Ultra-low Cycle Fatigue Fracture Performance Of Steel And Joints Under Different Stress States

Under the action of strong earthquakes,ultra-low cycle fatigue fracture failure often occurs in steel structure joints,that is,with very few cycles,the steel at the joints undergoes large plastic deformation,accompanied by crack initiation and propagation,finally the fracture failure phenomenon.The mechanical response and fracture performance of steel under ultra-low cyclic load are the basis for studying the seismic performance of steel structures.Therefore,it is necessary to study the fracture mechanism and prediction model of structural steel under ultra-low cyclic load in order to accurately evaluate the fracture resistance of steel structures under earthquake.Previous studies mainly focus on tensile fracture failure mode,but ignore the influence of shear effect in fracture failure.A fracture model considering the influence of Lode parameters is proposed based on the cyclic void growth model(CVGM),which is namely as LCVGM in this paper,and the model(LCVGM)is applied to predict and analyze the ultra-low cycle fatigue fracture of steel pipe welded joints.Taking Q235 B steel,Q345 B steel and ER50 weld metal as objects,a series of round bar specimens and plate specimens were designed and prepared to vary the stress state,the monotonic tensile tests and ultra-low cyclic tests of the specimens were carried out.The microscopic observation of the specimen section under different loading systems was carried out,the crack initiation position and fracture failure process of different types of specimens were studied,and the effects of loading systems on the bearing capacity and deformation capacity of the specimens were compared.Based on the test results and finite element analysis of different types of steel specimens,the parameters of hardening model and LCVGM of Q235 B steel,Q345 B steel and ER50 weld metal were calibrated.By using the user subroutine USDFLD of ABAQUS finite element software,LCVGM was used as fracture criteria,to analyze the fracture failure of various specimens under monotonic loading and ultra-low cycle fatigue.The development law of void growth index of various specimens under monotonic and cyclic loading was analyzed.The crack initiation position and fatigue life are obtained,which were in good agreement with the test results.Finally,a series of monotonic tensile and ultra-low cycle fatigue tests of X-shaped steel tube intersecting joints were carried out.The test results showed that the main pipe punching fracture occurred in all steel tube intersecting joints.The cracks all started at the saddle point of the main pipe,and gradually propagate along the thickness direction of the main pipe.With ABAQUS using LCVGM and finite element software,the evolution law of crack initiation of tubular intersecting joints under monotonic load and ultra-low cycle fatigue load was analyzed,and the ultra-low cycle fatigue fracture of tubular intersecting joints was numerically simulated.The prediction results were in good agreement with the experimental results.Besides,compared with the result of numerical simulation based on CVGM,the results showed that LCVGM takes into account the influence of shear effect through Lode parameter for the fracture failure with shear failure as the main pipe,so its prediction accuracy was more accurate than CVGM.