Fatigue Design of a Bolt in Bolt/Nut Assembly

(Estimation of ε₀ in Ishibashi's hypothesis)

Abstract

The final goal of this study is to estimate the fatigue strength of a bolt in bolt/nut assemblies from the fatigue strength of the material and the local stress conditions on the thread root to be failed. Ishibashi had presented a hypothesis concerning the relationship between the fatigue strength of the material and the local stress distribution, where the stress related to the fatigue strength is the one acting on the depth ε₀ which can be determined from the grain size of the material. This study aims to estimate ε₀ for fastener materials. Axisymmetric notched specimens with different root radius are manufactured by rolling and grinding. The ε₀ values determined reversely from the results of the fatigue test and the FE analysis are affected by manufacturing process. However, more accurate tests are necessary to confirm this conclusion and to quantify the thread rolling effect.