Threaded joints have sharp notches which generate a localized multiaxial stress field with high stress gradients. Fatigue strength of mechanical structures is a nonlocal phenomenon, and spatial distribution of stresses influence fatigue behaviour, not only the local value of stress. Therefore, the stress gradient approach is used for the assessment of fatigue behaviour due to high stress concentration and stress gradients. In order to predict the multiaxial highcycle fatigue, the stress based critical plane approach combined with Rainflow cycle counting and damage accumulation to estimate a multiaxial fatigue failure criterion are used. The stress gradients influence on the M10 bolt, strength class 10.9, highcycle fatigue are estimated with IABG, FemFat, Stieler and FKM-Guideline methods. Methods are compared mutually and with the results obtained without the influence of stress gradients. The fatigue endurance limit of a threaded joint at a very high survival probability is estimated with influence factors of size, surface roughness, stress gradients, statistics, mean stresses and mean stress rearrangements in case of local plastification with Neuber-hyperbola, and the Haigh diagram is modified with the stress gradient influence. A finite element model is created with nonlinear contact interactions between engaged threads and other parts. During the lifetime, the first load is due to the bolt tightening which is defined as 50, 70 and 90 % of the bolt material yield strength. Moreover, tensile eccentric forces with constant amplitudes are applied with a load ratio of 0.1, 0.5 and 0.9.
Keywords: threaded joint fatigue, stress gradients. multiaxial fatigue