| There are 85 percent of aircraft structure fatigue damage occurred at mechanical connections, such as the riveted and bolted joints. The interference fit is an effective technique to improve the fatigue life of mechanical connection structure.In aircraft assembly, the theoretical guidance is lacked for scientific and appropriate interference installation of hi-lock bolt. In this dissertation, by means of theoretical analysis, finite element (FE) simulation and experimental verification, a series of questions about hi-lock bolt interference connection are studied systematically, and the effects of process parameters on protuberance, residual stress, and mechanics performance of bolted joint are analyzed.In chapter 1, the application of interference reinforcement technology is introduced firstly. Then, the present research states of interference installation and mechanical performance of bolted joint are analyzed in detail, and the present research deficiency of interference bolted connection is indicated. Finally, the main contents and overall structure of this dissertation are listed.In chapter 2, considering the bolt as an elastic body, the theoretical analysis of interference fit connection is carried out. Using the thick wall cylinder theory of elastic-plastic mechanics, the formulas on contact stress and maximum elastic interference fit size are deduced in the plate’s elastic range, and the relationship between plastic zone size and interference fit size is established in the scope of plate’s elastic-plastic. Then the FE model of single plate is developed. Through the example, the theoretical solutions and FE results are compared, and the relative errors are acceptable.In chapter 3, a two-dimensional (2D) axis-symmetric FE model is established, and static installation processes of hi-lock bolt are simulated under a series of interference fit sizes. At the same time, the corresponding test is carried out. By comparing insertion force and protuberance, the FE model is verified. Then the rule of residual stress fields and hole deformation (including hole bending and protuberance) changed with the increasing interference fit size are analyzed. The essential of the phenomenon of the concavity appeared around the hole and the protuberance crackle is revealed by the stress analysis. Fanally, for the installation of M8 hi-lock bolt in Al-alloy lap plates, the application principal that the limit of interference fit should be controlled with in 2% is provided by comprehensive analysis of the stress distribution and protuberance size at the hole exit.In chapter 4, the effects of process parameters of interference installation on protuberance and insertion force are studied respectively. Using the slip line field theory, generation mechanism of protuberance is analyzed qualitatively, and the generated process of protuberances is presented by FE simulation. Then the effects of the parameters such as interference fit size, friction coefficient, inner hole diameter of supporting fixture, hole exit chamfer and preloading force on the protuberance are studied, and the optimized parameter values for decreasing protuberance are obtained. Moreover, the effects of the factors such as interference fit size, plate thickness, bolt import part and friction condition on the insertion force are analyzed.In chapter 5, the influence of interference fit size on residual stress is analyzed systematically. When the stresses distribute uniformly and without concentration, the interference fit size is optimum. By FE results, the residual (Mises) stress variations of the hole and the bottom surface of lap plates are analyzed respectively, and the optimized value of interference fit size is obtained. Moreover, the hoop (tangential) stress variation of the hole and faying surface between upper and lower plate are studied, and the rule of hoop stress variation with the increase of interference fit size is indicated.In chapter 6, in order to intuitively present the effect of interference installations on the mechanical properties of bolted joint, the three-dimensional (3D) FE model is developed, and the processes of the interference installation and tensile loading are continuously simulated. The single lap joint specimens are designed, and the tests including hi-lock bolt interference insertion, nut tightening, bolted joint bearing tensile load are carried out. By comparison of the insertion force and the strain, the 3D FE model is validated. Considering interference fit size as the key parameter, the stress variation of the hole and upper plate faying surface is analyzed when the tensile load reaches the peak. In addition, the effects of other parameters such as clamping force, friction coefficient, geometrical size of lap zone and external loads on local stress are studied. Finally, the single lap specimens undergoes tensile damage test, and also corresponding FE analysis is carried out.In chapter 7, summary of the whole work in this dissertation is given, and the future work is put forward. |
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