized stress concentrations or notches of different geometries. The investigation is composed of an experimental part and a numerical part. In the former, three types of micro-notches or “artificial defects” are compared: spherical, elliptical and circumferential. All types have the same basic dimensions, the difference being the 3D geometry. The notches were machined on the surface of smooth cylindrical specimens made of mild steel. The fatigue limits under reversed tension (push–pull) and reversed torsional loading conditions for different micro-notch sizes have been experimentally determined. In the numerical part, finite elements simulations using a cyclic elasto-plastic material behaviour law show that the mechanical state ahead of the different stress concentrations change drastically with the loading mode and the geometry of the artificial defect. From a fatigue point of view, it is shown that a stress gradient correction is required for all the loading, size and geometry configurations. Once the gradient correction is made and a proper multiaxial criterion is used, it appears that the size effect due to increasing the loaded surface area at the notch tip for the different geometries is negligible compared to the gradient effect.