Numerical Analysis of Loosening Phenomena of Bolted Joints under Shear Loads Using Helical Thread Models

Loosening of threaded fasteners is one of the most fundamental causes leading to serious accidents of various machines, structures and electronic devices. There are two types of loosening mechanism, i.e., loosening with/without rotation of threaded fasteners. Loosening with rotation frequently causes such issues as bolt force reduction, falling of threaded fasteners and fatigue failure of bolted joints. In this paper, loosening mechanism with rotation is analyzed in detail by FEM using helical thread models with accurate geometry. The target joint is subjected to shear loads perpendicular to the bolt axis. It shows quantitatively how the bolt force decreases due to alternating shear loads. The numerical results have also clarified that the loosening of bolt-nut connections progresses when a slip occurs in the order of the plate interface, the thread surface and the nut bearing surface, and the contact conditions at each surface behave in an irreversible manner.