Analysis of the Tightening Process and the Cyclic Stress Amplitude of Studs and Tap Bolts

When subjected to alternating external forces, a fatigue failure of bolted joint is most likely to occur around the first bolt thread for the case of bolt-nut connections. It has been substantiated in the previous paper by three-dimensional finite element analysis, in which the numerical models of bolted joints are constructed so as to accurately take account of the effect of thread helical geometry. In the cases of bolted joints clamped by studs and tap bolts, however, fatigue failures sometimes occur at other than the first bolt thread and frequently initiate around the far end female threads. In this paper, using the FE models with correct helical thread geometry, stress amplitude distributions along the thread root including the thread run-out are evaluated for both male and female threads. It is shown that the maximum stress amplitude in the male threads occurs at the thread root slight away from the first thread and the maximum value in the female threads is generated near the far end of the engaged threads. Also shown is the contact pressure distribution pattern at the interface between the plate and the block, which is inherent to the bolted joints clamped by studs and tap bolts and may be the major source of the specific stress amplitude distributions.