Estimation of the residual stress on the thread root generated by thread rolling process

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