抄録
In order to predict the fatigue strength of a bolt, two hypotheses had been presented Ishibashi's hypothesis is concerning the relationship between the local stress at the thread root of a bolt and the nominal stress, and Yoshimoto's one is concerning the effect of the axial residual stress at the first thread root of a bolt. However, these have not yet been verified since the local stress and the residual stress which may exist locally around the thread root cannot be measured or estimated with sufficient accuracy. This study aims to clarify the relationship between the residual stress and the fatigue strength quantitatively. In this report, simple 3-D FE analysis was done to calculate the axial residual stress distribution by using the interim FE model through the thread rolling process of the leadless bolt specimen, and the fatigue tests were performed for the leadless bolt specimens thread rolled before heat treatment (SV) and thread rolled after heat treatment (SG). Calculated result shows that the axial compressive residual stress about 440 MPa level is generated at the thread root at ε_0 = 30 μm layer of SG leadless bolt specimen. On the other hand, the difference between the fatigue strengths of SG leadless bolt specimen and SV specimen in the fatigue tests can be explained by the Ishibashi's and Yoshimoto's hypotheses if the axial compressive residual stress of 1200 MPa exists at the thread root of the SG specimen. Considering the residual stress distributions for grooved rolled specimen of the same material simulated and measured by X-ray diffraction method, the elements are too coarse around the root of the leadless bolt specimen, and can be improved by re-modeling.
