Study on Ductility Fracture of Welding Connections in Steel Structures

Welding is one of the most widely used techniques for connections in steel constructions. Materials in the heat affected zone ( HAZ) may be hardened due to phase transformation during the welding. Welding defects, such as cracks, may also exist in the HAZ. Therefore, the HAZ is often the most vulnerable link of welding connections. To investigate the ductile fracture at the HAZ of welding connections in beam-to-column joints and beam split joints, two specimens were made of Q355D steels, one for cross welding connections and the other for butt welding connections. The specimens were tested under monotonic tension. Perfect finite element ( FE) models and imperfect ones with a blunt notch at the stress concentration location were setup for the test specimens. Ductile fracture of the test specimens was predicted by the void growth model ( VGM) based on nonlinear FE analysis. A VUMAT subroutine was programmed for the simulation of ductile fracture propagation of the specimens by deleting fractured elements based on fracture criteria of the VGM during the FE analysis. FE models with different mesh sizes were established and mesh sensitivity analysis was conducted for ductile fracture simulation by the VGM. The tests showed that both specimens fractured at the HAZ with obvious plastic deformation after the peak load. The fracture displacements of the cross welding and the butt welding test specimens obtained by the tests are 15. 9 and 18. 3 mm respectively, indicating that both the welding connections have superior plasticity. The fracture locations predicted by the VGM based on the perfect FE models agree very well with the test observation of both specimens. However, the predicted fracture displacements of the two specimens are 30. 5 and 29. 1 mm respectively, which are much larger than the test results, indicating that welding defects have considerable adverse effect on ductile fracture of the welding connections. Based on the imperfect FE models with consideration of the defects, the fracture displacements of the two specimens predicted by the VGM are 14. 9 and 17. 2 mm, which agree well with the test results. The agreement indicates that introduction of blunt notches is a feasible approach of considering welding defects in ductile fracture analysis. The fracture paths obtained with the VUMAT subroutine are consistent with the test results, which verified the applicability of the VGM for the simulation of ductile fracture propagation of welding connections. Mesh sensitivity analysis showed that, because both specimens have low stress and strain gradient at the fracture location, ductile fracture simulation based on FE model with relatively coarse meshes can give accurate results at much lower computational cost.