Finite Element Analysis of the Fatigue Strength of Threaded Fasteners Using Helical Thread Models

A fatigue failure of bolted joint frequently leads to serious accidents of machines and structures. It is well known that the fatigue failure is likely to occur around the first thread root of bolt adjacent to nut loaded surface and the run-out of bolt thread. That is because high stress amplitudes are generated there due to alternating external forces. Accordingly, it is significantly important to evaluate the stress amplitudes along the thread root in order to elucidate the fatigue failure mechanism of bolted joints. In this study, stress amplitude distributions along the thread helix including the thread run-out are analyzed by three-dimensional finite element analysis, where the numerical models of bolted joints are constructed so as to accurately take account of the effect of thread helical geometry, using the modeling scheme proposed in the previous paper. The analytical objectives are bolted joints with axi-symmetric geometry except for the helical-shaped threaded portions, and they are subjected to axi-symmetric external forces. It has been substantiated, based on the stress amplitude distributions along the thread helix, that the fatigue failures are likely to originate from the first bolt thread, as in the case of the maximum stress, and the run-out of threads. It has also been shown that a bolt with reduced shank is effective to lower the stress amplitude, by comparing the numerical results to those for normal bolts.