Ductile crack initiation and propagation of structural steels under cyclic combined shear and normal stress loading

Ductile fracture preceding brittle fracture was observed in welded steel moment resisting frames during past strong earthquakes. Studies on ductile fracture were mainly focused on steels under normal stress loading, while seldom for those under combined shear and normal stress loading, which is also a common stress state for structural components, e.g., fillet welded joints and partial penetration welded joints. Ductile fracture of structural steels under combined shear and tension has been investigated through monotonic experiments in a previous study, and this paper aims to clarify the corresponding cyclic loading cases. Two series of specimens are manufactured, where they are respectively under pure shear, combined shear and normal stress loading. Meanwhile, a coupled fracture model considering material deterioration is proposed to simulate both ductile crack initiation and propagation, where a method to simply obtain the material parameter of the propagation rule is also proposed and validated through comparison between experimental and numerical results.