| The life of an aircraft depends mainly on the fatigue-life of the connection parts of the fuselage structure,and as a technology to strengthen fasteners,bolt interference can effectively enhance the fatigue strength of fasteners to avoid the occurrence of fatigue damage and improve the service life of the aircraft.However,the research on the installation of interference countersunk bolts is relatively scattered,the actual installation process lacks systematic theory and experimental guidance.In this paper,the theoretical analysis and finite element method are used to analyze the installation process of the countersunk bolt,the influence of different process parameters on the bump and residual stress during the installation is obtained,and the optimum value of process parameters are given in the end.At first,the research status of fatigue of bolted joints at home and abroad is summarized,and the research and application of interference strengthening technology,especially interference fit rivet and interference fit bolt connection are introduced.The shortcomings of existing research on bolt interference are put forward.Based on this,the main research contents are given.After the installation of the interference,the distribution and properties of the residual ring-shaped stress on the laminated plate have a great influence on the formation of the fatigue crack.In this paper,the bolt is regarded as elastic body and the theory of thick-walled cylinder is used to deduce the relationship between the ring-shaped stress and the radius of the plate and the installation interference in the range of elastic deformation and elastic-plastic deformation.At last,the finite element model of the bolt interference installation is established and by comparing the finite element and the theoretical formula,the theoretical formula is verified.According to the established two-dimensional axisymmetric finite element model,the static indentation process of the countersunk bolts under different interference value is simulated.The change law of distribution of the stress field after the installation and the deformation of the hole wall of the laminated plate and the formation of the protrusions with the interference are analyzed.The residual stress distribution has great influence on the fatigue strength of the structure,based on the established finite element model,the effects of the residual stresses under different interference values of the hole wall and the bottom surface of the laminated plates after installation are studied.Then the change rule of the residual stress distribution along the hole wall of the laminated plate and the interface between the upper and lower plates is studied.Finally,based on the above research,the optimal range of interference value for the installation of countersunk bolts in practical aircraft assembly engineering is given.The bumps generated during the interference installation process of the countersunk bolts tend to become stress concentration sources and cause fatigue crack after the structure is subjected to the alternating load resulting in the reduce the fatigue life of the structure.The influence of different process parameters such as interference,countersunk angle and depth of pit on the size of the bump was studied by finite element method.The orthogonal experiment was used to analyze the process parameters which had the greatest influence on the size of the bump,and an optimal combination of multi-factor process parameters is given to guide the actual installation. |
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