3-D FEM stress analysis of screw threads in bolted joints under static tensile loadings

The stress concentration factor (SCF) for the roots of screw threads in bolted joints under static loadings is analyzed using 3-D elastic FEM taking account the spiral of screw threads. At first, the stress states at the roots of screw threads in initial clamping state in a bolted joint where two hollow cylinders were clamped with a bolt and a nut were analyzed in initial clamping. The elastic FEM result of SCF for the first root was obtained as SCF=3.2. When the bolt was clamped in initial clamping (preload) at the 60 % of bolt yield stress, the plastic deformations were found at the first and the second roots, and non-engaged screw threads. It was found that as the external tensile loads increased, the development in plastic deformation region increased from the first root to the other roots as well as the non-engaged screw threads. It was found that the rupture occurred from the non-engaged screw threaded part while the plastic deformation increased at each root of screw threads. The numerical result was coincided with the experimental result. In the experiments, it was observed that the rupture occurred from the non-engaged screw thread and not from the first root of screw thread. Also, the bolt fatigue was predicted from FEM and it was shown that a fatigue fracture occurred from the first root.