Study On Static And Ultra Low Cycle Fatigue Properties Of High Strength Steel And Welded Joints

Compared with ordinary strength steel,high strength steel(yield strength ?460Mpa)has obvious advantages of high strength,light weight and economy,but at the same time,the yield strength ratio of high strength steel increases,the deformation capacity decreases,and its fracture problem is more prominent.Under the action of strong earthquake,the ultra low cycle fatigue fracture of steel structure joints often occurs,that is,the steel at the joints experiences large plastic deformation and fracture under the condition of few cycles.The ultra low cycle fatigue fracture analysis of high strength steel and welded joints can provide an important basis for evaluating the fracture failure of high strength welded steel structures under strong earthquake.In this paper,the domestic Q690 D high strength structural steel and ER80 G solid welding wire weld metal were taken as the research objects,21 round bar and plate specimens reflect different stress states were designed,and the static and ultra-low cycle fatigue tests of the specimens were carried out.In order to analyze the local welding structure of beam column full welding and end plate connection,the specimens of cruciform welded joint,T-shaped welded joint and hourglass T-shaped welded joint were designed,and the fracture failure tests were carried out under monotonic loading and ultra-low cycle loading.Based on the theory of micro fracture mechanics and ABAQUS finite element software,the fracture of the sample was simulated and analyzed.The results are as follows:(1)The experimental results show that the strength of Q690 D high strength steel is higher than that of ordinary steel,but the deformation ability and ultra low cycle fatigue life significantly decreased.Based on the SEM test,the fracture surfaces of Q690 D steel and weld metal round bar specimens show obvious equiaxed dimples,while the fracture surfaces of pure shear specimens show obvious shear dimples.(2)Based on the experimental results of Q690 D steel and weld metal round bar and plate shaped specimens,the model parameters are calibrated,then the ultra low cycle fatigue life of pure shear specimen predicted by means of finite element analysis,and simulation resultes show good prediction accuracy compared with test result.(3)The test results show that the cross joints section present shear failure morphology,the T-shaped joints and hourglass T-shaped joints section present tension-shear failure morphology.The influence of weld form,web width and bolt number on the fatigue life of T-shaped welded joints are not obvious,while the influence of fillet weld and penetration weld on the bearing capacity of the specimens are small.Therefore,considering the construction economy and construction difficulty,the fillet weld form is recommended for the welded joint of Q690 D high strength steel in the project.In addition,the cumulative energy consumption of hourglass T-shaped welded joint is much greater than that of T-shaped welded joint,and the viscous damping coefficient is also larger than that of T-shaped welded joint,which can better dissipate energy under earthquake action and is more beneficial to the seismic performance of the structure.(4)Finally,the LCVGM model transplanted into the finite element software with the ABAQUS user material subroutine.Based on the LCVGM model of Q690 D steel and weld metal,the finite element simulation method is used to predict the fracture of the three kinds of welded joints.The simulation results,crack initiation location,fracture location and fracture morphology,are consistent with the experimental results,and the predicted ultimate displacement and fatigue life of the welded joint are also agree well with the experimental results,which verifies the applicability of LCVGM model for ultra-low cycle fatigue fracture prediction of high strength welded joint.In addition,by comparing the prediction results of LCVGM model and CVGM model,it can be seen that CVGM model without considering the influence of Lode parameter will overestimate the deformation capacity of welded joint,which makes the prediction results more dangerous.