707 Finite Element Analysis of Bolted Joints using Helical Thread Model

Abstract

Conventional analytical and numerical methods to study the response of a bolted joint is relied on many assumptions and approximations and thus hardly yield satisfied results. 3D finite element model of bolted joints with real helical thread geometry is established and meshed with refined hexahedral elements. This finite element model is used to analyse specific characteristics of stress distribution, stress concentrations, contact pressure distributions, load sharing of the bolted joint components. 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 bolt thread helix, between flanges, on bolt head and at the nut loaded surface have been studied. The peak stress occurs at the first engaged bolt thread root from nut loaded surface and gradually decreases towards the free face 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 most of the load is shared by flanges when the external applied axial load is up to 30% of preload, and beyond this, bolt starts sharing more 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 stiffness, bolt load and thread stress predictions. These critical observations provide insight for optimization of bolted flange joint to meet the structural requirements and weight optimisation.