Loosening mechanism of threaded fastener for complex structures

Threaded fasteners are widely used in mechanical structures primarily owing to their easy disassembly for maintenance and low cost. However, the loosening mechanism of threaded fasteners due to dynamic loading has remained unclear for the past six decades. Current researches on complex structures comprising three or more components are insufficient. The two most common failure modes of threaded fasteners subjected to dynamic loading are fatigue and vibration-induced loosening. This study focuses on the failure of threaded fasteners by vibration-induced loosening due to dynamic shear loads. This study comprises experimental analysis and numerical analysis. The loosening mechanism of threaded fasteners for complex structures is analytically and experimentally identified. This work provides the equations and assessment method for the loosening, and the criteria of primary and secondary loosening are established. To verify the proposed loosening mechanism, tightening and loosening experiments are conducted for three types of bolted joints. The primary and secondary loosening forces of each bolt are thus obtained, and the proposed loosening mechanism can be verified for complex structures. In numerical analysis, a three-dimensional finite element (FE) model for tightening and loosening analysis is proposed. A FE model is used to study the loosening process which is characterized by a decline of the preload and moving distance for predicting loosening states. The model seems to be well agreement in comparison with theoretical and experimental results. As a result, the assessment method shows good performance in predicting loosening state. It is expected to verify the safety of bolted structures at the design stage. The FE model is expected to be used for the effective and safe design for joint components in various industrial fields such as wheel assemblies and other mechanical components under dynamic vibration.