Study On Weld Seam Damage Cumulative Evolution Law Of Welded Joints In Steel Tubular Truss Structure

The steel tube truss with direct welding joints has been widely used in public buildings because of its simple and quick joints,no joint plate and other auxiliary components,which can meet the requirements of the construction design.However,because of the complexity of the weld seam at the intersecting line in the welding process,the existence of micro holes and micro cracks is inevitable,and the joint weld is more vulnerable to cumulative damage than the parent material.The damage history will undoubtedly have a significant impact on the resistance and residual life of the structure in subsequent service.A large number of practice shows that most of the failure of the steel tube truss structure under the earthquake load begins with the welded joints,and the constitutive model considered the damage effect for the welded material which can be used for seismic and finite element analysis is not sufficient.The study on the damage accumulative evolution law of weld at joints is of great significance for the accurate characterization of the mechanical behavior of steel tube truss structures.For this reason,a series of studies are carried out on the damage accumulative evolution law of welding material of direct welded joints in steel tube truss structures:(1)The influence of weld modeling and constitutive relation selection on the mechanical performance analysis of directly welded K and T joints of steel tube truss structure was discussed.The results show that the numerical simulation results are much closer to the test results when establishing the weld model and taking into account the constitutive relation of welding material and base metal.On this basis,the parameter analysis of joints and the calculation formula of its bearing capacity are fitted with the optimization model considering the welding material and the material properties of the welding material,and the results prove that it has good applicability.(2)the T type fillet weld and butt weld of the steel tube truss structure direct welded joints are made,and the standard specimen is extracted from the tensile test for low cycle fatigue test.The damage cumulative evolution rule of welding material under cyclic loading is studied.The stress-strain hysteresis curve,?m-? curve and ?-? are obtained.The effects of loading method,material type,welding method and sampling position on the cycle life,hysteresis and damage cumulative evolution of the welding material are studied.The hysteretic behavior of welding material,pre welding steel and heat affected zone steel under different working conditions is compared and analyzed.The results show that the welding material is a cyclic softening material,and the stress amplitude,unloading stiffness and energy dissipation capacity degenerate with the increase of cycle,and the damage accumulation under cyclic loading leads to the degradation of the welding material.Loading method,material type,welding method and sampling position have certain effects on the damage evolution law.Compared with the steel products,the cycling characteristics of the welding material and the steel are obviously different.The steel is a cyclic hardening material,and the cycle life of the welding material is less than that of the steel.The ?m-? curve shows that Corresponding with the process of fatigue crack,the stress amplitude degradation of welding material and steel are both divided into three stages,but there is a significant difference between the welding material and steel.What's more,affected by the high temperature welding process,material properties of the steel in affected zone deteriorate,its cycling characteristic is closer to the welding material,and the cycle life is obviously reduced.The basic law of cumulative damage evolution of welding material during cyclic loading has been obtained from test,at the same time,it shows that there is a significant difference in cyclic characteristics between the welding material and its parent material.the damage accumulation of the welding material is more serious and complex than that of its parent material,and the damage accumulation should be considered in the structural analysis.(3)On the basis of the experimental research,combined with the damage mechanics theory and the related research status,the damage cumulative evolution equation is established by using the concept of damage variable with considering two factors of energy and deformation.The energy part is no longer limited to the ideal elastoplastic condition,and the damage variable value is always less than 1 before destroyed.The existing research situation about hysteresis model of metal materials under cyclic loading is further analyzed.Then based on Ramberg-Osgood model and Masing criterion,a hysteresis model containing damage cumulative effect is established.Then the parameters?,?,?,g,h,K'and n' were fitted through the experimental data obtained by low cycle fatigue test.(4)The hysteresis curve of the welding material is calculated by the hysteretic model and the results are compared with the test results.The results show that the model has good applicability and can well reflect the damage and degradation law of the welding material during the cycle process.