Fatigue Performance And Topology Optimization Of Cast Steel Joint With Branches

With the development of architecture technology,the structure system and the joint formation are becoming more and more complex.Due to the integral casting,the cast steel joint with complex shape and inner cavity can be produced according to the force characteristics,so that the shapes,sections and structures of the joints are reasonably distributed.And it makes the joint has a beautiful and streamlined shape while improving the stress distribution.It solves the problem that the conventional joints cannot solve in large force and complex structure.Therefore,it is becoming more and more widely used in the large span space structure.However,under the action of wind load,seismic load and other alternating loads,the cast steel joint is prone to fatigue cumulative damage.It will cause fatigue failure of joints at the level below the design load due to the reciprocating action of alternating load,and resulting in catastrophic accidents and huge property losses.In this paper,the fatigue performance of the cast steel joint with branches under cyclic alternating load was studied.Taking the cast steel joint with three branches as the object,the fatigue problem of the joints and the weld seam of the connection between joint and component is analyzed.The topology optimization of the cast steel joints with branches under fatigue constraints is carried out too.Firstly,the ANSYS Workbench is used to study the fatigue performance of the cast steel joint with three branches under cyclic alternating load.The distribution position of the dangerous area of the joint is analyzed,and the minimum cycle time of the joint is calculated.The influence of different geometric parameters on the fatigue life of cast steel joint with three branches and the location of fatigue risk areas with different geometrical parameters are discussed by using the control variable method.The geometric parameters such as angle ?,diameter to thickness ratio ?,diameter ratio ?,fillet radius R1 and fillet radius R2 have great influence on the fatigue life of joints.Secondly,the AR method is used to simulate and calculate the pulsating wind speed time history curve of the cast steel joint with branches.By using the quasi steady assumption will change as the wind speed corresponding to the wind load.The load spectrum of the wind load at the joint is obtained,and the fatigue life of the joint under wind load is calculated by MSC.Fatigue.The effect of different geometric parameters on the fatigue life of the cast steel joint with branches under wind load is studied by using the control variable method.The effect of wind load on the fatigue performance of the cast steel joints with branches is significant.The minimum number of wind loads and the lowest safety were on the remittance of the core area between the main pipe and branch pipe,it is the weakest link of the whole joint.The geometrical parameter of ? has the most significant effect on the fatigue life of the joint.Thirdly,using spectral analysis method,the wave load is equivalent to a stationary random process.Using the modified P-M spectrum,the power spectral density function of wave is established.The stress response power spectrum and the fatigue life of the dangerous points of the cast steel joint with branches were carried out.Based on the finite element calculation results,the fatigue strength of dangerous points in cast steel joint with branches was checked.The results show that the fatigue damage to the structure has a very large proportion of the total fatigue damage when the sea condition is low but the wave height is large.The safety factor of fatigue strength of dangerous point can reach above 3.Fourth,the fatigue analysis of the ring-shaped butt weld of the cast steel joint with branches is carried out by the equivalent structural stress method.The insensitive structural stress of grid is determined by equivalent structural stress field and notch stress field.The effects of crack growth,notch stress and fatigue life on the transformation of the equivalent structural stress are taken into consideration.The fatigue life of the weld is calculated with the main S-N curve of fatigue design.Based on ABAQUS static analysis,the fatigue life of girth butt weld of cast steel joint with branches was analyzed by the software FE-SAFE/VERITY.The results show that the number of cycles in the most dangerous of the girth butt weld at the main pipe of the cast steel joint with branches is 105.76.The application of VERITY to automatically identify the field group located in the weld seam can be used to analyze multiple welds or multiple failure modes.The equivalent structure stress method has the advantage of mesh insensitivity.Finally,based on the fatigue life analysis principle,the fatigue constraint is transformed into stress constraint,and the topology optimization analysis is carried out for the cast steel joint with branches under fatigue constraints.Topology optimization of cast steel with branches under fatigue constraint was carried out by using the software OptiStruct in HyperMesh.It is found that the topology optimization of the cast steel joint with branches under the vertical and horizontal loads is different.Under the action of vertical load,the low stress area is mainly concentrated on the main pipe and the branch pipe,which is the key part of the optimization.Under the horizontal load,the low stress region is mainly distributed on the three branch pipse,and the optimal part of the pipe is distributed unevenly along the circumferential ring.