Nonlinear Finite Element Analyses on the Bolt Bending Deformation

Abstract

Joining and assembly processes in mechanical design are performed by using permanent or non-permanent methods. Among the non-permanent techniques, threaded fasteners are commonly used mechanical components providing disassembly of the connections. The connections consisted of bolt and nut, which are externally and internally threaded members, are widely used in many engineering products because of the replaceable feature. Although the bolted joints are particularly designed to carry axial forces, they can be subjected to not only concentric loads but also eccentric loadings. Some of the eccentric loadings may lead to prying action which raises the axial force carried by the bolt due to the contact occurring between the connected members. In addition to the bending moment developed due to the eccentric loading, the prying action affects the bolt by increasing excessively the axial force carried by the bolt. The strength of the bolt in the joint decreases with regards to the level of the eccentricity under the prying action with a combination of the two facts which are bending moment and excessive tensile force. This additional tensile load is considered in the studies investigating the bolted joints under the prying action, however, the bending moment that occurred due to the eccentricity of the joint is not always an interested phenomenon. Outcomes about the bolt bending moment occurred under prying action in the literature are usually limited by only comments, and there are still unaddressed points about the bolt bending deformation such as the location of the maximum bending moment and effects of the preloading on the bending-tension interaction of the bolt. In this thesis, the bending deformation mechanism of a bolted joint under prying action was studied comprehensively by performing static structural finite element analysis considering the material, geometric and contact nonlinearities. After introducing a brief background about the bolted joints under the eccentric loadings and prying action, a comprehensive literature survey was presented. The studies in the literature about the three-dimensional finite element modelling techniques of the bolted joints, bolt bending, and prying action were explained in detail. The experimental and numerical study programs were planned in accordance with the objectives. Experimental studies were performed prior to finite element analyses to provide required inputs such as material properties and displacement boundary conditions. Before presenting the analysis results, the construction of the finite element analysis models was represented in detail. The results obtained from the finite element analyses were presented in four different sections. The force-displacement behaviour, strain-force variations throughout the loading, and results of the mechanical parameters for all configurations and modelling techniques were compared with the experimental results. After comparing the experimental and numerical results, the modelling techniques were analysed in terms of the bending deformation capability, and this feature was evaluated by using the bolt bending moment results throughout the clamped region and bending-tension interaction curves. The bolted joint configurations were also analysed by using three different pre-tightening force levels. In conclusion, the structural response of a bolted joint involving a partially threaded fastener under prying action was comprehensively evaluated by considering the bending deformation capacity. For the flexural features of the studied bolted joint type under prying action, finite element analysis modelling issues and design considerations were presented.