Effect of Incomplete Threads of Nut on the Local Stress on the Thread Root of a Bolt

There have been many efforts to estimate the fatigue strength of a bolt in bolt/nut assembly from the material strength and the local stress acting on the thread root to be failed by fatigue. Recent development of computer technology enables to obtain the local stress distribution on the thread root by using FE analysis on PC. However, FE analysis is normally performed using 2D axi-symmetric model due to the constraints such as numbers of elements and processing time to analyze complex contact problems. In 2D axi-symmetric model, the thread lead (angle) does not exist, and the effect of the incomplete thread at the bearing surface of the nut cannot be taken into account. In this study, a 3D FE model is proposed which contains only two leads including the first thread lead, and is cut out from 3D bolt/nut assembly model. The possibility is confirmed through the discussions of the minimum numbers of elements necessary and how to make a 3D model.