Study On Performance Of Circular Tubular Joints Based On Fracture Mechanism

Tubular joints are now widely used in space structures due to their excellent properties.Existing experimental studies have shown that fracture failure is an important failure mode of the tubular joints.However,it is not possible to make a reasonable and accurate prediction of the fracture behavior of the joints in finite element analysis,which makes it impossible to predict the failure mode and limit state of the joint reasonably when using the finite element method.And this leads to a degree of error in the evaluation of the performance of the joint.In this paper,the influence of weldingdetail on the fracture prediction of the joint is discussed by using method and the micromechanical fracture models and the fracture propagation model with finite element method.And then,the fracture behavior and ultimate loading capacity of the XK-joints under monotonic loading,X-joints bearing bending moment outside the plane underultra low cycle fatigue load and N-joints bearing axial force underultra low cycle fatigue load are analyzed respectively.The accuracy of the finite element simulation was evaluated by comparing the results of the analysis with the experimental data.The results show that it is very important to consider the weld size reasonably in the fracture prediction of the joints and the applicability of the VGM model to predict the ductile fracture of a coherent joint under monotonic loading is verified.The XKjoints are likely to undergo either strength failure or excessive plastic failure under monotonic loading.The failure mode of the joint depends on the geometric configuration of the joint and loading conditions of brace members.The micromechanicalfracture model CVGM can accurately predict the fracture behavior of X-joints under the out-of-plane bending moment and once the crack is produced,it indicates that the X-joint is reaching the limit state under the action of the outside bending moment.For the X-joints bearing out-of-plane bending moment,the failure occurs in the rising section of the skeleton curve.Therefore,the extreme points of the load-displacement curves can not be predicted by the original method as the ultimate bearing capacity and the state corresponding to the moment that the bearing capacity deduced to 85% can not be regarded as the limit state neither.It is necessary to use the fracture criterion to predict the seismic performance of joints.The micromechanicalfracture model CVGM can accurately predict the cracking behavior of the N-joint underultra low cycle fatigue load,however,the moment when joint cracks does not corresponding to the limit state.The fracture model based on crack propagation can effectively simulate the crack initiation behavior of the N-joint under reciprocating load,and thus can predict the limit state of the joint more accurately.