Multiaxial Low-Cycle Fatigue Life Prediction And Seismic Damage Evaluation Of Beam-to-Column Connection Of Steel Frame

This dissertation is mainly foucs on two issues: multiaxial low-cycle fatigue life prediction of metallic materials and seismic damage evaluation of beam-to-column connection of steel frame.For the multiaxial fatigue predictions,a new critical plane parameter is proposed from the experimental observation of different types of metallic materials fatigue test.To investigate the multiaxial fatigue strengthen of structural steels,the low-cycle fatigue experiments are conducted on Q235 B mild-carbon structural steel and its related welded metal by using thin-walled tubular specimen under multiaxial loading.The evaluation methods of fatigue properties for metallic materials are aslo studied.For the seismic damage evaluation of beam-to-column connection under the strong earthquake excitation,the extremely low-cycle fatigue failure mechanism is firstly investigated by using the fatigue data of welded structural detals,then the extremely low-cycle fatigue damage is considered as the combined effect of the ductile damage and the fatigue damage.Accordingly,the seismic damage evaluation of beam-to-column connection uncer the strong earthquake excitation is study based on a steel frame example.The main research contents are as follows.A new non-proportional influence factor considering the dual influence of the material additional cycle hardening and the rotation of principal stress/strain axes caused by non-proportional loading path on multiaxial fatigue is proposed,which can be adopted for a modification to FS critical plane approach.Fatigue data of various metallic materials from thin-walled tubular specimens under axial-torsional straining using sinusoidal wave forms in the exciting literature are collected for the model verification.Results shows that comparing with FS parameter,the proposed critical plane damage parameter can significantly improve the accuracy of multiaxial fatigue lifetime prediction under non-proportional loading condition.The multiaxial mechanical behavior and low-cycle fatigue strength of Q235 B mild-carbon structural steel base-metal and welded-metal is contrastive study based on the fatigue tests under axial,torsional,in-phase and 90°out-of-phase loading conditions.Results show that the ductility and fatigue resistance of welded-metal specimens are significantly lower than that of base-metal.Both the base-metal and welded-metal exhibit additional cycle hardening and lower fatigue lives under 90°out-of-phase loading comparing with uniaxial and in-phase loading.Also,a majority of observed fatigue lives are found to be in good correlation with predicted ones based on the proposal critial plane parameter under 90°out-of-phase loading for base-metal and welded-metal specimens in the present study.The evaluation methods of fatigue properties and the non-proportional hardening effect for metallic materials are studied based on the statistical method.A segmentation fitting method for fatigue properties evaluation is proposed based on the Brinell Hardness.Fatigue data are collected for the experimental verification,and results shows that the proposal method can greatly improve the accuracy of multiaxial fatigue lifetime prediction.Considering the experimental measurement of the fatigue properties is relatively difficult for various metallic materials,this part of studies can provide a simplified fatigue analysis procedure for the preliminary fatigue design of steel structure,and can guarantee a certain prediction accuracy.The extremely low-cycle damage is considered as the combined effect of ductile damage and the fatigue damage based on the study of Kuroda and Tateishi.The Continuum Damage Mechanics method is adopted for the ductile damage analysis,and the critical plane method is used for the fatigue damage analysis.A linear superposition method is employed to consider the combined effect of the two type damages.The user defined material constitutive subroutine(UMAT)considering the ductile damage evolution is adopted for the ductile damage analysis based on the ABAQUS computing platform.Thus,multiscale analysis method of seismic ductility damage of beam-to-column connections of steel frame is studied.Cumulative fatigue damage of the random seismic response is calculated by means of the Miner linear cumulative criterion.An 8-story box-column to H-beam steel frame is employed as an enginnering example for the seismic damage analysis under strong eqrthquake excitation.Multiscale FE simulation results of the bottom corner beam-to-column connection shows that the failure of the welded connection mostly occurs at the welded root of the bottom flange weld.During the strong earthquake,the fatigue crack initiates after few strain cycles.Subsequently,macroscopic fatigue crack propagation becames a dominate factor for the safty of the beam-to-column connections,which is coincide to the failure observation of welded connection under strong earthquake actions.