Research On Ultimate Bearing Capacity Of X-shaped Intersecting Joints Of Circular Steel Pipes Based On Micro-mechanism

Plane X-joints tubular are common joints,which are widely used for their good mechanical properties and simple manufacturing process.It is important for engineering design to study its mechanical properties.Ultimate bearing capacity is an important index to evaluate the mechanical properties of plane X-joints.At present,there are three methods to study the ultimate bearing capacity of plane X-joints:experimental research,finite element simulation and theoretical derivation.In this paper,the finite element simulation method is used to model and analyze in ABAQUS software.The model considers the change of the outer diameter ratio?of the branch pipe,the wall thickness ratio?of the main branch pipe and the size h_fof the welding leg.At the same time,the VGM fracture criterion of the micromechanical model is introduced to process the analysis results and obtain the ultimate bearing capacity.In this paper,the VGM fracture criterion is introduced into the space XKT type joints to study their ultimate bearing capacity.The main research work and achievements are as follows:(1)The VGM fracture criterion is introduced to deal with the finite element results,and the ultimate bearing capacity of X-joints tubular is analyzed systematically.The results show that other factors,such as the stress Lode angle(the function of the third invariant of deviator stress tensor)and the multiaxial complex stress state,should be taken into account when VGM fracture criterion is used to analyze the ultimate bearing capacity of joints.(2)Based on the VGM fracture criterion and the ultimate deformation criterion,the influences of the ratio?of the outer diameter of the main and branch pipes,the ratio?of the wall thickness of the main and branch pipes and the size h_fof the welding leg on the ultimate bearing capacity of the X-joints intersecting are analyzed.According to the VGM fracture criterion,with the decrease of parameter?,the change of ultimate bearing capacity of the joint is more complex,and the general trend is to reduce,and the reduction amplitude is not obvious.However,according to the ultimate deformation criterion,with the increase of parameter?,the ultimate bearing capacity decreases first and then increases.According to VGM criterion and ultimate deformation criterion,with the increase of parameter?,the ultimate bearing capacity of the joint increases gradually.According to the ultimate deformation criterion,with the increase of the weld leg size h_f,the ultimate bearing capacity of the joint increases gradually,but the increasing range is not obvious.(3)Based on the VGM fracture criterion,the ultimate bearing capacity of the space XKT type joints is studied by using the micromechanical model VGM.Under the condition of constant geometry size,the fracture displacement and fracture load increase with the increase of the boundary constraint of the web member.When the boundary constraint is constant,the fracture displacement and the fracture load decrease with the increase of the geometric size of the web.According to the limit deformation criterion,the boundary constraint of web member and the geometric dimension of web member have little influence on the ultimate bearing capacity of space XKT type joints.(4)For XKT type joints,the boundary constraint of web has little effect on the triaxial stress.The geometric dimension of web has a great influence on the triaxiality of joint stress,but the boundary constraint and geometric dimension have little influence on the peak value of triaxiality of joint stress.With the increase of the boundary constraint and the geometric size of the web,the equivalent plastic strain of the joint increases faster,and the joint is easier to reach the VGM fracture criterion.(5)When the boundary constraint of web member is less and the geometric dimension of web member is smaller,the plane X-joints with the same geometric dimension as the main support can be used to predict the ultimate bearing capacity of space XKT joint under monotonic load.