Influence of Microstructure and Micro Notches on the Fatigue Limit

To achieve a high fatigue limit the most common approach is to increase the static strength and hardness of a material. This is realized mostly by decreasing microstructural dimensions as the grain size or the spacing between phases in more phase materials. According to the Hall-Petch-relationship the static strength is increased by that way. Simultaneously, the fatigue limit, which is defined by the non-propagation of cracks, increases with decreasing microstructural dimensions. The reason is that microstructurally small cracks are stopped at microstructural barriers so that smaller distances between neighboring barriers reduce the length of non-propagating cracks and increase the intrinsic fatigue limit. But this increase of the fatigue limit is confined by the dimensions of the flaws within the material, such as non-metallic inclusions or geometrical notches, which reduce the fatigue limit of high strength materials significantly.The trade-off between small microstructural dimensions and the dimensions of the flaws will be discussed on the basis of experimental results in three examples. Firstly, commercial pure titanium shows a distinct change of the fatigue limit not before the notches are larger than the grain sizes, although the stress intensity at the notches is significant [1]. Secondly, micro notches in high strength materials always result in a huge decrease of the fatigue limit [2]. Thirdly, as bulk metallic glasses do not possess microstructural barriers like grain boundaries notches are a crucial factor in this class of materials [3].On the basis of these examples and the modified Kitagawa-Takahashi diagram [4] a new approach to explain the microstructure and notch influence on the fatigue limit will be presented. This could be the basis for future developments to increase the fatigue limit of metals.