Yoshimoto had been presented a hypothesis concerning the fatigue strength of a bolt in bolt/nut assembly using Ishibashi's hypothesis on the relationship between the fatigue notch factor and the local stress distribution on the first thread root of a bolt mated with nut. By using Yoshimoto's hypothesis, the effect of the manufacturing process (sequence or order of thread rolling and heat treatment processes) on the fatigue strength can be explained in connection with the axial residual stress at the first thread root of a bolt as a change of mean stress level. However, this hypothesis has not yet been verified quantitatively since the axial residual stress which may exist locally around the thread root could neither be measured nor estimated with sufficient accuracy. This study aims to quantify the axial residual stress on the thread root generated by thread rolling process. For this purpose, a method for simulating thread rolling process by using 3D elastic-plastic FEM was proposed to estimate the axial residual stress distribution by using precisely determined material property. Calculated results for two types of test specimens, one is grooved specimen with larger root radius and the other is leadless bolt specimen having the same thread profile as M10×1.25 bolts, show that the axial compressive residual stress of 1000 MPa level is generated at the root of the specimens. Finally, the validity of the simulation was confirmed through the X-ray stress measurement for grooved specimen, and the fatigue tests for leadless bolt specimen.

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.

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Bolt/double nuts assemblies are used for preventing the self loosening when the clamp force is relatively low. In general, the fatigue load acting on the bolt increases in the bolted joint with low clamp force due to the separation at the contact plane. However, the effect of the locking force on the fatigue strength of the bolt has not yet been clarified. This study aims to evaluate the fatigue strength of a bolt in bolt/double nuts assemblies quantitatively. Fatigue tests were performed for bolt/double nuts assemblies by taking the locking force and the nut height as parameters. FE stress analysis was also performed to predict the fatigue strength using Ishibashi's and Yoshimoto's hypotheses concerning the relationship between the local stress and the fatigue strength of the material. The results show that the locking force has the effect to decrease the local mean stress of the fatigue loading on the first thread root of the bolt mating with the lower nut, and it leads to the increase of the fatigue strength of the bolt expressed by the nominal stress.

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