| With the development of aerospace technology and the using evolution of bolt connection in aircraft structure,high-lock bolts,as a type of bolt with large life gain to aircraft structure,are widely used in various aerospace equipment.In addition,the single shear lap joint is the main form of bolt connection,and it bears alternating loads during the service of the aircraft,leading to the fatigue failure.At present,aircraft accidents with fatigue fracture as the main cause of structural fracture are common.The problem of bolt fatigue has become a difficult problem in the aerospace field.Therefore,the study on the factors affecting the fatigue life of high-lock bolt lap joints is an important task in the field of aviation manufacturing.The existing research on the factors influencing the fatigue life of high-lock bolt joint is mainly focused on the double-shear lap joints installed by a single bolt.In the actual situations,the bolts are mostly used in groups,and their force modes are very complicated.In addition,with the rapid development of computer application technology and finite element numerical calculation theory,the fatigue strength design based on a large number of test data and finite element numerical calculation method oriented to life prediction have emerged,which can be applied to the fatigue life prediction of joints.The four-bolt lap joint structure is the research object of this thesis.A combining method of computer simulation and test is adopted to study the factors influencing structural fatigue life,including load grades,fit types(i.e.,clearance fit and interference fit),preload and bolt structure(i.e.,with or without sleeve),etc.And then influence mechanism of different factors on the structural fatigue life is investigated.Moreover,the main factors influencing the structural fatigue life in the process of installation and manufacturing is studied,including hole offset,hole-axis inclination,mounting direction,preloading sequence,etc.The fatigue life is predicted under the test conditions and the phenomena of the testing results are explained.The thesis mainly includes the following research contents:(1)The fatigue test of single-shear lap joints of high-lock bolt is designed and carried out.Then the influences of fit type and load grade on the fatigue life of the joint are studied for the preparation of the subsequent finite element analysis and fatigue analysis.The test results show that the interference fit under high load has no significant effect on life gain compared with the clearance fit.In addition,the fatigue fracture is briefly analyzed.(2)The three-dimensional finite element model of high-lock bolt lap joint is established using ABAQUS.Then,the continuous process of bolt interference installation,bolt pre-tightening and cyclic loading is simulated.By extracting the stress history,the reason why the load grade and fit type in the test affect the fatigue life of the joints is explained,and the validity of the finite element model is verified.In addition,the established finite element model is used to study the influence of factors such as interference amount,preload and structure with or without sleeve on fatigue life.The results show that interference fit under low load has a gain effect on the fatigue life of the single splicing joint,and the optimal interference amount under low load should be less than 2%.Moreover,high preload and sleeve structure also has a gain effect on fatigue life.(3)The fatigue life of high-lock bolt lap joint is predicted in FE-SAFE based on different multi-axis fatigue life calculation models.The results show that the prediction results of fatigue life using Brown-Miller model are consistent with the test results.The feasibility of using this method to predict the life of single splicing joint of high lock bolt is verified.(4)The influences of common problems on the fatigue life of high lock bolt when using high-lock bolts in groups,such as connection plate processing and bolt installation,are studied.The results show that the machining errors such as hole offset and hole-axis inclination reduce the fatigue life of the joint.In addition,bolt mounting direction and preloading sequence have little influence on joint fatigue life. |
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