Conventional analytical and numerical methods for the mechanical properties of helical threads are relied on many assumptions and approximations and thus hardly yield satisfactory results. In this paper, an effective mesh generation scheme is used which can provide accurate helical thread model to analyse specific characteristics of stress concentrations and contact pressure distributions caused by the helical thread geometry. Sector model of bolted flange joint has been analysed for pretension alone and combination of pretension and axial load. Using the finite element (FE) model with accurate thread geometry with pretension, the thread root stresses, contact pressure along the helix and at the nut loaded surface in the circumferential direction have been studied. The peak stress occurs at the first engaged bolt thread root from nut loaded surface. This stress at the thread root gradually decreases towards the free face of the nut. The contact pressure at nut bearing surface varies in the circumferential direction because of the circumferential variation of the stiffness of engaged threads adjacent to the nut loaded surface. The axial load along the engaged threads gradually decreases from nut loaded surface to zero towards the free surface of the nut.
Results from analysis with pretension and axial load indicate that the contact separation starts at the inner radius of flange and grows towards outer diameter of flange as the axial load is increased in the bolted flange joint. It is observed from the analyses that the load is shared by flanges when the external applied axial load is up to 15% of preload, and beyond this, bolt starts sharing external load. The maximum stress occurs at the first engaged bolt thread root. Most of the bolt failures are at the first engaged thread. The study suggests that it is necessary to consider threads in FE model to obtain accurate contact pressure, thread stress, stiffness and bolt load predictions. These critical observations provide insight for optimization of bolted flange joint to meet the structural requirements and weight optimisation.
