Prise en compte de l'influence de la distribution des contraintes et des effets d'echelle pour une utilisation pratique en fatigue

The transferability of fatigue data from specimens to components or from one component to an other is often very difficult due to two major phenomenons : the volumetric distribution of stresses (stress gradients) and the so called "size effect". In the literature examples show the influence of these factors on the fatigue strength by the example of round specimens with different diameters and notch factors. This is useful for demonstrating tendencies, but in most cases components don't have nor a circularly shape neither a notch factor can be defined so that the data transfer is impossible. Furthermore, for the same size and geometry of a specimen or a component it is well known in. fatigue that the load type has an influence on the endurance limit (differences between traction, rotative bending and plane bending for instance). To design against fatigue it is necessary to take into account the effects of the previous factors but in many cases they are coupled. The fatigue strength of t he material is usually known from experimental data obtained with specimens and the component have a asize and notches different from these specimens. For this purpose the method of the volume influencing crack initiation V* and the highly stressed volume V90 per cent are useful tools for considering together the geometrical and statistical size effect in practical use. These two complementary methods are based on volumetric approaches. The first method is able to distinguish load types and the effect of the stress distribution. With the second one, a. relation is proposed to consider both the stress gradient and the size effect. This contribution demonstrates the handling of the methods and their implementation.