INVESTIGATION OF THE AXIAL STIFFNESS OF A BOLT USING A THREE-DIMENSIONAL FINITE ELEMENT MODEL

The axial stiffness of a bolt plays a critical role in the prediction of the self-loosening process of threaded connections subject to oscillatory excitation. Since self-loosening is one of the most frequent failure modes for threaded connections, it is important to understand the mechanisms of this behavior. In this paper a three-dimensional finite element model of a single bolt threaded into a plate is used to determine values for the axial stiffness of the bolt. The model is loaded by pulling the nodes around the outside of the bolt head in the axial direction while holding the bottom of the plate fixed. Several analyses are performed to investigate the axial stiffness of the bolt. Quasistatic analysis predicts a linear relationship between the reaction force and the axial stretch of the bolt, indicating that the axial stiffness is constant. Dynamic analyses yield similar results if the prescribed accelerations are limited, though the predicted stiffness values are lower than those from the quasistatic analyses. Different dynamic loading patterns also yield slightly different stiffness values. Investigations from this research can be used to better predict the self-loosening process.