| Aluminum alloy has the advantages of easy processing,corrosion resistance,light weight,green environmental protection and non-magnetic.It can not only be used in the maintenance of the structure,but also directly bear the load as the main stress component of the structure.According to the data,80%of the failures of building structures under cyclic loading are caused by fatigue failure.At present,there are few researches on the fatigue performance and fatigue life assessment of beam column joints of aluminum frame structures under cyclic loading at home and abroad,so its development is slow.Therefore,with the aluminum alloy structure widely used in the field of civil engineering,it is very important to study the fatigue performance and predict the fatigue life of aluminum alloy beam column joints.In this paper,the fatigue crack initiation analysis,fatigue crack propagation analysis and fatigue life assessment of 6061-T6 aluminum alloy beam column joints under cyclic loading are carried out by means of experimental research,finite element analysis and theoretical calculation.The main research contents are as follows:(1)The tensile specimens of 6061-T6 aluminum alloy were designed for material properties test,and the mechanical parameters and stress-strain curves of the material were obtained statistically.The constitutive relationship of the material was obtained according to Ramberg-Osgood model;The fatigue specimens of 6061-T6 aluminum alloy were designed for constant amplitude low cycle fatigue test.The results show that the fatigue life decreases with the increase of stress level.According to the test data,the S-N(stress-fatigue life)curve expression was obtained,and the fatigue limit value of 6061-T6 aluminum alloy was 122 MPa.(2)Combined with the data of material property test,the finite element software was used to model the aluminum alloy beam column joint,and the fatigue crack initiation stage was studied.Combined with the stress nephogram,the stress variation law of the joint under cyclic load is clarified;The nominal stress method,the hot spot stress method,the notch stress method and the local strain method are used to evaluate the fatigue performance of the joints at three fatigue risk locations,and the fatigue crack initiation life is predicted,the results show that the fatigue crack initiation life of HG-3 is the shortest,which indicates that the welding heel is the high-risk area of the joint.Finally,the prediction method of fatigue crack initiation life of aluminum alloy beam column joints is given.It is suggested that the defect stress method be used to analyze the fatigue crack initiation stage of aluminum alloy beam column joints.(3)The fatigue crack growth stage of welded joints of aluminum alloy beam and column is studied by XFEM(extended finite element method).An edge crack model of aluminum alloy plate under uniaxial tension was established to verify the effectiveness and accuracy of XFEM simulation of fatigue crack growth;By introducing an initial crack at the root of the joint,it is found that the fracture parameters of the beam column joint are distributed in a "U" shape along the flange width direction,When the initial crack is 1 mm,the fatigue crack growth life of the joint is 1.4×105,and the crack length is 4.1 mm.XFEM can better reflect the direction and rate of fatigue crack growth.(4)In order to obtain the S-N curve which can be used to predict the overall fatigue life of aluminum alloy beam column joints,the safety factor k=1.15 is used to modify the S-N curve obtained from the fatigue test,and the error of the modified S-N curve combined with the equivalent structural stress method is less than 5%.Therefore,the formula can provide a reference for the study of fatigue performance of aluminum alloy beam column joints. |
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