Characterization of ductile fracture behavior for Q355 steel under complex stress states

To reveal the fracture initiation mechanisms and establish the correlation between the ductile fracture mechanism and macroscopic mechanical behaviors in Q355 steel, ductile behavior of the Q355 steel was comprehensively examined across a broad spectrum of stress states. Seven types of specimens were utilized to generate various stress states, consisting of smooth and notched round bar, flat grooved plate, compression, shear, shear-compression, and shear-tension specimens. Finite element models of the fracture specimens were established, with good agreement between the simulated and measured force-displacement curves demonstrated. Through the experiments and simulations, the fracture strain, stress triaxiality, and Lode angle parameter were determined. The fracture mechanisms are strongly governed by stress state: dimple-dominated fracture prevails under high triaxiality, whereas a mixed shear-dimple mode dominates at low triaxiality. Based on test data, a recently developed fracture model that incorporates both stress triaxiality and Lode angle parameter was calibrated to construct 3D fracture locus. Furthermore, a validation test was carried out to evaluate robustness and calculation accuracy of present model. The results demonstrate strong consistency between experiment and simulation in terms of fracture displacement and fracture morphology. This work provides a theoretical foundation and practical insight for the fracture-resistant design and application of steel structures.