Numerical Simulation And Theoretical Analysis Of Fatigue Crack Growth Of Steel Pipe-welded Hollow Spherical Joints

The installation of suspension crane on flat grid structure is a great innovation in engineering.Repeated loads from suspended cranes may lead to fatigue failure of grid structures,and the key to fatigue of grid structures is the fatigue of joints.Steel tube-welded hollow spherical joints(referred to as "pipe-spherical" joints)are one of the common lifting points for suspended crane flat grid structures.It is necessary to study their fatigue performance in depth.Based on the extended finite element method(XFEM)in numerical simulation software ABAQUS,the effect of element mesh size on stress intensity factor(SIF),a key parameter of fatigue,is discussed in this paper for the recurrence of fatigue crack growth in constant-amplitude fatigue test of tubular-spherical joints.The fatigue life of joints is estimated theoretically by the change rate of stress intensity factor and the increment of fatigue crack length.The main research work and conclusions are as follows:1.On the premise that the initial crack size is 0.5 mm,the effects of five different crack tip element sizes —0.5 mm,1.0 mm,1.5 mm,2.0 mm and 3.0mm on the stress intensity factors of cracks in specimens are discussed.The results show that the crack stress intensity factor is sensitive to the mesh size of the element and has a reverse relationship with it.When the crack tip element size does not exceed the initial assumed crack size,the stable stress intensity factor can be obtained by less integration paths;2.Based on the numerical simulation software ABAQUS and the Paris formula of fracture mechanics,the stress intensity factor at the crack tip and the increment of crack propagation are analyzed and calculated.The fatigue crack propagation life at different locations of the weld seam is obtained.The results show that the fatigue crack propagation life at the weld toe of the tube-ball joint is the smallest;3.Aiming at the fatigue crack propagation at the weld toe of pipe-to-ball joint in joint specimens,the characteristics of crack propagation at different stages are analyzed and discussed,which are as follows: stage I-before crack penetrates the wall thickness of pipe.The crack propagates slowly along the circumferential and radial direction,and then in the later stage,the rate of circumferential propagation is higher than that of radial propagation until the penetrating wall thickness;the crack propagating life in the whole stage accounts for 60%~80% of the total propagation life of the specimens.stage II-the wall thickness of the crack penetrating tube.The annular crack growth rateaccelerates obviously until the specimen fails.4.Based on the fatigue failure of the weld toe of the tube-to-sphere joint in the joint specimens,the displacement amplitude growth rate w history curve at the loading end of the joint specimens is analyzed and discussed.The concrete manifestations are as follows: in the initial stage of loading,w changes steadily,less than 1%,and the corresponding cycle life accounts for about 70% of the total propagation life;when the crack penetrates the pipe wall thickness,w increases sharply,reaching about 5%;when the specimen fails,w increases to about 60%,and the corresponding propagation life can be neglected.Considering the safety of specimens,it is suggested that the failure criterion of tubular-spherical joints should be w=15%.