Effect of bending moment on the fatigue strength of a bolt in bolt/nut assembly

This study describes an approach to quantify the effect of the bending moment on the fatigue strength of a bolt in bolt/nut assembly. To confirm the validity of the conventional fatigue design methodology based on the nominal stress acting on the specified nominal stress area, both the fatigue tests with various bending stress ratios (nominal bending stress/ nominal total axial stress) and the 3D-FE stress analysis for the corresponding conditions were conducted. The results from fatigue tests using a newly developed fatigue testing fixture clearly show as bending stress ratio increases, the (virtual) fatigue strength expressed by the nominal stress also increases. The results from 3D-FE analysis show that the magnitude of the local bending stress on the thread root is lower than the one estimated from nominal axial stress and the stress concentration factor for purely tensile loading. The results, however, also show that the magnitude of the local stress acting on the bolt thread root is affected by the angular position of nut against the loading axis due to the existence of the incomplete thread of nut at the bearing face side. Finally, it is concluded that the conventional fatigue design methodology is practically acceptable for tensile force combined with bending moment loading, albeit with results that are slightly conservative.