| Steel bridges have excellent mechanical properties and are easy to industrialize.With the increase in the steel production and labor cost in China,steel bridges have been rapidly developed in transportation engineering in recent years.China is an earthquake-prone country,and earthquakes seriously threaten the structural safety of steel bridges.However,theoretical research on seismic design of steel bridges is still relatively lacking.This paper takes the steel bridge pier as the research object,which is the bearing component of steel bridges and is also vulnerable in earthquakes.The main goal is to establish ultra-low cycle fatigue(ULCF)fracture prediction methods for steel piers.The research is carried out by means of experiments,theoretical analysis and numerical simulation.The main work and research results of the study are as follows:(1)The large strain loading tests were conducted on smooth round bar and notched specimens of Q345q C steel,the strain strengthening effect and cyclic peak stress response were analyzed,and ductile fracture and ultra-low cycle fatigue fracture memechanism at different stress triaxialities axiality were studied.The results show that under cyclic large strain constant amplitude loading,Q345q C steel initially undergoes significant strain strengthening,and then cyclic peak stress quickly reaches a stable state.The Ramberg-Osgood model can accurately simulate the skeleton curve of Q345q C steel under cyclic large strain loading,which provides a basis for hysteretic response analysis of steel structures.The ULCF fracture of steel is ductile cracking.The size and depth of dimples are related to stress triaxiality.(2)Based on test results of notched specimens in Chapter 2,the applicability of traditional Coffin-Manson formula for predicting ULCF life of steel under multiaxial stress conditions is verified.By establishing the quantitative relationship between parameters and stress triaxiality,a modified Coffin-Manson formula considering the effect of stress triaxiality is proposed.The results show that in the case of high stress triaxiality,the traditional Coffin-Manson formula greatly overestimates ULCF life of steel,and under moderate stress triaxiality loading cases,the traditional Coffin-Manson formula can relatively accurately predict ULCF life of steel under multiaxial stress.The modified Coffin-Manson formula considering the effect of stress triaxiality can accurately predict ULCF life of steel under different stress triaxialities.Therefore,considering the effect of stress triaxiality on ULCF fracture can significantly improve the prediction accuracy of Coffin-Manson formula,especially in the case of high stress triaxiality.(3)On the basis of cyclic void growth model(CVGM)and degradation significant plastic strain model(DSPS),improved mechanical damage models are proposed considering the dependence of damage degradation parameters on stress triaxiality,named by ICVGM and IDSPS.The research results show that values of model parameters?and?do not depend on the magnitude of stress triaxiality,and when calibrating?,replacing the instantaneous stress triaxiality with average stress triaxiality can reduce the dispersion coefficient of calibration values.The damage degradation parameters?CVGM and?DSPS of Q345q C steel show a non-monotonic relationship with stress triaxiality.As the stress triaxiality increases,?CVGM and?DSPS both decrease firstly and then increase.When stress triaxiality is very large,?CVGM and?DSPS tend to be the constant value.Compared with the original CVGM and DSPS models,the prediction accuracy of ICVGM and IDSPS models are increased by about 50%and 35%,respectively.Therefore,considering the dependence of damage degradation parameters on stress triaxiality can improve the prediction accuracy of the original CVGM and DSPS models,especially the CVGM model.(4)Based on the concept that ULCF damage is composed of ductile damage and cyclic damage,combining with the micromechanism that void grow under tension and void shrink under compression,a new single-parameter micromechanical damage index(MDI)is proposed.Then,based on test results of notched specimens,the validity of MDI was verified.The research results show that MDI can accurately assess ULCF damage of steel under different stress triaxialities.Compared with that of modified Coffin-Manson formula,ICVGM and IDSPS model,the parameter calibration of MDI only needs tensile test results of round bar specimens,and the parameter calibration process is very convenient,which can promote the application of MDI.(5)A pseudo-static test of thick-walled square-section steel bridge piers under horizontal bidirectional loading was carried out.Then,ULCF fracture mechanism of steel bridge piers was analyzed by scanning electron microscope.Based on the finite element sub-modeling technology,finite element models of steel piers are established.The ULCF fracture initiation life of steel piers is predicted by modified Coffin-Manson formula,ICVGM,IDSPS and MDI.The research results show that ULCF fracture is a failure mode of steel bridge piers under horizontal bidirectional earthquake actions,which is caused by crack initiation at the corners of the bottom of piers.The microscopic morphology of the crack initiation area of steel piers is a typical ductile dimple,which means ductile cracking.The ULCF crack propagation is a mixed mode of ductility and quasi-cleavage fracture,which is different from traditional low-cycle fatigue crack propagation.In the later stage of crack propagation,quasi-cleavage fracture dominates the propagation process,which leads to the failure of steel bridge piers due to a sudden drop in strength capacity.The modified Coffin-Manson formula,ICVGM,IDSPS and MDI can accurately predict ULCF fracture initiation life of steel piers,with an average error of only 7.14%.During ULCF fracture process,steel piers did not undergo local buckling.Therefore,it is necessary to check ULCF performance of steel piers in seismic design,especially for thick-walled members. |
PDF Full Text Request