Effect of the Residual Stress on the Fatigue Strength of a Bolt in Bolt/Nut Assemblies

(Verification of the hypotheses using an axi-symmetric bolt model)

Abstract

The residual stress at the thread root of a bolt is one of the major factors influencing the fatigue strength. Some hypotheses have been proposed to predict the fatigue strength from the local stress on the thread root. However, these have not yet been verified quantitatively since the residual stress induced by thread rolling and locally acting on the thread root can neither be measured nor estimated with sufficient accuracy. This study aims to clarify the relationship between the fatigue strength and the local stress around the thread root taking into account the effect of residual stress. For this purpose, a method is proposed where the residual stress, which can be estimated by FE analysis, is induced on the thread root of an axi-symmetrical bolt in bolt/adapter assembly model by preloading. The results from fatigue test show that the preloaded specimen has the higher fatigue strength than non-preloaded one. This shows that the compressive residual stress could improve the fatigue strength as anticipated from Yoshimoto's hypothesis. On the other hand, the fatigue strength of a bolt calculated by FE analysis based on the local stress distribution using Ishibashi's hypothesis was smaller than that of notch-free specimen. However, experimental investigation showed that the bending stress are acting on the bolt thread root due to the construction of threaded adapter or the fatigue test device. Finally, it can be concluded that the fatigue strength of the axi-symmetrical bolt model can be predicted by using the hypotheses taking into account the bending effect.