Simulation of fracture of a tubular X-joint using a shear-modified Gurson-Tvergaard-Needleman model

The full-range behaviour of materials and connections is important in the research of progressive collapse resisting mechanisms and structural seismic behaviour. There have been several recent studies on the fracture simulation of tubular joints. However, the reliability of the fracture models adopted for the tubular members, as well as for the weld materials, has not been examined under various stress states. This paper firstly describes an experimental study on square hollow section X-joint under axial loading. The joint is well tested into the post-ultimate range, and fracture occurs through the thickness of the chord. For the tubular sections and welds used in the joint, three types of coupon tests are carried out for each material, based on which a standard procedure for calibrating the parameters of the Gurson-Tvergaard-Needleman (GTN) model is proposed. Finite-element simulations of the joint test using the calibrated GTN model are then performed. The results show that the original GTN model can predict the initiation of cracks in the X-joint, but fails to predict the fracture propagation, under a shear-dominated stress state, with sufficient accuracy. A shear-modified GTN (referred to as SMGTN) model is developed by combining the GTN model with a maximum shear stress criterion, and shows better performance than the original GTN model, according to the simulated results, in predicting the entire fracture process.