Study On Toughness And Fatigue Life Of Q690CFD Butt Welded Joints Of High Strength Steel Plate

As a new building material,high strength steel is widely used in high-rise buildings,industrial plants,bridges and other projects.As one of the main connection processes of steel structure,welding inevitably has defects in practical engineering.The fracture and crack propagation in the failure process are closely related to the initial defects of welded structures,and the residual stress caused by welding has an important effect on the toughness and fatigue life of structures.Therefore,this paper takes the butt welded joints of Q690CFD high strength steel plate as the research object,systematically studies the factors affecting the fracture toughness,impact toughness and fatigue crack propagation characteristics,conducts quantitative analysis on many factors,and finally proposes the fatigue life assessment process of high strength steel with defects.The main contents are as follows.1.Through the tensile test of Q690CFD high strength steel welded joints,it is found that the ultimate ultimate tensile strength of joints can be increased by up to 12%with high strain rate.The fracture toughness and impact toughness of the welded joints are evaluated by plane strain fracture toughness test and Charpy impact test,and the fracture toughness and impact work of the welded joints under different delivery states,welding heat input and yield strength are obtained.The fracture toughness increases by20Mpa.m^1/2 and the impact energy decreases by 72J with the yield strength increases from 550MPa to 890MPa.Compared with TMCP,the fracture toughness of QT state is close to that of TMCP state,but the impact energy is increased by 85J.The fracture toughness of welded joints increases by 16%,while the impact toughness decreases with the heat input increases from 5k J/cm to 26k J/cm.The impact energy decreases by66J with the temperature drops from 20℃to-40℃.According to the fitted Boltzman curve,the brittle-toughness transition temperature of Q690CFD high strength steel is-0.7℃.2.The blind hole method is used to test the residual stress of Q690CFD high strength steel welded by CO₂ gas shielded welding and the distribution law of residual stress is obtained.3.The stress intensity factor K_app of Q690CFD high strength steel base metal CT specimen model under external load is solved by analytic formula method,contour integral method and extended finite element method（XFEM）,and the stress intensity factor K_res of welded joint CT specimen model under welding residual stress is solved by weight function method and finite element method.4.Based on Abaqus analysis platform,numerical simulation of fatigue crack growth of Q690CFD high strength steel welded joints is carried out.Firstly,the influence of stress ratio R and load period T is considered through parameter analysis.The results show that the Paris constant C and n of Q690CFD high strength steel are1.103×10^-5 and 0.991 respectively.When R ranges from 0 to 0.5,the maximum stress ratio increases the crack growth rate by 100%compared with the minimum stress ratio.The influence of the cyclic period of load on the crack growth rate is within 5%,which can be ignored.Secondly,the virtual crack closure technique VCCT is used to calculate the stress intensity factor K_tol in the composite stress field considering the redistribution of residual stress.Finally,the Walker formula is modified to obtain the fatigue crack growth rate formula.5.Based on the above research,the fatigue life evaluation process of high strength steel welded joints under compound stress field considering residual stress field and external load stress field is summarized.