| Under the earthquake action,the seismic performance and seismic damage degree for beam-to-column joints of steel frame are analyzed from the macro level of structural components,which plays an important role in guiding the fatigue design of steel frame beam-to-column joints.However,due to the lack of attention to the elastic-plastic response of the local vulnerable parts of the welded joints,it is difficult to grasp the fatigue damage mechanism and failure mode of the local vulnerable parts.This dissertation,based on the local concept,mainly focuses on the fatigue damage evaluation of welded beam-to-column connection when the steel frame subjected to the strong earthquake excitation.Firstly,the low-cycle fatigue tests for two types of welded structural details are carried out,and the low cycle fatigue performance,fatigue strength and fatigue damage mechanism of welded structural details of beam-to-column joints are discussed.Secondly,according to characteristics of seismic disaster of welded beam-to-column connection,the low cyclic loading tests for box-column connection are performed on large displacement amplitude,the evolution process and the damage mechanism of crack initiation,propagation and coalescence of weld seam are analyzed,and the quantitative criterion for crack initiation of box-column connection is proposed.Thirdly,based on the fatigue performance parameters of the welded structural details and the strain response of fatigue critical point of the welded connection,the fatigue damage of the welded connection is analyzed combined with the multiaxial fatigue analysis method.Finally,the finite element software ABAQUS is adopted to establish the solid model of beam-to-column connection of steel frame,and the numerical analysis are compared with the experimental results.The main research contents are as follows.(1)According to the published fatigue recommendation,and combined with the local structural features of welded joint of steel frame structure,two types of welded structural details are designed to conduct low cycle fatigue test.The test results are presented,and the fatigue failure mode,cyclic performance,fatigue performance and fatigue strength of welded structural details using Q235 structural steel are investigated.The fatigue S-N curves of the similar welded structural details in IIW fatigue recommendation are adopted to correlate the fatigue resistance of the two types of welded structural details,and failure mechanism of welded structure under low-cycle loading conditions is studied.(2)The cyclic loading tests are performed on box-column to I-beam connection of Q235 structural steel under different cycle displacement range greater than yield displacement to investigate the fatigue failure behavior,such as the destruction patterns of crack initiation and propagation in weld meatal area,crack type,crack macroscopic coalescence mode,as well as the strain distribution law of beam flange weld.The paper also summarized the damage mode of welded connection crack,standard failure mode and mixed failure mode.(3)As for cyclic loading test,test parameters(load drop,variation rate of load drop and loading system)on the effects of the crack initiation and propagation for the welded connection are analyzed,the damage and deterioration mechanism under the cyclic loading controlled by large displacement amplitude is also discussed,and then the quantitative criterion for crack initiation of box-column connection is proposed.(4)Based on multiaxial fatigue critical analysis method and the strain history of fatigue critical point from the cyclic loading test,the fatigue damage critical plane is determined through weight average processing,then normal and shear strain response on the critical plane are calculated to formulate the multiaxial fatigue damage parameter using the von Mises criterion.The fatigue damage of the joint is assessed by using the multiaxial fatigue life prediction model,combined with Miner linear cumulative damage criterion calculation.Subsequently,the results are compared with the results calculated by the modified FS model,the fatigue crack initiation life calculation method and the fatigue damage assessment model for box-column welded joint of steel frame structure are proposed.(5)For the multiaxial fatigue life prediction FS model revised by the non proportional impact factor,the paper puts forward the method of equivalent convex path to simplify the complex strain path when calculating the non proportional additional hardening effects of strain path,and the moment of inertia is used to describe the contribution of non proportional additional hardening effect for an arbitrary point in area formed by arbitrary strain path,so the non-proportionality of strain path calculated in this way has definite physical meaning.(6)The finite element software ABAQUS is adopted to establish the multi-scale model of steel frame structure and the solid model of beam-to-column connection,the mechanical character of fatigue critical point of steel frame structure are analyzed,and the fatigue damage calculation results of finite element model are compared with the experimental results. |
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